Astronomy & Astrophysics manuscript no. ms_AA_revision_correction_v03 ©ESO 2020
November 30, 2020
A population of hypercompact H ii regions identified from young
H ii regions
A. Y. Yang1, J. S. Urquhart2, M. A. Thompson3, K. M. Menten1, F. Wyrowski1, A. Brunthaler1, W. W. Tian4, 5, M.
Rugel1, X. L. Yang6, S. Yao6, M. Mutale3
1 Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121, Bonn, Germany
e-mail: ayyang@mpifr-bonn.mpg.de
2 Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
3 Centre for Astrophysics Research, School of Physics Astronomy & Mathematics, University of Hertfordshire, College Lane,
Hatfield, AL10 9AB, UK
4 CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012
5 University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China
6 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
Received , ; accepted ,
ABSTRACT
Context. The derived physical parameters for young H ii regions are normally determined assuming the emission region to be optically
thin. However, this assumption is unlikely to hold for young H ii regions such as hyper-compact H ii (HC H ii) and ultra-compact H ii
(UC H ii) regions and leads to underestimation of their properties. This can be overcome by fitting the SEDs over a wide range of
radio frequencies.
Aims. The two primary goals of this study are (1) to determine the physical properties of young H ii regions from radio SEDs in the
search for potential HC H ii regions, and (2) to use these physical properties to investigate their evolution.
Methods. We used the Karl G. Jansky Very Large Array (VLA) to observe the X-band and K-band with angular resolutions of ∼ 1.7′′
and ∼ 0.7′′, respectively, toward 114 H ii regions with rising-spectra (α5 GHz1.4 GHz > 0). We complement our observations with VLA
archival data and construct SEDs in the range of 1−26 GHz and model them assuming an ionization-bounded H ii region with uniform
density.
Results. Our sample has a mean electron density of ne = 1.6 × 104 cm−3, diameter diam = 0.14 pc, and emission measure EM =
1.9 × 107 pc cm−6. We identify 16 HC H ii region candidates and 8 intermediate objects between the classes of HC H ii and UC H ii
regions. The ne, diam, and EM change, as expected, but the Lyman continuum flux is relatively constant over time. We find that about
67% of Lyman-continuum photons are absorbed by dust within these H ii regions and the dust absorption fraction tends to be more
significant for more compact and younger H ii regions.
Conclusions. Young H ii regions are commonly located in dusty clumps; HC H ii regions and intermediate objects are often associated
with various masers, outflows, broad radio recombination lines, and extended green objects, and the accretion at the two stages tends
to be quickly reduced or halted.
Key words. ISM: H ii regions –ISM: evolution–radio continuum: stars–stars: massive–stars: formation
1. Introduction
One key question regarding massive star formation in the
youngest H ii region relates to how accretion proceeds against
the outward pressure therein (e.g., Keto & Wood 2006), as mas-
sive stars reach the main sequence while still accreting (e.g.,
Zinnecker & Yorke 2007; Motte et al. 2018). However, many
details of the earliest stages of H ii regions are unclear. Simple
analytic models suggest that the H ii region can be created by ei-
ther the inner, ionized part of the inflowing material (Keto 2002,
2003) or the ionized photoevaporative outflow (Hollenbach et al.
1994) fed by accretion (Keto 2007). The onset time for the de-
velopment of a H ii region is found to be early in the McKee &
Tan (2003) and Peters et al. (2010) turbulent core and ionization
feedback models, but the models of Hosokawa & Omukai (2009)
and Hosokawa et al. (2010) for a bloated protostar suggest that
this onset is later on. After the birth of H ii regions, the subse-
quent expansion has been modeled as uniform spherical bubbles
(Spitzer 1978), or asymmetrical flows into outflow-driven cav-
ities (Peters et al. 2010), and the expansion rates predicted by
different models could also be different (e.g., Bisbas et al. 2015).
Detailed observations toward the youngest H ii regions are cru-
cial to investigate their initial development and constrain theo-
retical models (Thompson et al. 2015, 2016).
The two youngest H ii region stages are commonly known
as hyper-compact H ii (HC H ii) and ultra-compact H ii (UC H ii)
regions (e.g., Kurtz 2005). The youngest is the HC H ii region
with a typical physical size (diam) of diam . 0.05 pc, an elec-
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tron density (ne) of ne & 105 cm−3, an emission measure (EM) of
EM & 108 pc cm−6, and a radio recombination line (RRL) with
a line width of ∆V & 40 km s−1 (Kurtz et al. 2000; Sewilo et al.
2004; Hoare et al. 2007; Murphy et al. 2010). The UC H ii region
is thought to be the next evolutionary stage after the HC H ii re-
gion, with diam . 0.1 pc, ne & 104 cm−3, EM & 107 pc cm−6,
and ∆V ∼ 25 − 30 km s−1 (e.g., Wood & Churchwell 1989; Af-
flerbach et al. 1996; Hoare et al. 2007). The defining character-
istics of these two stages (i.e., diam, ne, and EM) are somewhat
arbitrary, as the evolution from HC H ii regions to UC H ii re-
gions is thought to be continuous (e.g., Garay & Lizano 1999;
Yang et al. 2019). Compared to the hitherto discovered ∼ 600
UC H ii regions (Urquhart et al. 2007, 2009b; Lumsden et al.
2013; Urquhart et al. 2013; Cesaroni et al. 2015; Kalcheva et al.
2018; Djordjevic et al. 2019), only 16 HC H ii regions have been
identified in previous studies (summarized by Yang et al. 2019
and references therein). It is not yet clear at what stage and how
an HC H ii region evolves into an UC H ii region. Given the fact
that the observed sizes of young H ii regions are found to vary
with observing frequency (Panagia & Felli 1975; Avalos et al.
2006), it has been suggested that the classical quantitative crite-
ria for identifying HC H ii regions should be modified in order
to consider the variations (Yang et al. 2019), which could lead
to a better understanding of the intermediate object between an
HC H ii region and an UC H ii region. However, to understand
the relation between the two classes, and eventually to under-
stand the early stages of newly formed massive stars, reliable
properties toward a large sample of HC H ii regions and UC H ii
regions are needed to be determined.
Although young H ii regions around massive stars are heav-
ily obscured by a thick cocoon of molecular gas, they can nev-
ertheless be studied at radio wavelengths thanks to the ability of
radio radiation to penetrate the dense molecular gas. Therefore,
most studies of young H ii regions are based on radio contin-
uum observations (e.g., Wood & Churchwell 1989; Kurtz et al.
1994; van der Tak & Menten 2005; Gibb & Hoare 2007). The
radio continuum spectrum of an H ii region with spectral in-
dex α (S ν ∝ να) varies from +2 (optically thick) at low fre-
quency to −0.1 (optically thin) at high frequency. The turnover
frequency between the optically thick and thin regimes for ther-
mal bremsstrahlung is essentially a linear function of the electron
density (Mezger & Henderson 1967). A younger H ii region with
higher density will remain optically thick at higher frequencies.
For instance, UC H ii regions have a typical turnover frequency
of νt ∼ 5 GHz, while HC H ii regions have νt = 10 to 100 GHz
(e.g., Beltrán et al. 2007; Hoare et al. 2007; Keto et al. 2008;
Zhang et al. 2014). Therefore, young H ii regions with spectra
still rising in a higher frequency are potentially young and dense,
which might correspond to an early stage of UC H ii region or a
stage connecting UC H ii and HC H ii regions.
The physical properties of young H ii regions have been
measured in several previous studies (e.g., Wood & Church-
well 1989; Murphy et al. 2010; Urquhart et al. 2013; Kalcheva
et al. 2018; Medina et al. 2019), either by a targeted multi-band
observation on small samples of UC H ii regions (e.g., Murphy
et al. 2010) or using single-band surveys assuming that the gas
is optically thin to free-free emission (e.g., Urquhart et al. 2013;
Kalcheva et al. 2018). The assumption that H ii regions are opti-
cally thin would give unreliable physical properties if the H ii re-
gion is actually optically thick at the observed frequency. There-
fore, multi-band data taken over a large range of frequencies are
crucial in order to reliably determine the physical properties of
young H ii regions.
Table 1. Observed 114 rising spectra H ii regions. Columns, in order,
show source name, flux density at 1.4 GHz and 5 GHz respectively (see
Yang et al. 2019 for details), heliocentric distance, and bolometric lu-
minosity, and the reference these values are drawn from. Uncertainties
on the fluxes and distances are estimated to be 10%, and those on lumi-
nosity, 20%.
Name S 1.4GHz S5 GHz Dist Lbol [Ref.]
(mJy) (mJy) (kpc) (L)
(2) (3) (4) (5) (6)
G010.3009−00.1477 426.2 631.4 3.5 5.2 [1]
G010.4724+00.0275 31.3 38.4 8.5 5.7 [1]
G010.6223−00.3788† 327.6 483.3 2.4 5.7 [1]
G010.6234−00.3837 571.3 1952.2 5.0 5.7 [1]
G010.9584+00.0221 47.9 196.0 2.9 4.0 [1]
G027.9782+00.0789 89.3 124.0 4.8 4.2 [2]
G028.2003−00.0494 − 161.0 6.1 5.1 [1]
G028.2879−00.3641 410.9 552.8 11.6 5.9 [1]
G028.6082+00.0185 168.2 210.2 7.4 5.0 [1]
G029.9559−00.0168 1610.8 3116.2 5.2 5.7 [1]
G049.3704−00.3012 252.9 414.4 5.4 5.1 [3]
G048.6099+00.0270 56.5 131.2 9.8 5.1 [4]
Notes. Only a small portion of the data is provided here, the full table
is presented in Table A.1 and will be available in electronic form at the
CDS. Source names appended with a † refers to the sources observed
that could not be imaged. References: [1] Urquhart et al. (2018), [2]
Cesaroni et al. (2015), [3] Urquhart et al. (2013), [4] Kalcheva et al.
(2018).
In this work, we present the results of multi-band observa-
tions with the Karl G. Jansky Very Large Array1 (VLA) in X-
band (8–12 GHz) and K-band (18–26 GHz) of a sample of 114
young H ii regions. These sources were selected from a sample
of H ii regions with rising spectra between 1.4 GHz and 5 GHz,
that is, α5 GHz1.4 GHz > 0 (Yang et al. 2019). Together with archival
VLA data at 1.4 GHz and 5 GHz (see Sect. 2.1 for details), we
measure the spectral energy distribution (SED) between 1 and
26 GHz for each source in the sample, which covers both op-
tically thick and thin portions of their radio spectra. We model
every SED to find the best estimates for the physical properties.
This paper is organized as follows: Section 2 describes the
details of the sample, observation, and data reduction. Section 3
presents and discusses the observational results, the modeled
SEDs, and the radio properties of the sources and their distri-
butions. In Section 4, we discuss HC H ii region candidates, plus
a small sample of objects considered to be in an intermediate
phase between HC H ii and UC H ii regions. We use our obser-
vations to derive the physical properties (ne, diam, EM, Lyman
continuum flux) of H ii regions and compare our multi-band re-
sults to those estimated using the optically thin assumption. In
Section 5 we discuss the relations and distribution of all of the
UC H ii and HC H ii regions. We present a summary of this work
and highlight our conclusions in Section 6.
2. Observation
2.1. Sample selection
In Yang et al. (2019), we constructed a parent sample of 534
objects with rising radio spectral indexes between 1.4 GHz and
5 GHz using three JVLA surveys, THOR (The HI, OH, Recom-
bination line survey of the Milky Way, Bihr et al. 2016; Beuther
1 The Karl G. Jansky Very Large Array of the National Radio Astron-
omy Observatory: https://science.nrao.edu/facilities/vla
Article number, page 2 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Table 2. Summary of VLA observation parameters.
Parameter
Project VLA18B-065
Frequency (GHz) X-band (8 – 12) & K-band(18–26)
Array configuration C
Observing mode continuum
Bandwidth per channel 128 MHz
No. Channels 30 & 60
Primary beam ∼ 4.2′ & ∼ 1.9′
Synthesized beam ∼ 2.0′′ × 1.4′′ & ∼ 0.7′′ × 0.6′′
Observing dates 2019 Feb 07 & 2019 Feb 26
Time on-source per source ∼ 1 min
No. Targets 114
Total observing time 2h & 2.5h
Flux density calibrator (Jy) 3C286 (4.5 Jy)
Phase calibrators (Jy) J1832-1035 (1.28 Jy)
J1851-0035 (1.10 Jy)
J1922+1530 (1.0 Jy)
et al. 2016), MAGPIS (The Multi-Array Galactic Plane Imaging
Survey, White et al. 2005; Helfand et al. 2006), and CORNISH
(Coordinated Radio “N” Infrared Survey for High-mass star for-
mation, Hoare et al. 2012; Purcell et al. 2013). From an anal-
ysis of the combined radio, infrared, and submillimeter emis-
sion properties (Yang et al. 2019), we identified 120 young H ii
regions from the parent sample. This sample not only recovers
previously known HC H ii regions, but also includes broad RRL
objects with line widths of ∆V > 40 km s−1 and a number of
UC H ii regions with positive spectra (Yang et al. 2019). We ob-
served 114 young H ii regions in X- and K-band data taken with
the VLA. We use the data from archives and the literature for
the four sources in the initial sample that have not been observed
in the project, marked with a star in Tables 1 and 7. The final
sample includes 118 young H ii regions.
The flux densities and angular diameters of the 118 observed
sources are given in Table 1. The 1.4 and 5 GHz flux densities
are taken from Yang et al. (2019) and references therein. The
distances and bolometric luminosities are mainly drawn from
the results reported in Urquhart et al. (2018) 2, which includes
105 objects of the sample. For the remaining 13 sources with
no measurements in Urquhart et al. (2018), their distances and
bolometric luminosities are taken from three studies, namely Ce-
saroni et al. (2015), Urquhart et al. (2013), and Kalcheva et al.
(2018). The kinematic distances were computed by fitting the
radial velocity of each source to the Galactic rotation curve.
The kinematic distances near/far ambiguity (KDA) for sources
within the solar circle was resolved by CO emission line data and
H i absorption (e.g., Urquhart et al. 2013; Cesaroni et al. 2015;
Yang et al. 2016; Kalcheva et al. 2018) or using a combination
of H i analysis, maser parallax, and spectroscopic measurements
(Urquhart et al. 2018). The bolometric luminosity of the sam-
ple was taken from the same reference as the distance and was
determined by integrating the SED from near-infrared to submil-
limeter wavelengths (e.g., König et al. 2017).
2 This study by Urquhart et al. (2018) is based on the ATLASGAL
compact source catalog, which consists of ∼10 000 clumps showing
submillimeter wavelength emission from dust (Contreras et al. 2013;
Urquhart et al. 2014).
2.2. Observations and data reduction
Observations of 114 young H ii regions were carried out using
the VLA in C configuration. Instrument parameters used are
shown in Table 2. The observations were made at X-band (8–
12 GHz) and K-band (18–26 GHz), split into two subbands with
30 channels at X-band, and four subbands with 60 channels at
K-band, each channel with a bandwidth of 128 MHz, full stokes.
The synthesized beams in C configuration at X-band and K-band
are ∼ 1.8′′ and ∼ 0.7′′, and the FWHM primary beams sizes are
∼ 4.2′ and ∼ 2′, respectively. The typical on-source time for
each target is about one minute and the total observation time is
4.5 hours. The phase calibrators (J1832-1035, J1851-0035, and
J1922+1530) were observed every half hour at X-band and ev-
ery 12 minutes at K-band to correct the amplitude and phase of
the interferometer data by atmospheric and instrumental effects.
The pointing corrections at the high-frequency K-band were de-
termined by observing the nearby phase calibrators in interfero-
metric pointing mode. The absolute flux density scale at X-band
and K-band was calibrated by comparing the observations of the
standard flux density scale calibrator J1331+305 (3C286) with
its models provided by the NRAO.
Standard calibration and data reduction were performed us-
ing the Common Astronomy Software Applications package
(CASA, McMullin et al. 2007). Raw VLA data were calibrated
and reduced by running the CASA pipeline. We discarded the
first 3 s of data of every scan for calibrators to exclude the an-
tenna settling time. Flux and phase calibrator data were care-
fully examined to ensure high-quality data. A calibration table
was produced and applied to all targeted data. Each target was
inspected by eye to flag bad data such as phase scatters, errant
amplitudes, system-temperature spikes, which resulted in a mean
on-source integration time of ∼ 50 s for each source.
Images were constructed using the default Briggs robust pa-
rameter of zero, which provides a good trade-off between the
low thermal noise of natural weighting and the high resolution
of uniform weighting. Because of short on-source time (∼50 s),
we adopted to widest possible frequency ranges for each image
to do the clean task in CASA. In order to measure flux den-
sity at different frequencies, we produced multi-band images at
X-band and K-band. At X-band, three images were produced at
central frequencies of 9 GHz (8–10 GHz), 10 GHz (8–12 GHz),
and 11 GHz (10–12 GHz). Also, at K-band, three images were
produced at central frequencies of 20 GHz (18–22 GHz), 22 GHz
(18–26 GHz), and 24 GHz (22–26 GHz). The final beam size of
images at the central frequency of X-band, namely 10 GHz,
and at the central frequency of K-band, that is 22 GHz, are
∼ 2.1′′ × 1.4′′ and ∼ 0.7′′ × 0.6′′, respectively. Sources with
θ < 1.8′′ (X-band) and θ < 0.8′′ (K-band) are considered to be
unresolved. Sources with angular size θ > 1.8′′ (X-band) and
θ > 0.8′′ (K-band) are considered to be resolved and the decon-
volved sizes are given in Table 3.
3. Results and analysis
3.1. Observational results
In Fig. 1, we present images of three sources that show the typ-
ical variation in emission structure observed in our sample. The
contour levels shown in these images were determined using a
dynamic range power-law fitting scheme to meaningfully repre-
sent both high and low dynamic range images (e.g., Thompson
et al. 2006; Urquhart et al. 2009b; Yang et al. 2018). This has
been slightly altered from the scheme described by Thompson
Article number, page 3 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
18h51m21.60s22.08s
RA (J2000)
14.4"
07.2"
-0°12'00.0"
D
ec
 (J
20
00
)
G032.7441-00.0755
Cband(0.34'×0.34')
1
3
4
m
Jy
/b
ea
m
18h51m21.60s22.08s
RA (J2000)
14.4"
07.2"
-0°12'00.0"
D
ec
 (J
20
00
)
G032.7441-00.0755
Xband(0.34'×0.34')
1
5
9
13
m
Jy
/b
ea
m
18h51m21.60s22.08s
RA (J2000)
14.4"
07.2"
-0°12'00.0"
D
ec
 (J
20
00
)
G032.7441-00.0755
Kband(0.34'×0.34')
3
11
20
29
m
Jy
/b
ea
m
18h55m33.60s34.08s34.56s
RA (J2000)
+2°19'04.8"
12.0"
19.2"
D
ec
 (J
20
00
)
G035.4669+00.1394
Cband(0.34'×0.34')
9
18
27
m
Jy
/b
ea
m
18h55m33.60s34.08s34.56s
RA (J2000)
+2°19'04.8"
12.0"
19.2"
D
ec
 (J
20
00
)
G035.4669+00.1394
Xband(0.34'×0.34')
3
12
22
31
m
Jy
/b
ea
m
18h55m33.60s34.08s34.56s
RA (J2000)
+2°19'04.8"
12.0"
19.2"
D
ec
 (J
20
00
)
G035.4669+00.1394
Kband(0.34'×0.34')
2
4
6
m
Jy
/b
ea
m
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Cband(0.34'×0.34')
59
119
179
m
Jy
/b
ea
m
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Xband(0.34'×0.34')
52
195
339
483
m
Jy
/b
ea
m
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Kband(0.34'×0.34')
28
108
187
266
m
Jy
/b
ea
m
Fig. 1. Example images of three radio sources at C-band (left-column), X-band (middle-column), and K-band (right-column). The position of the
H ii region is marked with a plus. In the upper, middle, and lower rows, we show the maps for the compact H ii region G032.7441−00.0755, the
extended H ii region G035.4669+00.1394, and the H ii region G043.1665+00.0106 located in a cluster (see Sect. 3.1), respectively. C-band images
are from the CORNISH survey. The white contour levels of each image are equally spaced by 5σ and start at a level of 5σ. The green outline
shown in the lower row shows the polygon that was manually drawn around the H ii region located in a cluster. The image size and beam size are
shown in the upper-middle and lower-left of each image. The C-band, X-band, and K-band images for the whole sample are shown in Appendix
Figure B.1.
Table 3. Observational results of 112 young H ii regions at X-band (8–12 GHz) and K-band (18–26 GHz). Columns: (1) Source name; (2) and (3)
peak flux density and local RMS at X-band; (4-6) flux density at 9 GHz, 10 GHz and 11 GHz, respectively; (7) deconvolved source size at X-band;
(8) and (9) peak flux density and RMS at K-band; (10-12) flux density at 20 GHz, 22 GHz, and 24 GHz, respectively; (13) deconvolved source size
at K-band. The uncertainties in the flux measurements are estimated to be 10%.
Source name S Peak(X) σ(X) S 9 GHz S 10 GHz S 11 GHz θs(X) S Peak(K) σ(K) S 20 GHz S 22 GHz S 24 GHz θs(K)
(mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′) (mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
G010.3009−00.1477⊕ 88.1 3.1 700.3 686.7 661.4 6.8×6.6 15.9 0.7 433.4 419.6 392.6 6.5×6.4
G010.4724+00.0275 82.5 1.6 100.7 105.9 115.4 1.4×0.4 66.9 0.8 156.8 159.9 172.0 1.6×0.5
G010.6234−00.3837⊕ 1099.9 7.9 3071.4 3072.1 3314.8 4.2×3.8 572.3 14.6 2884.8 2857.2 2851.5 3.1×3.0
G010.9584+00.0221 186.3 1.5 258.3 256.2 265.0 1.2×0.9 91.3 1.2 210.7 202.6 200.6 1.0×0.7
G011.0328+00.0274 3.9 0.2 4.8 4.3 4.1 1.3×0.9 1.7 0.1 3.4 2.9 2.8 0.9×0.4
G011.1104−00.3985⊕ 70.5 0.7 350.4 334.8 327.7 9.5×9.4 15.6 0.4 136.1 123.3 126.1 2.2×1.7
G011.1712−00.0662⊕ 4.1 0.1 95.1 92.7 100.1 11.9×8.6 0.6 0.1 - - - −
· · · · · · · · · · · ·
G011.9368−00.6158⊕ 306.7 2.0 1116.4 1083.5 1098.3 3.4×3.2 76.4 2.0 656.1 652.4 629.6 2.8×1.8
G011.9446−00.0369⊕ 85.7 2.0 709.6 691.4 724.6 6.3×4.7 20.0 0.6 307.2 291.6 289.9 4.3×2.1
G012.1988−00.0345 29.6 0.4 66.0 64.8 63.9 2.0×1.9 6.5 0.2 59.6 54.7 55.9 2.0×2.0
G012.2081−00.1019 88.0 0.8 212.5 209.3 206.5 2.3×1.9 27.2 0.6 159.0 142.1 140.8 2.0×1.2
Notes. The ‘-’ symbol means no measurement is available. ⊕ indicates the sources are extended and their K-band flux densities should be consid-
ered to be lower limits. Only a small portion of the data is provided here, the full table is shown in Table A.2 and will be available in electronic
form at the CDS. † refers to the added 5 UC H ii regions with rising spectra between C and X band, see Sect 3.1.
Article number, page 4 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Log[Sint (mJy)]
0
10
20
Nu
m
be
r
X-band
K-band
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Log[Speak (mJy/beam)]
0
10
20
30
Nu
m
be
r
X-band
K-band
0 2 4 6 8 10 12 14 16 18 20 22 24 26
[ (′′)]
0
10
20
30
40
Nu
m
be
r
X-band
K-band
Fig. 2. Distributions of observation results such as integrated flux density S int, peak flux density S peak, and angular size θ, of 116 young H ii regions
at X-band (blue solid line) and K-band (red solid line). The bin sizes are 0.5 dex, 0.5 dex, and 1′′ for S int, S peak, and θ, respectively.
et al. (2006) and can be described as the following relationship
D = 5 × Ni + 5, where D is the dynamic range of the map (de-
fined as the ratio between the peak brightness and the 1σ RMS
noise), N is the number of contour lines, and ‘i’ is the contour
power-law index. Here, the minimum power-law index was one,
which resulted in linearly spaced contours starting at 5σ and in-
creasing in steps of 5σ. The starting contour level we adopted
for each target is variable, ranging from 5σ to 7σ according to
the RMS level of each image. The RMS noise level (σ) of each
image was determined using the standard deviation (STDEV =
1.4826×MADFM), where MADFM is the median absolute devi-
ation from the median (MADFM = median(|Xi −median(X)|),
where X is one element in the data set), in order to reduce the ef-
fects of outliers on noise measurement (e.g., Purcell et al. 2013).
The short on-source integration time of the target observation
(∼ 50 s) could lead to a rather high RMS level on the observed
field for some sources located in complex star formation regions.
The compact sources in the sample are directly fitted by 2D
Gaussian models using the imfit task in CASA (see upper pan-
els of Fig. 1). The resolved UC H ii regions are classified into a
variety of morphologies ranging from spherical to irregular (e.g.
Wood & Churchwell 1989; Urquhart et al. 2007, 2009b, 2013;
Purcell et al. 2013). The properties of the extended sources (see
the middle row of Fig. 1) and UC H ii regions located within a
cluster (see the lower row of Fig. 1) are determined from the flux
enclosed within a polygon fitted around the emission profile of
the source; this is determined by the noise level for an extended
source manually fitted around the emission for a cluster source,
which follows the same strategy used in the construction of the
CORNISH survey catalog (Purcell et al. 2013). The observa-
tional results of the extended sources or cluster sources in the
sample such as flux density (defined as the difference between
the aperture summed flux and background flux density divided
by the beam-area) and angular diameter (defined as intensity-
weighted diameter), as well as their uncertainties, can be mea-
sured by aperture photometry (for details of the aperture pho-
tometry method that we used see Sect. 5.3.2 of Purcell et al.
2013).
Analysis of the poor-quality X-band and K-band data for
seven young H ii regions (marked by † in Table 1) revealed that
their images are too confused to obtain reliable results and so
these have been excluded. We also add five sources identified as
UC H ii regions in CORNISH by Purcell et al. (2013) that are lo-
cated within our fields and have rising spectra between C-band
and X band in this work. Thus, the final observed sample con-
sists of 112 H ii regions. In Table 3, we give the observational
results and the derived properties for all of these sources.
In Table 4, we provide a statistical summary of the observed
and derived properties for each source at both the X-band and
Table 4. Summary of observational results and the derived physical pa-
rameters for 116 young H ii regions. In Columns (2-5) we give the mini-
mum, maximum, mean± standard deviation, and median values, respec-
tively, of each parameter.
Parameter xmin xmax xmean ± xstd xmed
observational properties at X-band
log[S int (mJy)] 0.58 4.20 2.20±0.76 2.20
log[S peak (mJy/beam)] 0.40 3.04 1.71±0.66 1.68
Angular size θ′′ 1.68 21.25 4.54±3.76 2.4
observational properties at K-band
log[S int (mJy)] 0.33 3.80 1.99 ± 0.74 2.0
log[S peak (mJy/beam)] -0.40 2.88 1.23±0.79 1.68
Angular size θ′′ 0.6 10.24 2.12 ± 1.95 1.34
Physical properties
log[ne(cm−3)] 3.14 5.65 4.20 ± 0.05 4.10
diam [pc] 0.004 0.81 0.14 ± 0.01 0.08
log[EM (pc cm−6)] 5.96 9.05 7.28± 0.06 7.09
log[NLy (s−1)] 45.37 49.82 47.81± 0.09 47.95
νt [GHz] 0.56 16.67 3.29 ± 0.31 1.95
Dust absorption fraction fd 0.14 0.99 0.67 ± 0.03 0.75
K-band. We estimate the uncertainties on the flux density and
angular size at both frequencies to typically be ∼10% by con-
sidering the calibration errors and errors of the measurement
method (e.g., Murphy et al. 2010; Sánchez-Monge et al. 2013).
In Fig. 2, we show the distributions of the derived parameters.
The distributions of integrated flux S int and peak flux density
S peak in the left and middle panels of Fig. 2 are similar at X-
band and K-band, which suggests that the majority of sources
are optically thin between these frequencies. The X-band shows
a slightly higher peak value of S int and S peak than K-band, some
of which may be due to the majority of sources having a turnover
frequency below X-band and the fluxes start to decrease after-
wards following the power-law of S ν ∝ ν−0.1 at the optically thin
regime of an H ii region. Some sources may be due to the larger
beam at X-band collecting more flux. The X-band has a larger
field of view and is more sensitive to larger angular scales than
K-band, which is why a larger proportion of the sources detected
at X-band are more extended in the right panel of Fig. 2.
3.2. Radio properties from the SED models
The physical characteristics of H ii regions (e.g., EM, ne, Lyman-
continuum flux NLy) can be estimated by the observed angular
sizes and flux densities at a given frequency, assuming that the
continuum emission comes from a homogeneous, optically thin
Article number, page 5 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
100 101 102
Frequency (GHz)
10 1
100
101
102
Fl
ux
 (m
Jy
)
ne = 2.79e+05
diam(pc) = 0.011
G032.7441-00.0755
100 101
Frequency (GHz)
102Fl
ux
 (m
Jy
)
ne = 8.40e+03
diam(pc) = 0.172
G035.4669+00.1394
Fig. 3. The radio SED fitting to flux density points for for ex-
ample compact and extended sources. Upper panel: SED fitting to
flux density points between 1 and 26 GHz for the compact example
G032.7441−00.0755 (upper row of Fig. 1). Lower panel: SED fitting
to flux density points between 1 and 11 GHz for extended example
G035.4669+00.1394 (middle row of Fig. 1) by excluding K-band flux
measurements. The best-fitting results for the electron density ne and
physical linear diameter diam are shown in the upper-left corner of each
plot. The best-fitting SEDs for the whole sample are shown in Fig B.1.
ionized gas (e.g., Urquhart et al. 2013; Kalcheva et al. 2018).
However, one should keep in mind that the physical properties
of young H ii regions might be underestimated or overestimated
by using a single frequency observation for two reasons: (i) The
young H ii region might be optically thick at the observed fre-
quency (e.g., Cesaroni et al. 2015); and (ii) the apparent angular
size depends on the observing frequency (e.g., Panagia & Felli
1975; Avalos et al. 2006; Yang et al. 2019). Therefore, to deter-
mine the properties of young H ii regions, it is essential to know
their spectral energy distribution (SED) over a wide frequency
range that covers both optically thick and thin emission (e.g.,
Murphy et al. 2010).
We use our multi-wavelength VLA data to construct SEDs
for the free-free emission in order to measure the radio proper-
ties of our sample of young H ii regions. We model each SED
for an ionization-bounded H ii region using the standard uniform
electron density model given by Mezger & Henderson (1967).
In this standard model, the integrated flux density at a given fre-
quency ν is given by S ν =
2kν2ΩTe(1−e−τ)
c2 using the Rayleigh Jeans
approximation, where Ω is the solid angle related to the physical
diameter diam and distance d of each source. The optical depth τ
of free-free radiation can also be represented as a function of fre-
quency (Mezger & Henderson 1967; Dyson & Williams 1997),
τ ∝ T−1.35e ν−2.1 n2e diam, where we assume an electron tempera-
ture Te = 104 K (Dyson & Williams 1997). Therefore, the radio
SED of an H ii region from the standard model is expected to
have a rising spectrum at low frequencies sν ∝ ν+2 (τ  1) and
a flat spectrum at high frequencies sν ∝ ν−0.1 (τ  1). Based
on the distances d in Table 1 and the observed fluxes sν in Ta-
ble 2, the SED model of each source has two free parameters:
the electron density ne and the physical diameter diam. The best
estimate for the two parameters can be obtained by fitting the
radio-frequency continuum spectrum of each source. The un-
certainties on flux measurements at these points are taken into
account in the fitting process. For compact and spherical H ii re-
gions in the sample, the derived density ne and diameter diam
from SED fitting represent averaged properties over the ionized
gas that are responsible for the free-free emission between 1 and
26 GHz. For the H ii regions with non-spherical geometry, this
spherical morphology model might introduce additional uncer-
tainty into the determination of the geometry-dependent param-
eters such as the electron density and diameter. Ideally, the cal-
culation should consider the three-dimensional structure of the
volume responsible for the radio emission; however, we do not
know the internal structure and any model of the source geome-
try would introduce additional unknown parameters. Moreover,
the morphologies of the nonspherical H ii regions are variable
between X-band and K-band as shown in Fig. B.1. To avoid the
complication when calculating the geometry-dependent parame-
ters, the peak physical properties averaged over the beam rather
than the entire source are commonly used for these nonspheri-
cal and irregular H ii regions in previous studies (e.g., Wood &
Churchwell 1989; Kurtz et al. 1994). In this work, the uniform
spherical model is sufficient to match the SEDs of the nonspher-
ical H ii regions, and the SED of each source takes into account
the multi-band radio emission of the entire source. Therefore,
the fitted ne and diam represent averaged properties over the en-
tire emission gas at multi-bands and can be used to shed light
on the physical condition of these nonspherical H ii regions as a
whole.
Figure 3 shows examples of the fitted SEDs for a
compact source G032.7441−0.076 and an extended source
G035.4669+00.1394. Owing to the lack of short baseline spac-
ings, the K-band flux measurements have been excluded from
the SED fitting of the extended sources in the sample. Includ-
ing the four sources with data from archives and references (see
Sect. 4 and Table 8), the SEDs and best-fitting models of all 116
H ii regions are presented in Appendix Fig. B.1. The EM of each
H ii region is then calculated using EM = ne2 × diam. Consid-
ering a mean error of ∼10% both in the flux density at each fre-
quency and the distance measurement, this gives typical errors
of ∼20% in ne, ∼10% in diam, and ∼40% in EM. The typical er-
rors that we adopted refer to the uncertainty on measurements, as
in previous studies (e.g., Sánchez-Monge et al. 2013; Kalcheva
et al. 2018), and would be larger if the uncertainty on the as-
sumptions in the model were considered.
The fitted parameters from radio SEDs are given in Table
5 along with the physical parameters derived from the analy-
sis presented in the following section. In panels (a), (b), and (c)
of Fig. 4, we present the distributions of the fitted parameters.
The physical sizes peak at 0.02 pc in panel (a), and 57% of the
sources (66/116) have physical diameters of less than 0.1 pc, as
shown in the subplot of that panel. This is consistent with the
majority of these being classified as UC H ii regions or smaller.
There are 9 sources with diam < 0.01 pc and the mean diameter
Article number, page 6 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
diam (pc)
0
10
20
30
40
Nu
m
be
r
0.00 0.02 0.04 0.06 0.08 0.100
2
4
6
8
10 diam<0.1pc
3.0 4.0 5.0 6.0
Log[ne (cm 3)]
0
10
20
Nu
m
be
r
6.0 7.0 8.0 9.0
Log[EM (pc cm 6)]
0
10
20
30
Nu
m
be
r
(a) (b) (c)
-0.5 0.0 0.5 1.0 1.5
Log[ t (GHz)]
0
10
20
Nu
m
be
r
t > 5 GHz : 20
t < 5 GHz : 96
45.0 46.0 47.0 48.0 49.0 50.0
Log[NLy (s 1)]
0
10
20
Nu
m
be
r
0.0 0.2 0.4 0.6 0.8 1.0
fd
0
10
Nu
m
be
r
(d) (e) (f)
Fig. 4. Distributions of the derived physical properties of 116 young H ii regions. Panels (a), (b), and (c): Distributions of the physical linear
diameter diam (a), the electron density ne (b), and the EM (c). The bin sizes are 0.05 pc, 0.25 dex, and 0.25 dex for diam, ne, and EM, respectively.
Panels (d), (e), and (f): Distributions of the turnover frequency νt (d), the Lyman continuum flux NLy (e), and the dust absorption fraction fd (f).
The bin sizes are 0.1 dex, 0.1 dex, and 0.5 dex for νt, fd, and NLy, respectively.
is diam =0.006 pc, corresponding to ∼ 1000 AU. This physical
scale implies that the sample could have coincidences with ra-
dio jets and jet candidates from massive young stellar objects
(MYSOs; Purser et al. 2016).
Figure 4 (b) shows the distribution of ne, which peaks at
104 cm−3. About 60% (70/116) of the sources have high densities
with ne > 104 cm−3. The 70 high-density H ii regions are com-
pact with a mean diameter of diam = 0.06 pc, implying that there
might exist small-scale and high-density objects in the sample
such as HC H ii regions (Kurtz 2005) and MYSO jets (Purser
et al. 2016).
Figure 4 (c) shows the distribution of EM, which peaks
at 107 pc cm−6, and most sources have EM between 3.2 ×
106 pc cm−6 and 1.0 × 108 pc cm−6. There are two groups in the
distribution of EM: one with EM < 108 pc cm−6 and the other
with EM > 108 pc cm−6, which indicates that there are sources
in the sample connected to the very early stages of H ii regions.
The median values of diameter (diam =0.08 pc), electron
density (ne = 1.3 × 104 cm−3), and EM (EM = 1.9×107 pc cm−6)
of our sample are consistent with typical values for UC H ii re-
gions. About 10% of the sources have ne > 105 cm−3, 36%
of the sample show diam < 0.05 pc, and 17% of them have
EM > 108 pc cm−6, which fulfill the standard quantitative cri-
teria of HC H ii regions. We discuss the potential HC H ii regions
in the sample in Section 4.
3.3. Derived physical characteristics
3.3.1. Turnover frequency νt
As a dividing line between the optically thin and thick regimes
of the radio spectrum of H ii region, the turnover frequency νt is
defined as the frequency where τ = 1 (Kurtz 2005). The flux den-
sity of H ii region peaks at ν > νt, and decreases as the square
of frequency at ν < νt. Using the formula provided in Mezger
& Henderson (1967) for a homogeneous H ii region, the opti-
cal depth can be expressed as a function of observing frequency
ν, electron temperature Te, which is assumed to be 104 K, and
emission measure EM:
τ = 0.082 ×
[Te
K
]−1.35
×
[
ν
GHz
]−2.1
×
[
EM
pc cm−6
]
, (1)
Setting τ = 1, the turnover frequency can be expressed as (Kurtz
2005):
[
νt
GHz
]
= 0.082 ×
[Te
K
]−1.35
×
[
n2e × diam
cm−3 pc
]0.476
(2)
The typical error for the νt is 30% by considering the typical
20% error in density estimation and 10% in diameter measure-
ments. Panel (d) of Fig. 4 presents the distribution of the turnover
frequency νt for this sample of young H ii regions (i.e., young
UC H ii regions), which peaks at νt ∼ 2 GHz and has a mean
value of νt ∼ 3.3 GHz. Both of the peak and mean turnover fre-
quencies of this sample of young H ii regions are lower than the
expected value of ∼ 5 GHz of UC H ii regions in Kurtz (2005)
with typical ne ∼ 3 × 104 cm−3 and diam ∼ 0.1 pc. This lower
turnover frequency found in the sample may be due to a large
fraction of detected emission from the optically thin low-density
region surrounded by a H ii region, as suggested in Steggles
(2016) and Steggles et al. (2017). Alternatively, many of these
H ii regions are simply optically thin.
The Fig. 4 (d) indicates two populations of H ii regions: one
with νt < 5 GHz and the other with νt > 5 GHz, which are re-
ferred to as optically thin and optically thick H ii regions in this
work, respectively. The optically thick H ii regions are found to
Article number, page 7 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
have higher density, higher emission measure, and smaller phys-
ical linear size compared to optically thin H ii regions, as shown
in Table 6.
3.3.2. Lyman continuum flux
For an optically thin H ii region in the photoionization equilib-
rium, the Lyman continuum ionizing flux NLy emitted by the
embedded massive star can be calculated from the radio con-
tinuum flux and heliocentric distance to the source (Sánchez-
Monge et al. 2013), as
[
NLy
s−1
]
= 8.9 × 1040
[
S ν
Jy
] [
ν
GHz
]0.1 [ Te
104K
]−0.45 [ d
pc
]2
, (3)
where S ν is the integrated flux density at frequency ν, Te is elec-
tron temperature assumed to be 104 K, and d is the distance to the
source. For each source in the sample, we use the S ν measured
in the optically thin part of the radio SED to calculate the Ly-
man continuum flux. The distance for each source is taken from
the literature (as discussed in Sect. 2.1). The typical error of the
derived Lyman continuum flux is ∼ 40% considering the error
in both kinematic distance and the integrated flux measurement
(e.g., Urquhart et al. 2013).
The distribution of the derived Lyman continuum flux is
shown in Fig. 4 (e), which peaks at 1048 s−1 and ranges from
1045.4 s−1 to 1049.9 s−1. The corresponding spectral types of the
zero-age main sequence (ZAMS) stars are between B0 and O4
listed in Table 5, assuming that a single star is responsible for
the ionization and there is no dust in the ionization-bounded H ii
region (e.g., Garay et al. 1993; Wood & Churchwell 1989). The
derived spectral type of the ZAMS star would be earlier or later
(e.g., Wood & Churchwell 1989), if multiple stars are respon-
sible for the ionization or if there is dust absorption within the
H ii region (e.g., Garay et al. 1993). For instance, the presence of
dust may lower the flux by a factor of two or more as the dust
absorption fraction ranges from ∼50% to ∼90% for UC H ii re-
gions (e.g., Wood & Churchwell 1989; Garay et al. 1993; Kurtz
et al. 1994), but if the emission was from a cluster then the spec-
tral type would be typically earlier by a subclass or two (Wood
& Churchwell 1989; Urquhart et al. 2013). The effects of cluster
and dust on determining the spectral type are probably compa-
rable and counterbalance each other. Therefore, the values we
estimated are reliable within a few subclasses.
3.3.3. Dust within H ii regions
Previous studies found that a significant fraction of the Lyman
continuum photons are absorbed by the dust within H ii regions
(Garay et al. 1993; Wood & Churchwell 1989; Kim & Koo
2001). By assuming that a single star is responsible for the ob-
served luminosity and the observed Lyman continuum flux of an
H ii region, the fraction of UV photons absorbed by dust within
H ii regions is defined as fd = 1− N′c/N?c (e.g., Wood & Church-
well 1989), where N′c is the number of observed ionizing photons
and N?c the predicted Lyman continuum photons derived from
spectral type based on the total infrared luminosity. As discussed
in previous studies (e.g., Garay et al. 1993; Wood & Churchwell
1989), fd should be taken as an upper limit as it is very likely to
be overestimated if the expected Lyman continuum photons are
excited by clusters of young stars rather than by a single star. For
instance, at a given total luminosity, the spectral type estimated
assuming a cluster that provides the entire infrared luminosity is
typically two or three subclasses later than the spectral type es-
timated assuming a single star (Wood & Churchwell 1989), and
thus leads to a lower expected Lyman continuum flux N?c than
derived assuming a single star. The observed N′c would be domi-
nated by the earliest spectral type in the clusters as the properties
of O-type stars change so dramatically between two subclasses
(e.g., Panagia 1973; Wood & Churchwell 1989), which has also
been found by Urquhart et al. (2013) who suggested that the
most massive stars within clumps dominate the observed proper-
ties. The upper limit of the fraction of Lyman continuum photons
absorbed by dust within H ii regions can range from 50% (Garay
et al. 1993; Kim & Koo 2001) to 90% (Wood & Churchwell
1989; Kurtz et al. 1994).
There is evidence of dust existing in the H ii regions in our
sample: all of them show bright 24µm emission in the MIPSGAL
survey (Carey et al. 2009) and strong 70µm emission in the Hi-
GAL survey (Molinari et al. 2010), at a high angular resolution
(∼6′′). After excluding ∼40% of the sources with Lyman excess
(see Sect. 5.2), the upper limit of the mean fraction absorbed by
dust within H ii regions for our sample is fd = 0.67±0.03, which
is consistent with previous results (e.g., Garay et al. 1993; Kim
& Koo 2001; Wood & Churchwell 1989), as shown in panel (f)
of Fig. 4. Among the 67 H ii regions with dust absorption, 43%
(29/67) of the sources with physical diameters diam < 0.1 pc
have a mean of fd = 0.79 ± 0.04 , and 57% (38/67) of the
sources with diam > 0.1 pc have a mean of fd = 0.58 ± 0.04.
This indicates that the dust absorption fraction tends to be more
significant for the more compact and presumably younger H ii
regions compared to the larger and more evolved H ii regions,
which agrees with the model in Arthur et al. (2004) who suggest
that the fraction of ionizing photons in H ii regions absorbed by
dust decreases with time.
4. Classification and properties of the optically
thick H ii regions
In Sect. 3.3.1 we identified 20 young optically thick H ii regions
with turnover frequencies larger than 5 GHz. As the turnover fre-
quency of an UC H ii region is ∼5 GHz (Kurtz 2005), the 20 opti-
cally thick H ii regions are very likely to be in the HC H ii region
stage or in an intermediate stage connecting the HC H ii region
and UC H ii region stages. The quantitative criteria for HC H ii
regions, UC H ii regions, and the intermediate objects between
the two stages, as summarized from the literature (e.g., Wood
& Churchwell 1989; Kurtz et al. 1994; Afflerbach et al. 1996;
Kurtz 2005; Hoare et al. 2007), are presented in Table 7.
Among the 20 optically thick H ii regions, 7 sources are asso-
ciated with previously identified HC H ii regions that have been
summarized in Table 1 of Yang et al. (2019). In Fig. 5 we show
the distribution of the ne, EM, and diam of 18 optically thick H ii
regions, as we excluded two objects (G043.1652 & G043.1665)
in the optically thick sample that are associated with unrecovered
HC H ii regions listed in Table 8 and marked with an asterisk (see
Sect. 4.3). On this plot, we indicate the region of parameter space
where HC H ii regions are expected to reside (i.e., ne > 105 cm−3
and diam < 0.05 pc), and we show the evolutionary trend from
HC H ii region to the stage between HC H ii region and UC H ii
region in the physical parameter space. Of the optically thick H ii
regions, 14 satisfy these criteria. The remaining sources all sat-
isfy the size criterion for HC H ii regions but their electron den-
sities are too low and so these are considered to be intermediate
between the HC H ii and UC H ii region stages.
In Figure 6-8, we present three-color infrared maps of each
H ii region. In these maps, we include contours of the dust and
Article number, page 8 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Table 5. Derived physical properties of 116 young H ii regions.
Name ne diam EM νt logNLy Spectral fd
(105 cm−3) (pc) (107 pc cm−6) (GHz) (photons s−1) Type
(1) (2) (3) (4) (5) (6) (7) (8)
G010.3009−00.1477 0.09 0.119 0.92 1.69 47.94 O9.5 0.86
G010.4724+00.0275 1.43 0.022 45.2 10.77 48.11 O9 0.94
G010.6234−00.3837 0.16 0.166 4.39 3.55 48.9 O6.5 0.81
G010.9584+00.0221 0.36 0.029 3.78 3.31 47.35 B0 -
G011.0328+00.0274 0.13 0.014 0.24 0.89 45.57 B1 -
G011.1104−00.3985 0.07 0.145 0.62 1.4 47.94 O9.5 0.27
G011.1712−00.0662 0.09 0.053 0.45 1.21 46.91 B0 -
G011.9368−00.6158 0.07 0.155 0.86 1.63 48.12 O9 0.19
G011.9446−00.0369 0.17 0.075 2.2 2.56 47.84 O9.5 -
G012.1988−00.0345 0.07 0.148 0.65 1.43 47.98 O9 0.77
Notes. Only a small portion of the data is provided here, the full table is presented in Table A.3 and will be available in electronic form at the CDS.
Table 6. Summary of the derived physical parameters for the 96 op-
tically thin H ii regions (νt < 5 GHz) and the 20 optically thick H ii
regions (νt > 5 GHz). Columns (2-5) provide the minimum, maximum,
mean± standard deviation, and median values, respectively, of each pa-
rameter.
Parameter xmin xmax xmean ± xstd xmed
The 20 optically-thick H ii regions sample
log[ne(cm−3)] 4.37 5.65 5.11± 0.07 5.12
diam [pc] 0.004 0.23 0.035 ± 0.01 0.023
log[EM (pc cm−6)] 8.00 9.05 8.50± 0.08 8.58
log[NLy (s−1)] 46.21 49.55 47.77± 0.20 47.80
νt [GHz] 5.28 16.67 9.73 ± 0.80 9.94
Dust absorption fraction fd 0.16 0.99 0.81 ± 0.05 0.88
The 96 optically-thin H ii regions sample
log[ne(cm−3)] 3.15 4.69 4.02± 0.03 4.01
diam [pc] 0.006 0.81 0.16 ± 0.02 0.11
log[EM (pc cm−6)] 5.96 7.73 7.16 ± 0.04 7.02
log[NLy (s−1)] 45.37 49.83 47.82± 0.1 47.97
νt [GHz] 0.56 3.60 1.91 ± 0.08 1.80
Dust absorption fraction fd 0.14 0.97 0.62 ± 0.03 0.66
Table 7. Quantitative criteria for HC H ii regions, UC H ii regions and
intermediate objects (HC H ii→ UC H ii ) between the two stages, sum-
marized from the literature.
Parameters HC H ii HC H ii→ UC H ii UC H ii
Size (pc) . 0.05 ∼ [0.05 − 0.1] . 0.1
ne (cm−3) & 105 ∼ [104 − 105] & 104
EM (pc cm−6) & 108 ∼ [107 − 108] & 107
RRL ∆V (km s−1) & 40 ∼ [25 − 40] < 40
radio emission and any coincident masers so that we can in-
vestigate their environments and associations with other star-
formation tracers. We individually discuss the properties of the
optically thick H ii regions with respect to their environment,
their association with dense gas, and star-formation tracers in
the following sections, and we follow the order that is presented
in Table 8.
10 2 10 1
diam (pc)
105
n e
(c
m
3 )
G024.7898
G034.2581
G043.1657
G028.2003
G045.0712
G032.7441
G034.2573
G045.4656
G010.4724
G030.0096
G061.4770
G034.2572
G045.0694G049.3666
G051.6785
G060.8842
G030.5887
G033.1328
diam = 0.05 pc
EM = 1E8 pc cm 6
ne = 1E5 cm 3
Known HCHII
HCHII & HCHII candidate
HCHII-to-UCHII
108
EM
(p
cc
m
6 )
Fig. 5. Distribution of properties of 18 optically thick HC H ii regions
identified in Sect. 4. The vertical and horizontal dotted and dashed lines
indicate the standard quantitative criteria of an HC H ii region. The red
filled circles show HC H ii regions and HC H ii region candidates iden-
tified in this work while the red filled circles with lime circles identify
the previously known HC H ii regions. The filled blue circles show the
intermediate objects between the HC H ii and UC H ii region stages. The
magenta arrow shows the evolutionary trend.
4.1. HC H ii regions and candidate HC H ii regions identified
in this work
G010.4724+00.0275: This source is located in the
G10.47+0.03 complex region that hosts three UC H ii re-
gions (Wood & Churchwell 1989), water masers (Hofner
& Churchwell 1996), 6.7 GHz methanol masers (Pestalozzi
et al. 2005), various complex molecules (Hatchell et al. 1998),
and massive molecular outflows along the NE–SW direction
(López-Sepulcre et al. 2009). This object is resolved into two
compact sources, G10.47+0.03A and G10.47+0.03B, in Wood
& Churchwell (1989) with a resolution of 0.4′′, which is also
seen in the K-band emission shown as contours in the upper-left
panel of Fig. 7 with two blended compact components. The
radio source is positionally coincident with methanol and water
masers, a bright mid-infrared point source and is embedded
in a dense molecular clump as traced by the ATLASGAL
emission, and therefore clearly associated with star formation
activity. Its physical properties such as ne = 1.43 × 105 cm−3,
diam = 0.022 pc, EM = 4.52×108 pc cm−6, and logNLy = 48.11,
Article number, page 9 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
Table 8. Summary of the physical parameters and the classification of the 20 optical thick H ii regions identified in this work. The classifications
given in col. 6 are HC H ii regions (Class: HC), HC H ii region candidates (Class: HC?), and intermediate objects (Class: HC-UC); these have
been assigned based on their electron density ne, physical diameter diam and emission measure EM, derived from the SED fitting method. We
also include four sources with νt ∼ 3.5 GHz , such as G010.9584+00.0221 and G035.5781−00.0305 that have previously been identified as
HC H ii regions but not recovered by this work (see Sect. 4.3), as well as G030.8662+00.1143 and G030.7197-00.0829 that show broad RRL with
∆V > 40 km s−1.
Name ne diam EM ∆V(RRL)[ref.] Class log[Lbol]log[NLy] Maser clump
(105 cm−3)(pc) (108 pc cm−6) (km s−1) (L) (s−1) emission?outflow?
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
HC H ii and candidate HC H ii identified in this work
G010.4724+00.0275 1.43 0.022 4.52 - HC? 5.7 48.11 Yes Yes
G024.7898+00.0833† 3.38 0.008 9.18 40 (H66α)[1] HC 5.2 47.52 Yes Yes
G028.2003-00.0494† 1.41 0.027 5.35 74(H92α)[2] HC 5.1 48.37 Yes Yes
G030.0096-00.2734 1.91 0.0043 1.56 - HC? 3.8 46.21 Yes Yes
G032.7441-00.0755 2.79 0.011 8.28 40.3(Hnα)[3] HC 5.0 47.69 Yes Yes
G034.2573+00.1523 3.55 0.0046 5.82 48.7(H42α)[3] HC 4.8 46.58 Yes Yes
G034.2581+00.1533† 3.01 0.0041 3.73 48.4(H76α)[2] HC 4.8 46.54 Yes Yes
G043.1657+00.0116† 1.57 0.046 11.32 63.9(H66α)[4] HC 6.9 48.69 Yes Yes
G045.0712+00.1321† 1.22 0.040 5.89 40(H76α)[5] HC 5.7 48.73 Yes Yes
G045.4656+00.0452 1.02 0.023 2.36 47.8(H39α)[3] HC 5.0 47.88 Yes Yes
G061.4770+00.0892 4.45 0.0040 7.88 - HC? 5.1 46.81 Yes Yes
G030.5887−00.0428? 2.08 0.009 3.91 56.2(H40α)[3] HC 4.0 47.28 Yes Yes
Intermediate objects (HC H ii→ UC H ii regions)
G034.2572+00.1535 0.52 0.038 1.01 22.8(H76α)[2] HC−UC 4.8 48.03 Yes Yes
G045.0694+00.1323 0.74 0.026 1.40 16.7(H92α)[2] HC−UC 5.7 47.76 - Yes
G049.3666−00.3010 0.94 0.031 2.69 34.5(H40α)[3] HC−UC 5.1 48.05 Yes Yes
G051.6785+00.7193 0.64 0.026 1.07 - HC−UC 5.0 47.68 Yes Yes
G060.8842−00.1286 0.93 0.012 1.08 - HC−UC 4.2 46.40 Yes Yes
G030.7197−00.0829? 0.22 0.093 0.45 43.0(H40α)[3] HC−UC 4.7 48.44 Yes Yes
G030.8662+00.1143? 0.37 0.031 0.43 44.9(H39α)[3] HC−UC 4.1 47.46 Yes Yes
G033.1328−00.0923? 0.21 0.10 0.46 43.0(H39α)[3] HC−UC 5.0 48.54 Yes Yes
Previously identified HC H ii regions not resolved in the current work
G010.9584+00.0221∗ 0.36 0.029 0.38 43.8(H92α)[2] HC 4.0 47.35 Yes Yes
G035.5781−00.0305∗ 0.22 0.093 0.45 50.0(H42α)[3] HC 5.3 48.36 Yes Yes
G043.1652+00.0129∗ 0.88 0.053 4.15 53.7(H66α)[2] HC 6.9 48.91 Yes Yes
G043.1665+00.0106∗ 0.24 0.22 1.22 48.6(H66α)[2] HC 6.9 49.55 Yes Yes
References: 1. Beltrán et al. (2007); 2, Sewilo et al. (2004); Sewiło et al. (2011); 3, Kim et al. (2017); 4, De Pree et al. (1997);
5, Keto et al. (2008); 6, (Zhang et al. 2014). Notes: for G032.7441−00.0755, the RRL Hnα indicates n=39,40,41,42. Symbols †
and ∗ indicate the known HC H ii regions summarized in Table 1 of Yang et al. (2019). Symbol ? represents the four H ii regions
with data from the literature and archives.
imply that it is likely an HC H ii region. Its natal clump has
a mass of 2.57 × 104 M and a bolometric luminosity of 5.0
×105 L (Urquhart et al. 2018). Its spectral type of O5.5 derived
from the bolometric luminosity is earlier than O9 derived from
Lyman continuum flux, which supports the hypothesis that
this source is located in a cluster, as reported in Pascucci et al.
(2004).
G024.7898+0.0833: This source is an HC H ii region identi-
fied by Beltrán et al. (2007), which is found to be associated
with many CH3OH masers (Surcis et al. 2015; Bartkiewicz et al.
2016) and OH masers (Forster & Caswell 2000; Caswell et al.
2013), H2O masers (Caswell et al. 1983; Forster & Caswell
2000), and outflows traced by CO (Furuya et al. 2002; Beltrán
et al. 2011) and SiO (Codella et al. 2013). Its physical properties
such as ne, diam, and EM (Table 8) are consistent with previ-
ous results (Beltrán et al. 2007; Cesaroni et al. 2019). Its natal
clump has a mass of 7.64 × 103 M and a bolometric luminosity
of 1.58 × 105 L (Urquhart et al. 2018). The spectral type of this
HC H ii region O6.5 derived from the infrared luminosity (Table
1) is much earlier than O9.5 derived from the Lyman contin-
uum flux which includes contributions from the nearby UC H ii
region G024.7889+00.0824 in the field. One possible explana-
tion for the discrepancy of spectral type is that this source is
located in a cluster and/or a significant amount of Lyman contin-
uum photons are absorbed by the surrounding dust, with an up-
per limit on the dust absorption fraction of fd= 92% (see Sects.
3.3.2 and 3.3.3). As this HC H ii region shows extended 4.5 µm
emission, it is associated with an extended green object as de-
fined by Cyganowski et al. (2008).
G028.2003−0.0494: This source is a known HC H ii region
identified by Sewilo et al. (2004), which is found to be asso-
ciated with the 37.7 GHz CH3OH maser (Ellingsen et al. 2011),
OH masers (Argon et al. 2000; Caswell et al. 2013), and H2O
masers (Urquhart et al. 2011). Its physical properties such as
ne, diam, and EM listed in Table 8 are consistent with previ-
ous results (Sewiło et al. 2011). Its natal clump has a mass of
4.45 × 103 M and a bolometric luminosity of 1.30 × 105 L
(Urquhart et al. 2018), which is associated with molecular out-
Article number, page 10 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
10.94°10.96°10.98°
Galactic Longitude
+00.00°
+00.02°
+00.04°
G
al
ac
tic
 L
at
itu
de
G010.9584+00.0221
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
OH
CH3OH
H2O
24.78°24.80°
Galactic Longitude
+00.06°
+00.08°
+00.10°
G
al
ac
tic
 L
at
itu
de
G024.7898+00.0833
3'×3'8.0 m
4.5 m
3.6 m
10"×10"
0.1pc
OH
CH3OH
H2O
28.18°28.20°28.22°
Galactic Longitude
-00.06°
-00.04°
G
al
ac
tic
 L
at
itu
de
G028.2003-0.0494
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
CH3OH
OHH2O
35.56°35.58°35.60°
Galactic Longitude
-00.04°
-00.02°
G
al
ac
tic
 L
at
itu
de
G035.5781-0.0305
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
OHH2O
43.14°43.16°43.18°
Galactic Longitude
+00.00°
+00.02°
G
al
ac
tic
 L
at
itu
de
G043.1657+00.0116
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
OH
CH3OH
H2O
G043.1665
G043.1657
G043.1652
45.06°45.08°
Galactic Longitude
+00.12°
+00.14°
G
al
ac
tic
 L
at
itu
de
G045.0712+00.1321
3'×3'8.0 m
4.5 m
3.6 m
G045.0712
G045.0694
20"×20"
0.1pc
OH
CH3OH
H2O
Fig. 6. Three-color composition image (or RGB image) from Spitzer GLIMPSE 8 µm (red), 4.5 µm (green), and 3.6 µm (blue) bands (Benjamin
et al. 2003; Churchwell et al. 2009) for the HC H ii regions discussed in Sect. 4. Lime or red circles show the radio sources in the field from the
CORNISH survey. The upper-right zoomed-in images for each panel show the peak position of H2O maser (magenta cross) and OH maser (black
cross), and the linear scale-bar of 0.1 pc in white. Gray contours in the image show 870 µm emission from ATLASGAL (Schuller et al. 2009), and
the lime (or red) contours show K-band 22 GHz emission presented in this work. The red contours in the bottom-right panel show X-band 10 GHz
emission as the K-band emission is missing for source G045.0694. The FWHM beam of GLIMPSE (2′′) and K-band observations are indicated
by the white circles shown in the lower-left corner of each image.
flows (Maud et al. 2015; Yang et al. 2018). Its spectral type O6.5
derived from the bolometric luminosity is earlier than O7.5 de-
rived from the Lyman continuum flux that includes the contri-
bution from its nearby UC H ii region G028.1985−00.0503 with
NLy = 5.0×1047. This could be the result of this source being lo-
cated in a cluster, as shown in the middle-left panel of Fig. 6, or
could be due to the fact that about 43% of the Lyman continuum
photons are absorbed by the surrounding dust.
G030.0096−00.2734: This compact radio source, located in
the W43 star-forming complex (e.g., Blum et al. 1999; Med-
ina et al. 2019; Gao et al. 2019), is the first of the sample
that was found to be associated with an infrared dark cloud
(G030.01−0.27; Battersby et al. 2011), which itself is associ-
ated with many molecular lines (Schlingman et al. 2011) as
well as methanol masers (Breen et al. 2015). Its natal clump,
AGAL030.008−0.272, is associated with a molecular outflow
identified by Yang et al. (2018), which has a maximum out-
Article number, page 11 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
10.46°10.48°
Galactic Longitude
+00.02°
+00.04°
G
al
ac
tic
 L
at
itu
de
G010.4724+00.0275
3'×3'8.0 m
4.5 m
3.6 m
10"×10"
0.1pc
CH3OH
H2O
30.00°30.02°
Galactic Longitude
-00.28°
-00.26°
G
al
ac
tic
 L
at
itu
de
G030.0096-00.2734
3'×3'8.0 m
4.5 m
3.6 m
10"×10"
0.1pc
CH3OH
34.24°34.26°34.28°
Galactic Longitude
+00.14°
+00.16°
G
al
ac
tic
 L
at
itu
de
G034.2572+00.1535
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
OH H2O
CH3OHG034.2581
G034.2573
G034.2572
32.72°32.74°32.76°
Galactic Longitude
-00.10°
-00.08°
-00.06°
G
al
ac
tic
 L
at
itu
de
G032.7441-0.0755
3'×3'8.0 m
4.5 m
3.6 m
10"×10"
0.1pc
OH
CH3OH
H2O
45.44°45.46°45.48°
Galactic Longitude
+00.04°
+00.06°
G
al
ac
tic
 L
at
itu
de
G045.4656+00.0452
3'×3'
20"×20"
0.1pc
OH
H2O
61.46°61.48°61.50°
Galactic Longitude
+00.08°
+00.10°
G
al
ac
tic
 L
at
itu
de
G061.4770+00.0892
3'×3'8.0 m
4.5 m
3.6 m H2O
20"×20"
0.1pc
G061.4770
OH
Fig. 7. As described in Fig. 6, except in this case the sources include newly identified HC H ii regions and intermediate objects. The gray contours
in the image of G061.4770+00.0892 show the 500µm emission from Hi−GAL.
flow velocity of 4.5 km s−1. It is the only radio source in its na-
tal clump, and its spectral type B1 derived from the bolometric
luminosity is consistent with B0.5 derived from the Lyman con-
tinuum photons, indicating a lack of dust within this H ii region.
The radio emission is coincident with a compact mid-infrared
point source confirming it is associated with an embedded pro-
tostellar object. The physical properties of G030.0096−00.2734
are consistent with this source being an HC H ii region at a very
early evolutionary stage.
G030.5887−00.0428: This source shows compact radio emis-
sion at 5 GHz CORNISH, as shown in the middle-right panel
of Fig. 8. Its flux densities at high frequencies were obtained
in project VLA18A-066, with 217.70 mJy at 15.5 GHz and
223.53 mJy at 16.5 GHz. With flux densities at low frequency of
1.4 GHz and 5 GHz (summarized in Yang et al. 2019), its physi-
cal properties can be determined from the radio SED. Water, hy-
droxyl, and methanol maser sites (Argon et al. 2000; Pestalozzi
et al. 2005; Urquhart et al. 2011) are detected in its vicinity, and
molecular outflows (Yang et al. 2018) are found to be associated
with its natal clump. Its natal clump AGAL030.588−00.042 has
a mass of 758 M and a bolometric luminosity of 1.12 × 104 L
(Urquhart et al. 2018), and shows a broad millimeter RRL H40α
with ∆V = 56.2 km s−1 (Kim et al. 2017). It is the only radio
source in the parent clump, and its spectral type B0.5, obtained
from the bolometric luminosity, is consistent with that of a B0
star derived from the radio luminosity, indicating the absence of
Article number, page 12 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
dust within this H ii region. The broad RRL line, compact size,
and high electron density are consistent with this source being
classified as an HC H ii region.
G032.7441−00.0755: The radio emission associated with this
source is weak and very compact and there is bright emission
at 70µm from the Hi−GAL survey (Molinari et al. 2010), while
no counterpart is seen at mid-infrared wavelengths (8µm; see
middle-right panel of Fig. 7). This source was found to host H2O
masers (Caswell et al. 1983), OH masers (Caswell et al. 2013),
and CH3OH masers (Bartkiewicz et al. 2016), and is associated
with CO outflows (Yang et al. 2018), broad molecular lines such
as SiO (2-1) (Csengeri et al. 2016), N2H+, and HCO+ (Shirley
et al. 2013) and millimeter RRLs (∆V = 40.34 km s−1; Kim
et al. 2017). The blueshifted and redshifted methanol masers
spots mapped by Bartkiewicz et al. (2016) have a similar ori-
entation to the blueshifted and redshifted outflows mapped by
Yang et al. (2018). Its physical parameters (ne = 2.79×105 cm−3,
diam = 0.011 pc, EM = 8.28× 108 pc cm−6, νt = 14.37 GHz) are
consistent with other HC H ii regions and we therefore identify
this as a new mid-infrared-dark HC H ii region detection. Fig-
ure 7 shows that it is the only radio source in its natal clump.
Its spectral type O7 derived from the bolometric luminosity is
earlier than O9.5 derived from the Lyman continuum flux, indi-
cating that about 88% of the Lyman continuum photons were
absorbed by dust within this H ii region. It could be the best
example to trace the dynamics associated with the final stages
of accretion in massive star formation because it is still dark at
8µm and covers a significant broad component of ionized- (e.g.,
RRL), shocked- (e.g., SiO), and molecular gas (e.g., CO).
G034.2572, G034.2573 and G034.2581: These three H ii
regions lie in G34.26+0.15, a well-studied complex region
that contains three UC H ii regions (Wood & Churchwell 1989;
Sewilo et al. 2004): G34.26+0.15A (G034.2573+00.1523),
G34.26+0.15B (G034.2581+00.1533) and G34.26+0.15C
(G034.2572+00.1535); these are marked with red circles in the
middle-left panel of Fig. 7. This complex also hosts H2O masers
(Hofner & Churchwell 1996), OH masers (Forster & Caswell
1999; Ruiz-Velasco et al. 2016), CH3OH masers (Breen et al.
2015) and numerous molecules (Fu & Lin 2016; Kim et al.
2000), as well as infall/outflows traced by CO or water masers
(e.g., Wyrowski et al. 2016; van der Tak et al. 2019; Yang
et al. 2018; Imai et al. 2011). Broad radio recombination lines
(RRLs) are detected in G34.26+0.15B and G34.26+0.15C with
a line-width of ∆V > 40 km s−1 (Sewilo et al. 2004). This is
also found in their natal clump AGAL034.258+00.154 (Kim
et al. 2017, 2018). G34.26+0.15B is considered to be a HC H ii
region candidate (G034.2581+00.1533, Sewilo et al. 2004;
Yang et al. 2019), which is blended with G34.26+0.15C in the
C-band and X-band images, and is only resolved in the higher
resolution K-band image. G034.2572+00.1535 is associated
with G34.26+0.15C, which is an extended source, and can be
resolved into three compact sources, all of which have RRL line
widths of ∆V > 40 km s−1 (Sewilo et al. 2004).
G034.2572+00.1535 is very likely to host candidates in an
evolutionary stage between HC H ii region and UC H ii region.
The nearby source G034.2573+00.1523 is also likely to be asso-
ciated with an HC H ii region.
G043.1652, G043.1657 and G043.1665: These three sources
are located in the well-known star-forming region W49A com-
plex that is associated with CO outflows (Scoville et al.
1986). As shown in the bottom-left panel of Fig. 6, the three
sources are associated with three HC H ii regions W49A A
(G043.1652+00.0129), W49A B (G043.1657+00.0116), and
W49A G (G043.1665+00.0106) in the W49A complex (De Pree
et al. 1997, 2004; Sewilo et al. 2004), which are found to be asso-
ciated with many CH3OH (Bartkiewicz et al. 2014; Breen et al.
2015), OH (Argon et al. 2000), and H2O (De Pree et al. 2000;
Urquhart et al. 2011) masers. G043.1657+00.0116 (W49A B)
has ne = 1.57 × 105 cm−3, diam = 0.046 pc, EM = 11.32 ×
108 pc cm−6, and logNLy = 48.69, which is consistent with the
typical value of HC H ii regions, as reported by De Pree et al.
(2000).
G043.1652+00.0129 (W49A A) is resolved into two com-
pact components at higher resolution ∼ 0.05′′ (De Pree et al.
2000, 2004). Its physical properties such as ne = 0.88×105 cm−3,
diam= 0.053 pc, EM= 4.15×108 pc cm−6, and logNLy = 48.91,
are consistent with previous results in De Pree et al. (1997) for
W49 A at a similar resolution of ∼ 1′′. However, the derived
properties are slightly below the typical values of HC H ii regions
and also show smaller ne, smaller EM, and larger diam compared
to the results measured at higher resolution (0.05′′) with ne =
6.1 × 105 cm−3, diam = 0.056 pc, and EM = 83 × 108 pc cm−6
(De Pree et al. 2000). This might be due to the fact that our ob-
servation includes not only the two compact components but also
a larger fraction of optically thin emission around them.
G043.1665+00.0106 (W49A G) is also multiply peaked at
higher resolution ∼ 0.05′′ (De Pree et al. 2000, 2004). Its phys-
ical properties, such as ne = 0.24×105 cm−3, diam = 0.24 pc,
EM= 1.22×108 pc cm−6, and logNLy = 49.55, are consistent
with the results in de Pree et al. (1996) for W49A G. The ne
is slightly smaller compared to the measurements at higher reso-
lution with ne > 1.0 × 105 cm−3 for the two main compact com-
ponents (De Pree et al. 2000), which may result from the large
amount of optically thin emission around these compact compo-
nents.
G045.0712 and G045.0694: The radio emission consists of
two distinct sources: the stronger source G045.0712+00.1321
and the weaker source G045.0694+00.1323, offset by ∼6′′ (as
shown in bottom-right panel of Fig. 6). G045.0712+00.1321
was identified as an HC H ii region by Keto et al. (2008) and
Sewiło et al. (2011) (G45.07+0.13 NE). The physical proper-
ties of G045.0712+00.1321 indicate that this HC H ii region is
associated with a O6.5 type massive star, which supports the
previous results and classification by Sewiło et al. (2011). The
fainter of the two, G045.0694+00.1323, is likely to be transi-
tioning into an UC H ii region based on the distribution of ra-
dio properties shown in Fig. 5. Their radio emission is coinci-
dent with a bright extended infrared source and a dense submil-
limeter clump, AGAL045.071+00.132, in Urquhart et al. (2018).
The natal clump is associated with extended molecular outflows
aligned W to E (Yang et al. 2018). This source is also host to
H2O (Hofner & Churchwell 1996), OH (Argon et al. 2000),
and CH3OH masers (Kurtz et al. 2004). The presence of two
very young H ii regions, molecular outflows, and three different
species of masers would suggest that this clump hosts a young
proto-cluster.
G045.4656+00.0452: This compact radio source is embedded
in a dense molecular clump and is associated with an extended
mid-infrared source, as well as water (Forster & Caswell 1999)
and OH (Argon et al. 2000) maser emissions (see bottom-left
Article number, page 13 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
49.36°49.38°
Galactic Longitude
-00.32°
-00.30°
-00.28°
G
al
ac
tic
 L
at
itu
de
G049.3666-00.3010
3'×3'8.0 m
4.5 m
3.6 m
30"×30"
0.1pc
G049.3704
G049.3666
H2O
51.66°51.68°51.70°
Galactic Longitude
+00.70°
+00.72°
+00.74°
G
al
ac
tic
 L
at
itu
de
G051.6785+00.7193
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
CH3OH
H2O
60.86°60.88°60.90°
Galactic Longitude
-00.14°
-00.12°
G
al
ac
tic
 L
at
itu
de
G060.8842-00.1286
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
H2O
30.58°30.60°
Galactic Longitude
-00.06°
-00.04°
-00.02°
G
al
ac
tic
 L
at
itu
de
G030.5887-00.0428
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
CH3OH
OHH2O
33.12°33.14°
Galactic Longitude
-00.10°
-00.08°
G
al
ac
tic
 L
at
itu
de
G033.1328-00.0923
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
H2O
30.70°30.72°30.74°
Galactic Longitude
-00.10°
-00.08°
-00.06°
G
al
ac
tic
 L
at
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G030.7197-00.0829
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
H2O
30.86°30.88°
Galactic Longitude
+00.10°
+00.12°
G
al
ac
tic
 L
at
itu
de
G030.8662+00.1143
3'×3'8.0 m
4.5 m
3.6 m
20"×20"
0.1pc
H2O
Fig. 8. As described in Fig. 6, except in this case the sources are HC H ii regions and intermediate objects between HC H ii and UC H ii re-
gions. The gray contours in the image of G060.8842−00.1286 show the 500 µm emission from Hi−GAL. The lime contours in the images of
G030.7197−00.0829, G030.8662+00.1143, G030.5887−00.0428 and G33.1328−00.0923 show the 5 GHz emission from CORNISH survey.
Article number, page 14 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
panel of Fig. 7). Its natal clump AGAL045.466+00.046 is also
associated with bipolar outflows (Yang et al. 2018) and broad
H39α RRL (∆v = 47.8 km s−1; Kim et al. 2017). Cyganowski
et al. (2008) identified this source as an extended green ob-
ject associated with an infrared dark cloud. The physical pa-
rameters determined for this source (ne = 1.02 × 105 cm−3,
diam = 0.023 pc, EM = 2.36 × 108 pc cm−6, νt = 7.89 GHz)
are consistent with this being classified as an HC H ii region.
G061.4770+00.0892: This object is very compact with a de-
convolved size similar to that of the beam (∼ 0.7′′) at K-band,
and its radio emission is blended with a nearby cometary UC H ii
region detected both in 5 GHz CORNISH and X-band obser-
vations described in this work. However, the two sources are
separated in the high-resolution observations (∼ 0.4′′; Wood &
Churchwell 1989) and our K-band observations (∼ 0.7′′). As
shown in the bottom-right panel of Fig. 7, the near-infrared RGB
image of this source presents extended 4.5 µm emission and so
it could be associated with an extended green object (EGO) as
defined by Cyganowski et al. (2008). A bipolar molecular out-
flow aligned NE to SW (Phillips & Mampaso 1991; White &
Fridlund 1992) and water masers (Henkel et al. 1986; Svoboda
et al. 2016) are detected toward its parent cloud. Broad RRL
components (Garay et al. 1998) and strong OH (1665/67 MHz)
absorption (Sarma et al. 2013) are reported towards this source
and the other physical properties derived from radio emission
indicate that this source is likely to host an HC H ii region.
4.2. Intermediate objects between HC H ii and UC H ii
regions
According to their physical properties, there are eight objects
located in the evolutionary stages between HC H ii regions and
UC H ii regions in Table 8. Two out of the eight sources (i.e.,
G034.2572+00.1535 and G045.0694+00.1323) are associated
with clusters of H ii regions that have already been discussed in
Sect. 4.1; in the following sections we provide brief notes on the
other six intermediate objects.
G030.7197−00.0829: This source was resolved at 5 GHz by
CORNISH. The physical properties (ne = 0.22 × 105 cm−3,
diam = 0.09 pc, EM=0.45 × 108 pc cm−6, νt = 3.6 GHz) can
be determined from the radio SED based on flux densities of
464.58 mJy at 1.4 GHz (White et al. 2005), 969.33 mJy at 5
GHz (Purcell et al. 2013), and 570 mJy at 43 GHz (Leto et al.
2009). These results are consistent with the measurements in
Leto et al. (2009). Its natal clump AGAL030.718−00.082 has
a mass of 6.6 × 103 M, a bolometric luminosity of 5.5 × 104 L
(Urquhart et al. 2018), and a broad millimeter RRL H40α with
∆V = 43.0 km s−1 (Kim et al. 2017) , and is associated with CO
outflows (Yang et al. 2018). Its Lyman continuum flux agrees
with its bolometric luminosity, indicating a lack of dust within
this H ii region. Therefore, this source appears to be an interme-
diate object between HC H ii and UC H ii regions.
G030.8662+00.1143: The SED of this resolved source was
constructed from the flux densities of 137.17 mJy at 1.4 GHz
and of 255.2 mJy at 5 GHz from White et al. (2005), 306.0
mJy at 6.7 GHz, and 356.0 mJy at 8.4 GHz from Walsh et al.
(1998), as well as 560 mJy at 43 GHz from Leto et al. (2009).
Its physical characteristics measured from the radio SED, such
as ne =0.37×105 cm−3, diam =0.03 pc, EM=0.42×108 pc cm−6,
and νt =3.5 GHz, are consistent with previous measurements
(Leto et al. 2009). Water maser sites (Urquhart et al. 2009a,
2011) are detected in its vicinity and molecular outflows (Yang
et al. 2018) are found to be associated with its natal clump. Its na-
tal clump AGAL030.866+00.114 has a mass of 295 M, a bolo-
metric luminosity of 1.30 × 104 L (Urquhart et al. 2018), and a
broad millimeter RRL H39α with ∆V = 44.9 km s−1 (Kim et al.
2017). Its spectral type B0.5 obtained from the bolometric lu-
minosity is consistent with O9.5 derived from radio luminosity,
indicating the absence of dust in this H ii region. Therefore, this
source appears to be an intermediate object.
G033.1328−00.0923: This source shows extended emission
at 5 GHz CORNISH, shown as lime contours in the bottom-left
panel of Fig. 8. With flux densities of 173.43 mJy at 1.4 GHz
and 378.59 mJy at 5 GHz summarized in Yang et al. (2019), as
well as 461.2 mJy at 9 GHz and 675.3 mJy at 15 GHz measured
by Kurtz et al. (1994), we construct its radio SED between 1 and
15 GHz. Its physical properties from the SED fitting are consis-
tent with results in Kurtz et al. (1994). Water masers (Pestalozzi
et al. 2005; Kurtz & Hofner 2005) are detected in its vicinity and
molecular outflows (Yang et al. 2018) are found to be associated
with its natal clump. Its natal clump AGAL033.133−00.092 has
a mass of 5.0 × 103 M, a bolometric luminosity of 1.1 × 105 L
(Urquhart et al. 2014, 2018), and a broad millimeter RRL H39α
with ∆V = 43.0 km s−1 (Kim et al. 2017). As it is only one radio
source in the natal clump, its spectral type O7 obtained from the
bolometric luminosity is consistent with O7.5 derived from the
radio luminosity. Therefore, this source is likely to be an inter-
mediate object between HC H ii and UC H ii region.
G049.3666−00.3010: This object appears to have a nearby
UC H ii region to the east referenced as G049.3704−00.3012
(marked with a red circle in the upper-left panel of Fig. 8). Both
of these H ii regions are embedded towards the center of the
dense clump AGAL049.369−00.301, which has been associated
with a broad H40α RRL with ∆V = 34.5 km s−1 (Kim et al.
2017). The optically thick radio source is coincident with an ex-
tended mid-infrared source, and two water masers have been de-
tected in its vicinity (Valdettaro et al. 2001; Xi et al. 2015).
G051.6785+00.7193: This radio source is very compact at all
radio bands presented in this work, while it can be resolved into
two sources at high angular resolution ∼ 0.2′′ at 1.3 cm using the
VLA in Rodríguez-Esnard et al. (2012). The radio source is em-
bedded in a very compact and centrally condensed ATLASGAL
clump AGAL051.678+00.719 with a mass of 2.88×103 M and
is associated with a very bright mid-infrared point source that
has a luminosity of 1.0 × 105 L. The natal clump is also asso-
ciated with water and methanol masers (Sridharan et al. 2002;
Rodríguez-Esnard et al. 2012), and molecular outflows aligned
with extended mid-infrared emission going from NE to SW
(Beuther et al. 2004), as presented in the upper-right panel of
Fig. 8.
G060.8842−00.1286: This object is southwest of the two
H ii regions (see middle-left panel of Fig. 8) in the massive
star-forming region S87IRS1 (Barsony 1989), the other being
a nearby extended and weak H ii region (Purcell et al. 2013) that
has been resolved out at K-band in this work. The S87IRS1 is
associated with the clump JPSG060.886-00.129 in Eden et al.
Article number, page 15 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
(2017), which is itself associated with a molecular outflow (Bar-
sony 1989; Xue & Wu 2008). The radio source is associated with
bright mid-infrared emission and coincident with a water maser
(Kurtz & Hofner 2005). At high resolution ∼0.4′′, the clump is
found to be fragmented into multiple millimeter cores (Beuther
et al. 2018). Its bolometric luminosity agrees with its radio lumi-
nosity, suggesting a lack of dust within this H ii region.
4.3. HC H ii regions not resolved in this work
In addition to the optically thick radio sources identified in
this work, we include notes on another four HC H ii regions
that have been identified in previous studies (e.g., Wood &
Churchwell 1989, Sewilo et al. 2004 and Zhang et al. 2014)
but are unresolved in our observations. Two of the four
(G043.1652+00.0129 and G035.5781−00.0305) are unresolved
mainly due to the fact that our observations include their nearby
UC H ii regions as the resolution is not sufficient to resolve
the emission into individual sources. The remaining two re-
gions (G043.1665+00.0106 and G010.9584+0.0211) are not re-
covered by this work primarily because our observations in-
clude a large amount of surrounding ionized gas emission as
this diffuse gas is optically thin. Therefore, the derived proper-
ties in this work represent average values for sources with co-
existing emission from HC H ii and nearby UC H ii regions or
represent a complex weighted average over the compact sources
plus the surrounding diffuse ionized gas, and thus do not sat-
isfy the criteria for classification as HC H ii regions. However,
these sources have previously been identified as HC H ii regions
and we therefore include these sources in this section for com-
pleteness. The source names and derived properties are given
towards the end of Table 8. Two sources (G043.1652+00.0129
and G043.1665+00.0106) in the W49A complex region have al-
ready been discussed together in Sect. 4.1 and are therefore not
described again here. Images of the remaining two HC H ii re-
gions are presented in Fig. 6 and brief notes are provided below.
G010.9584+0.0221: This source is an HC H ii region and is
located in the western part of the G10.96+0.01 region and sur-
rounded by more diffuse ionized gas, as suggested by Sewilo
et al. (2004). Its physical properties, such as ne = 0.36 ×
105 cm−3, diam = 0.029 pc, EM = 0.38 × 108 pc cm−6 and
logNLy = 47.35, are all consistent with the results reported by
Sewilo et al. (2004) and Sewiło et al. (2011). In spite of the re-
ported broad H92α line with ∆V = 43.8 ± 1.5km s−1, the de-
rived properties are slightly below the typical values of HC H ii
regions, which might be due to the previous VLA observations
(Sewilo et al. 2004, Sewiło et al. 2011) and this work includes
a significant amount of optically thin emission from the diffuse
ionized gas around this source, and both results are likely to be
underestimates by averaging over the compact source plus its
surrounding ionized gas, as mentioned in Sewilo et al. (2004)
and Yang et al. (2019). Its natal clump has a mass of 398 M and
a bolometric luminosity of 1.0 × 104 L (Urquhart et al. 2018),
and is associated with high velocity outflow wings identified in
CO spectra from the SEDIGISM survey (Schuller et al. 2017).
In this case, the luminosity and Lyman continuum flux are both
contributed by the same source, meaning that the spectral type
derived from the bolometric luminosity is consistent with that
derived from the radio luminosity; B0.5 and B0, respectively.
G035.5781−00.0305: This radio emission can be resolved
into two extremely close sources at 2 cm and 3.6 cm with a res-
olution of < 1′′ (Kurtz et al. 1994): the source to the west has
been identified as an HC H ii region G35.578−0.030 (Zhang et al.
2014) and the source to the east as an UC H ii , G35.578−0.031
(Kurtz et al. 1994). These are seen as a single blended source
in our radio maps (see the middle-right panel of Fig. 6). This
source is associated with OH masers (Argon et al. 2000) and
H2O masers (Forster & Caswell 1999; Urquhart et al. 2011). The
physical properties for the blended source G035.5781−00.0305
are ne = 0.22 × 105 cm−3, diam = 0.093 pc, EM = 0.45 ×
108 pc cm−6, and logNLy = 48.36. Thus, G035.5781−00.0305 in
this work has smaller ne, smaller EM and larger diam compared
to the HC H ii region G35.578−0.030 in Zhang et al. (2014) with
ne = 3.3 × 105 cm−3, diam = 0.018 pc, EM = 1.9 × 109 pc cm−6.
Its natal clump has a mass of 6.8×103 M and a bolometric lumi-
nosity of 2.0×105 L (Urquhart et al. 2018), which is associated
with molecular outflows (Yang et al. 2018).
4.4. Summary
In Table 8 we summarize the physical properties of the sources
of our sample and the associated discussion in the preceding
text. Inspection of this table reveals that in addition to the phys-
ical properties (ne, diam, EM and RRL), which are typical for
HC H ii regions, all the sources of our sample are found to be
embedded towards the centres of dense molecular clumps and
are also commonly associated with various masers, molecular
outflows, broad RRLs, and extended green objects, all of which
are all signposts of active star formation. The bolometric lumi-
nosities tend to be higher than the radio flux suggests, which
is consistent with these being associated with a forming proto-
cluster. These optically thick H ii regions are therefore the best
examples to investigate the relation between HC H ii regions and
UC H ii regions, to study the birth of H ii regions, and therefore
to understand the final stages of accretion in massive star forma-
tion.
There are 13 HC H ii regions, 3 HC H ii region candidates,
and 8 intermediate objects listed in Table 8. Among them, four
HC H ii regions and three HC H ii region candidates are reported
here for the first time. Based on the classification of HC H ii re-
gions in Table 7, it is difficult to assess the completeness of the
sample of HC H ii regions and intermediate H ii regions identi-
fied in this study because there are four HC H ii regions, marked
with an asterisk in Table 8, that are in very close proximity to
other UC H ii regions that we were not able to resolve.
5. Discussion
5.1. Implications of the evolution of young H ii regions
As suggested by classical theoretical models (Dyson et al. 1995;
Mezger & Henderson 1967), H ii regions are expected to ex-
pand over time, which results in decreasing ne and EM and in-
creasing diam, as seen in Fig. 5. The plots shown in this fig-
ure display a clear evolutionary trend in ne, diam, and EM from
HC H ii regions to the intermediate objects between the HC H ii
and UC H ii region stages. The mean values of physical proper-
ties range from ne = 2.5×105 cm−3, diam = 0.012 pc, and EM =
5.5 × 108 pc cm−6 for HC H ii regions, to ne = 0.79 × 105 cm−3,
diam = 0.03, and EM = 1.58 × 108 pc cm−6 for intermediate ob-
jects, and thus ne tends to change quickly compared to the EM
and diam at the earliest times of H ii region stage.
Article number, page 16 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
100 101
t (GHz)
1045
1046
1047
1048
1049
1050
N L
y
(s
1 )
HCHII & HCHII candidate
HCHII-to-UCHII
UCHII in this work
106 107 108 109
EM (pc cm 6)
1046
1047
1048
1049
1050
N L
y
(s
1 )
HCHII & HCHII candidate
HCHII-to-UCHII
UCHII in this work
CORNISH UCHII
103 104 105
ne (cm 3)
1046
1047
1048
1049
1050
N L
y
(s
1 )
HCHII & HCHII candidate
HCHII-to-UCHII
UCHII in this work
CORNISH UCHII
10 2 10 1 100
diam (pc)
1046
1047
1048
1049
1050
N L
y
(s
1 )
NLy diam1.4 ± 0.1
HCHII & HCHII candidate
HCHII-to-UCHII
UCHII in this work
CORNISH UCHII
Fig. 9. The plots of the evolution and correlation of the derived physical parameters. νt vs. NLy (upper-left), EM vs. NLy (upper-right), ne vs.
NLy (bottom-left), and diam vs. NLy (bottom-right) for HC H ii regions (red dots), intermediate objects between HC H ii region and UC H ii regions
(blue), UC H ii regions in this work (green dots), and CORNISH UC H ii regions (gray dots). The CORNISH UC H ii regions sample refers to the
whole CORNISH UC H ii regions sample from Kalcheva et al. (2018) by excluding UC H ii regions in this work. The magenta arrow indicates the
evolutionary trend of the physical properties.
To investigate the evolution of physical properties of H ii re-
gions over a wide range of evolutionary stages, we add the COR-
NISH UC H ii regions from Kalcheva et al. (2018) that are pre-
sumably in a later stage compared to our sample. Evolution of
the Lyman continuum flux NLy, turnover frequency νt, and emis-
sion measure EM is presented in Fig. 9 for the three subsamples
discussed here and for the four subsamples by adding the more
evolved CORNISH UC H ii regions. We see that νt decreases as
the H ii region evolves, from 11.5 GHz for HC H ii regions to
6.4 GHz for intermediate objects, and to 1.8 GHz for UC H ii re-
gions, as expected from the theoretical model in Mezger & Hen-
derson (1967). It is interesting to note that there is no obvious
correlation between the Lyman continuum flux and the evolution
of the H ii regions. Furthermore, we find no significant correla-
tion between NLy and EM with ρ = −0.01 and p-value = 0.85,
and between NLy and ne with ρ = −0.07 and p-value = 0.3 in the
four subsamples. In addition, the mean value of NLy ∼ 1048 s−1
is consistent throughout the four evolutionary phases, from the
HC H ii region and HC H ii region candidates, to intermediate
objects, to UC H ii regions in this work, and to more evolved
UC H ii regions in CORNISH. These results suggest that there
is effectively no evolution of the Lyman continuum photon flux
with changes in the νt, ne, and EM, and by extension there is no
increase in NLy with evolution of the H ii region.
As shown in the bottom-left panel of Fig. 9, the positive
correlation between NLy and diam is significant with ρ = 0.5
and p-value  0.001, using a partial correlation test to con-
trol the distance dependence, giving a power-law relation of
NLy ∝ diam1.4±0.1. However, given the fact that there is little
evidence of any sort of significant correlation between Lyman
continuum flux and other parameters tracing the evolution of H ii
regions, such as νt, ne, or EM as discussed above, this correla-
tion is more likely to result from the fact that more luminous
H ii regions expand more rapidly in their early stages but that the
expansion speed will decrease over time, becoming similar to
less luminous H ii regions. The evolution shown in bottom-left
panel of Fig. 9 is therefore from left to right rather than diag-
onal from bottom-left to upper-right as suggested from the dis-
tribution. The flat evolution of NLy indicates that the value of
NLy remains constant as the H ii region develops, and by exten-
sion that the ionizing flux from a young massive star remains
constant during the evolutionary phases of H ii regions in this
sample. This result is in agreement with the classical expansion
model without gravity or the model with gravity in Keto (2002)
in which the NLy of the H ii region tends to stop increasing if
it reaches the critical ratios where the accretion is quickly re-
duced. Also, the constant NLy over time agrees with the results
of Hosokawa & Omukai (2009) and Hosokawa et al. (2010) who
Article number, page 17 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
103 104 105 106 107
Lbol(L )
1045
1046
1047
1048
1049
1050
N L
y(s
1 )
O4O5O6O7O9B0B0.5B1B2B3B5
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
pc
Fig. 10. Lyman continuum flux NLy vs. the bolometric luminosity Lbol
for young H ii regions with rising spectra. The black solid line refers to
the expected Lyman continuum photon rate from a single ZAMS star of
a given bolometric luminosity. The top axis lists the spectral type cor-
responding to a given bolometric luminosity taken from stellar models
(Thompson 1984; Panagia 1973; Davies et al. 2011). The error bars in
the bottom-right corner correspond to a 50% uncertainty for Lbol and
NLy (Urquhart et al. 2018). At the top, we show the color bar for the
physical size of the sources, indicating the physical diameter in parsecs.
The red circles and black squares refer to optically thick H ii regions
(νt > 5 GHz) and optically thin H ii regions (νt < 5 GHz). About 30%
of the sample is located in the forbidden region above the solid curve
considering a 50% uncertainty. The dotted line represents the empirical
relation between Lbol and NLy for ionized jets from YSOs, with a power-
law index of 0.64 derived by Purser et al. (2016). The red arrows for the
optically thick H ii regions indicate that the bolometric luminosities are
upper limits due to the presence of other H ii regions in the same clump.
showed that the luminosity and temperature of a bloated proto-
star remain almost unchanged in the last accretion phase. More-
over, the almost unchanged NLy may also support the model of
Peters et al. (2010) who proposed that a shrinking H ii region has
small fluctuations of 5%–7% in ionizing flux over time.
5.2. Lyman continuum−bolometric luminosity relationship
The measurements of Lyman continuum flux in the optically thin
regime presented in Sect. 3.3.2 and the bolometric luminosity of
the sample measured by previous studies (see Table 1) allow us
to discuss the relation between Lyman continuum photons (NLy
) and bolometric luminosity (Lbol), as well as Lyman continuum
flux excess phenomenon in the sample of young H ii regions.
There exists a significantly positive correlation between Lbol and
NLy with ρ = 0.54 and p-value 0.001 when using the par-
tial correlation test to remove the distance dependence, which is
consistent with the correlation (ρ = 0.69) calculated by Urquhart
et al. (2013) for a sample of ultra-compact and compact H ii re-
gions.
Figure 10 shows NLy as a function of Lbol. The color sym-
bols indicate the physical size of the sample and the black solid
line represents the upper limit of the expected Lyman continuum
photon rates at specific given bolometric luminosities for ZAMS
stars. About 40% of the sources in the sample are located in the
forbidden region above this black line, suggesting a Lyman con-
tinuum excess. Considering a 50% uncertainty on NLy and Lbol,
the fraction of Lyman excess sources in our sample is consis-
tent with ∼30% sources in previous work (Sánchez-Monge et al.
2013; Cesaroni et al. 2015). Those sources with Lyman excess
are more likely to be associated with young B-type stars (e.g.,
Sánchez-Monge et al. 2013; Lumsden et al. 2013; Urquhart et al.
2013).
Most of the optically thick H ii regions in the sample do not
show a Lyman continuum excess; these are marked with red cir-
cles in Fig. 10 and located to the right of the black solid line rep-
resenting the upper limit of the expected Ly continuum photons.
The main reason for this is that many are embedded in clusters
(as discussed in Sect. 4). Although it is possible that the Lyman
flux has been underestimated because of filtering of some of the
extended flux in the interferometric observations (e.g., Urquhart
et al. 2013), and because of absorption by dust in the H ii region
(e.g., Wood & Churchwell 1989; Garay et al. 1993), it is un-
likely these affects would be significant enough to result in these
objects having a Lyman excess (in many cases the Lyman flux
would need to have been underestimated by an order of magni-
tude or more).
It is possible that some of the optically thick objects we
have detected are ionized jets whose radio emission also has
positive spectral indices (Moscadelli et al. 2016; Purser et al.
2016), and because there are very weak (S int ∼mJy) and com-
pact (diam ∼ 1000 AU) sources (see Sect. 4). We include the
empirical relationship between bolometric luminosity (Lbol) and
Lyman flux (NLy) derived from young stellar objects (YSOs) in
Fig. 10 (dotted diagonal line; Purser et al. 2016). Given that it is
likely that the Lyman continuum flux has been underestimated
and the bolometric luminosity has been overestimated, only the
optically thick sources located to the right of this relation are
associated with radio jets; these are G030.0096, G060.8842,
G034.2573, G034.2581, and G061.4770. The radio emission of
the five sources are point-like as shown in Figs. 7 and 8, and
therefore no morphological evidence was found to indicate that
they are radio jets, which implies that they are more likely to be
HC H ii regions as discussed in Sect. 4. Further observations are
needed to reliably classify these objects.
In Fig. 10, there are seven young H ii regions in Table 8
located close to the black solid line, namely G010.9584,
G030.0096, G030.5887, G030.8662, G060.8842, G030.7197,
and G033.1328, which means that their Lyman continuum fluxes
agree well with their bolometric luminosities, and further indi-
cates the absence of dust within these H ii regions to absorb the
Lyman continuum photons. These seven objects are the only
radio sources in the observed field of this work and in their
parent clumps from Urquhart et al. (2018). Three of the seven
(G010.9584, G030.0096, and G030.5887) have been suggested
to be in the HC H ii region stage and the remaining four are
expected to be in the intermediate stage between HC H ii and
UC H ii regions. Except for three sources with no RRL infor-
mation, the remaining five sources show broad RRL with line
widths ∆V > 40 km s−1, and all of them are associated with
outflows and masers, as shown in Table 8. These dust-free and
young H ii regions are interesting cases to study the destruction
of dust in the very young H ii regions because H ii regions are
often expected to be dusty in the early stages, as discussed in
Article number, page 18 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Sect. 3.3.3 and in Arthur et al. (2004). Further investigations are
needed to understand the absence of dust in these young H ii re-
gions.
6. Summary and conclusion
In this work, we report the results of multi-band (8–12 GHz and
18–26 GHz), high angular-resolution (∼ 1.7′′ and ∼ 0.7′′), VLA
observations toward a sample of young H ii regions that are se-
lected on the basis of rising spectra between 1 and 5 GHz in Yang
et al. (2019). We construct their radio SED between 1 GHz and
26 GHz and measure their physical properties for 116 young H ii
regions by modeling each SED based on an ionization-bounded
H ii region with standard uniform electron density. The sample
has a mean electron density of 1.6 × 104 cm−3, a mean diameter
of 0.14 pc, a mean emission measure of 1.9×107pc cm−6, a mean
turnover frequency of 3.29 GHz, and a mean Lyman continuum
flux of 6.5×1047 s−1. Based on these properties, there are a total
of 20 HC H ii regions and 3 candidates reported so far after com-
bining our findings with the HC H ii region catalog summarized
in Yang et al. (2019). This sample consists of a large number of
HC H ii regions and UC H ii regions, which gives us a compre-
hensive picture of the physical condition and evolution of these
young H ii regions. The main results of our study can be summa-
rized as follows:
1. We identify 16 HC H ii regions and 8 intermediate objects lo-
cated between the class of HC H ii and UC H ii regions. Four
HC H ii regions and three candidates are newly reported in
this work, along with two new infrared-dark HC H ii regions.
2. We discuss how the physical properties of H ii regions
change as they evolve from HC H ii regions to UC H ii re-
gions and then to compact H ii regions. While ne, diam, EM,
and νt all change during this evolution, the Lyman contin-
uum flux stays relatively constant over time, suggesting that
the accretion tends to be quickly reduced or could be halted
at the earliest HC H ii region stage in our sample.
3. These young and compact H ii regions are located in dusty
clumps. The mean fraction of ionizing flux absorbed by dust
in H ii regions is 67%, and the absorption fraction tends to
be more significant for the more compact and younger H ii
regions. Nevertheless, about 40% of the sources show Ly-
man continuum excess and are preferentially associated with
young B-type stars.
In conclusion, young H ii regions are likely to be located in
dusty clumps. The youngest H ii regions, namely HC H ii regions
and intermediate objects between HC H ii and UC H ii, are found
to be associated with star-forming activity such as that found in
various masers, molecular outflows, broad RRLs, and extended
green objects. Accretion at the two earliest stages of H ii region
evolution tends to be quickly reduced or stopped, and therefore
these regions could be optimal tracers of the final stages of mas-
sive star formation.
Acknowledgements. We would like to thank the anonymous referee for the
helpful comments. A. Y. Yang thanks Yan Gong for his helpful discussion.
WWT acknowledges support from the National Key R&D Programs of China
(2018YFA0404203). This work has made use of the SIMBAD database (CDS,
Strasbourg, France). The VLA is operated by the National Radio Astronomy Ob-
servatory, which is a facility of the National Science Foundation operated under
cooperative agreement by Associated Universities, Inc.
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Article number, page 20 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Appendix A: Additional Tables
Article number, page 21 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
Table A.1. Sample of 118 rising-spectra H ii regions
Name S 1.4GHz S5 GHz Dist log Lbol [Ref.] Name S1.5 GHz S 5GHz Dist log Lbol [Ref.]
(mJy) (mJy) (kpc) (L) (mJy) (mJy) (kpc) (L)
±10% ±10% ±10% ±20% ±10% ±10% ±10% ±20%
G010.3009−00.1477 426.2 631.4 3.5 5.2 [1] G027.3644−00.1657 45.0 60.1 8.0 4.8 [1]
G010.4724+00.0275 31.3 38.4 8.5 5.7 [1] G027.9782+00.0789 89.3 124.0 4.8 4.2 [2]
G010.6223−00.3788† 327.6 483.3 2.4 5.7 [1] G028.2003−00.0494 − 161.0 6.1 5.1 [1]
G010.6234−00.3837 571.3 1952.2 5.0 5.7 [1] G028.2879−00.3641 410.9 552.8 11.6 5.9 [1]
G010.9584+00.0221 47.9 196.0 2.9 4.0 [1] G028.6082+00.0185 168.2 210.2 7.4 5.0 [1]
G011.0328+00.0274 3.7 5.7 2.9 2.7 [1] G029.9559−00.0168 1610.8 3116.2 5.2 5.7 [1]
G011.1104−00.3985 253.2 305.4 5.0 4.7 [1] G030.0096−00.2734 0.3 4.5 5.2 3.8 [1]
G011.1712−00.0662 83.2 102.2 2.9 3.2 [1] G030.5353+00.0204 553.6 710.4 2.7 3.9 [1]
G011.9368−00.6158 735.6 1155.9 3.4 4.8 [1] G031.0495+00.4697 10.7 13.6 2.0 3.5 [1]
G011.9446−00.0369 251.1 943.6 3.1 4.3 [1] G031.1596+00.0448 20.7 23.8 2.7 3.3 [1]
G012.1988−00.0345 47.6 62.7 11.9 5.0 [1] G031.2801+00.0632 144.5 268.9 5.2 4.8 [1]
G012.2081−00.1019 127.9 207.9 13.4 5.5 [1] G032.4727+00.2036 56.0 97.4 3.0 3.5 [1]
G012.4294−00.0479 24.4 45.2 2.6 3.2 [1] G032.7441−00.0755 0.3 7.9 11.7 5.0 [1]
G012.8050−00.2007 4332.2 12616.4 2.6 − [1] G032.7966+00.1909 1698.9 3123.4 13.0 6.1 [1]
G012.8131−00.1976† 907.7 1500.4 − − [1] G032.9273+00.6060 229.5 285.6 15.1 4.7 [1]
G012.9995−00.3583 10.5 20.1 1.3 2.9 [1] G033.4163−00.0036 57.6 75.2 5.4 4.1 [1]
G013.2099−00.1428 437.9 946.8 2.6 4.2 [1] G033.9145+00.1105 464.7 842.2 6.5 5.2 [1]
G013.3850+00.0684 139.2 603.9 1.9 3.5 [1] G034.2572+00.1535 370.8 1762.6 1.6 4.8 [1]
G014.7785−00.3328 15.4 18.2 3.1 3.0 [1] G034.2581+00.1533 − 35.9 1.6 4.8 [1]
G016.1448+00.0088 8.8 14.8 12.3 4.2 [1] G034.4032+00.2277 5.3 8.9 1.6 3.5 [1]
G016.3913−00.1383 40.8 124.3 1.9 2.5 [1] G035.0242+00.3502 5.3 11.4 2.3 4.1 [1]
G016.9445−00.0738 258.5 519.3 15.9 5.2 [1] G035.4669+00.1394 235.1 317.6 8.5 5.3 [1]
G017.0299−00.0696 2.0 5.4 10.1 4.1 [1] G035.5781−00.0305 38.0 187.8 10.4 5.3 [1]
G017.1141−00.1124 14.2 17.2 10.1 4.5 [1] G036.4057+00.0226 17.3 22.3 3.5 3.9 [1]
G018.1460−00.2839† 151.2 856.2 − − [1] G037.5457−00.1120 252.3 406.5 9.7 5.1 [1]
G018.3024−00.3910 846.8 1277.9 3.2 4.7 [1] G037.7347−00.1128 12.3 16.0 9.7 4.6 [1]
G018.4433−00.0056 56.2 81.3 11.9 4.5 [2] G037.7633−00.2167† 295.3 337.6 − − [1]
G018.4614−00.0038 128.2 342.1 11.8 5.4 [1] G037.8731−00.3996 1279.3 2561.2 9.7 5.7 [1]
G018.6654+00.0294 3.7 5.7 10.1 4.5 [1] G037.9723−00.0965 10.2 20.9 16.6 4.3 [2]
G018.7106+00.0002 41.0 107.5 2.4 3.1 [1] G038.8756+00.3080 191.2 311.3 14.2 4.7 [1]
G018.7612+00.2630 26.4 51.4 14.1 5.1 [1] G039.1956+00.2255 14.2 62.3 14.5 4.4 [1]
G018.8250−00.4675 9.1 11.4 5.0 3.6 [1] G039.7277−00.3973† 112.3 133.3 − − [1]
G018.8338−00.3002 108.4 131.4 12.7 4.9 [1] G039.8824−00.3460 247.0 276.9 9.3 4.6 [1]
G019.0754−00.2874 333.7 380.7 5.0 5.1 [1] G042.4345−00.2605 38.5 83.7 4.4 4.0 [1]
G019.4912+00.1352 269.3 415.1 13.7 5.1 [1] G043.1651−00.0283 564.3 2714.3 11.1 6.2 [1]
G019.6087−00.2351 855.6 2900.9 12.6 6.0 [1] G043.1652+00.0129 − 160.1 11.1 6.9 [1]
G019.6090−00.2313 126.5 259.9 12.6 6.0 [1] G043.1657+00.0116 − 98.2 11.1 6.9 [1]
G019.7407+00.2821 44.4 239.0 14.0 4.7 [1] G043.1665+00.0106 237.8 1365.7 11.1 6.9 [1]
G019.7549−00.1282 10.6 36.5 7.8 4.5 [1] G043.1778−00.5181 122.9 181.7 8.0 5.0 [1]
G020.0720−00.1421† 138.1 210.1 − − [1] G045.0694+00.1323 17.9 46.2 8.0 5.7 [1]
G020.0809−00.1362 104.4 498.2 12.6 5.7 [1] G045.0712+00.1321 61.6 146.7 8.0 5.7 [1]
G020.3633−00.0136 32.2 55.1 3.4 3.3 [1] G045.1223+00.1321 1346.0 2984.3 8.0 6.0 [1]
G021.3571−00.1766 18.5 24.9 10.0 4.6 [1] G045.4545+00.0591 492.5 1029.5 6.9 5.6 [2]
G021.3855−00.2541 51.1 113.9 10.0 4.9 [1] G045.4656+00.0452 28.9 62.3 6.6 5.0 [2]
G021.4257−00.5417 78.9 94.8 3.5 4.2 [1] G045.4790+00.1294 380.2 504.2 6.0 4.5 [2]
G023.2654+00.0765 55.6 88.6 5.9 4.3 [1] G048.6057+00.0228 6.6 36.2 10.8 5.6 [1]
G023.4553−00.2010 2.9 14.4 5.9 3.8 [1] G048.6099+00.0270 56.5 131.2 9.8 5.1 [4]
G024.5065−00.2224 153.7 205.6 5.8 4.7 [1] G048.9296−00.2793 66.9 185.4 5.6 4.2 [2]
G024.7898+00.0833 − 12.5 6.4 5.2 [1] G049.2679−00.3374 64.4 102.6 5.6 4.5 [2]
G024.9237+00.0777† 57.1 172.5 12.2 − [1] G049.3704−00.3012 252.9 414.4 5.4 5.1 [3]
G025.3948+00.0332 203.7 296.9 15.6 5.3 [1] G049.4905−00.3688 1165.6 3821.7 5.3 6.2 [1]
G025.3970+00.5614 93.7 121.2 13.9 5.1 [1] G050.3152+00.6762 38.7 81.3 1.8 3.3 [1]
G025.3981−00.1411 1351.5 2132.2 10.2 6.0 [1] G051.6785+00.7193 2.8 22.6 10.9 5.0 [1]
G025.7157+00.0487 15.7 20.8 9.4 4.6 [1] G052.7533+00.3340 264.0 386.0 9.0 4.4 [2]
G025.8011−00.1568 19.2 31.9 10.2 4.9 [1] G053.9589+00.0320 40.8 46.0 4.0 3.6 [1]
G026.5444+00.4169 301.0 413.4 9.8 5.2 [1] G060.8842−00.1286 − 18.7 2.5 4.2 [2]
G027.2800+00.1447 370.4 428.0 13.4 4.9 [1] G061.4763+00.0892 252.7 718.7 4.1 5.1 [2]
G030.5887−00.0428? 7.9 92.37 2.7 4.0 [1] G030.8662+00.1143? 137.17 255.2 2.7 4.1 [1]
G030.7197−00.0829? 464.58 969.33 5.2 4.7 [1] G033.1328−00.0923? 173.43 378.59 9.4 5.0 [1]
Notes. This table will be available in electronic form at the CDS. † refers to the sources in our observation with poor-quality images. ? refers to
the 4 sources with data from archives and the literature mentioned in Sect 4. Columns: (1) source name; (2) and (3) flux density at 1.4 GHz and
5 GHz taken from Yang et al. (2019); (4) heliocentric distance; (5) bolometric luminosity; (6) reference for heliocentric distance and bolometric
luminosity: [1] Urquhart et al. (2018), [2] Cesaroni et al. (2015), [3] Urquhart et al. (2013), [4] Kalcheva et al. (2018). The symbol ±10% refers to
an overall estimation of the percentage error at 1.4 GHz and 5 GHz.
Article number, page 22 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Table A.2. Observation results of 112 young H ii regions at X-band (8–12 GHz) and K-band (18–26 GHz).
Name S Peak(X) σ(X) S 9 GHz S 10 GHz S 11 GHz θs(X) S Peak(K) σ(K) S 20 GHz S 22 GHz S 24 GHz θs(K)
(mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′) (mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′)
±10% ±10% ±10% ±10% ±10% ±10% ±10%
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
G010.3009−00.1477⊕ 88.1 3.1 700.3 686.7 661.4 6.8×6.6 15.9 0.7 433.4 419.6 392.6 6.5×6.4
G010.4724+00.0275 82.5 1.6 100.7 105.9 115.4 1.4×0.4 66.9 0.8 156.8 159.9 172.0 1.6×0.5
G010.6234−00.3837⊕ 1099.9 7.9 3071.4 3072.1 3314.8 4.2×3.8 572.3 14.6 2884.8 2857.2 2851.5 3.1×3.0
G010.9584+00.0221 186.3 1.5 258.3 256.2 265.0 1.2×0.9 91.3 1.2 210.7 202.6 200.6 1.0×0.7
G011.0328+00.0274 3.9 0.2 4.8 4.3 4.1 1.3×0.9 1.7 0.1 3.4 2.9 2.8 0.9×0.4
G011.1104−00.3985⊕ 70.5 0.7 350.4 334.8 327.7 9.5×9.4 15.6 0.4 136.1 123.3 126.1 2.2×1.7
G011.1712−00.0662⊕ 4.1 0.1 95.1 92.7 100.1 11.9×8.6 0.6 0.1 − − − −
G011.9368−00.6158⊕ 306.7 2.0 1116.4 1083.5 1098.3 3.4×3.2 76.4 2.0 656.1 652.4 629.6 2.8×1.8
G011.9446−00.0369⊕ 85.7 2.0 709.6 691.4 724.6 6.3×4.7 20.0 0.6 307.2 291.6 289.9 4.3×2.1
G012.1988−00.0345 29.6 0.4 66.0 64.8 63.9 2.0×1.9 6.5 0.2 59.6 54.7 55.9 2.0×2.0
G012.2081−00.1019 88.0 0.8 212.5 209.3 206.5 2.3×1.9 27.2 0.6 159.0 142.1 140.8 2.0×1.2
G012.4294−00.0479 19.9 0.4 51.7 49.8 49.8 2.8×2.4 6.6 0.2 28.0 22.9 26.9 1.7×0.9
G012.8050−00.2007⊕ 858.6 11.0 16097.6 15749.0 16931.9 17.1×17.0 302.3 4.9 6400.9 6240.9 6252.3 8.1×1.1
G012.9995−00.3583 8.4 0.1 14.6 14.7 15.0 1.9×1.1 3.3 0.1 12.4 11.1 11.2 1.8×0.8
G013.2099−00.1428⊕ 57.4 0.4 1080.7 1065.6 965.8 11.7×11.6 10.7 0.3 385.9 359.6 354.2 9.0×9.0
G013.3850+00.0684⊕ 15.0 0.6 738.1 819.3 812.9 21.3×21.2 1.6 0.1 − − − −
G014.7785−00.3328 18.9 0.2 25.3 25.0 25.4 1.1×0.8 9.7 0.1 17.7 16.3 15.9 0.7×0.5
G016.1448+00.0088 13.7 0.1 16.6 16.6 16.6 0.9×0.8 7.7 0.1 14.3 13.4 13.0 0.7×0.5
G016.3913−00.1383⊕ 2.5 0.1 50.0 52.5 46.9 9.9×8.1 0.4 0.1 31.9 21.0 22.9 7.0×4.6
G016.9445−00.0738⊕ 138.3 0.4 545.2 548.9 546.6 3.5×2.5 30.6 0.3 529.2 513.4 515.6 3.5×2.7
G017.0299−00.0696 2.7 0.1 3.9 3.8 3.9 1.6×0.5 1.1 0.1 1.9 2.1 3.1 0.8×0.5
G017.1141−00.1124 4.9 0.1 16.5 15.9 16.3 3.1×2.5 1.0 0.1 10.3 8.2 11.5 2.5×1.6
G018.3024−00.3910⊕ 56.9 0.7 1281.9 1308.6 1180.6 15.5×13.3 12.7 1.0 − − − −
G018.4433−00.0056 38.2 2.6 73.0 73.9 95.3 1.8×1.5 6.3 0.7 72.4 57.4 76.1 2.2×1.8
G018.4614−00.0038 178.6 0.8 387.2 383.5 394.4 2.1×1.8 54.0 0.9 355.7 322.1 388.6 2.1×1.6
G018.6654+00.0294 4.9 0.8 6.9 6.7 6.6 1.7×1.4 2.5 0.1 5.3 5.4 5.5 0.8×0.7
G018.7106+00.0002 88.5 0.5 116.9 116.2 116.0 1.1×0.9 42.5 0.4 104.2 101.2 100.8 0.9×0.8
G018.7612+00.2630 42.6 0.2 55.7 55.3 55.4 1.1×0.9 18.9 0.2 45.6 47.6 50.4 0.9×0.8
G018.8250−00.4675 4.4 0.1 10.2 9.9 10.5 2.5×2.2 1.2 0.1 8.3 6.3 7.7 1.6×1.5
G018.8338−00.3002⊕ 56.0 0.5 132.6 135.2 139.0 5.8×5.6 52.8 0.3 65.9 66.4 65.1 0.4×0.3
G019.0754−00.2874⊕ 19.5 0.7 369.2 326.8 355.5 15.2×15.2 6.5 0.5 − − − −
G019.4912+00.1352⊕ 23.1 0.4 355.3 374.2 417.1 9.6×8.1 5.0 0.3 176.0 151.5 196.4 5.9×4.4
G019.6087−00.2351⊕ 249.2 3.5 3535.0 3145.6 3497.1 14.0×13.9 45.6 1.0 − − − −
G019.6090−00.2313 255.4 3.0 247.0 259.8 241.0 3.1×1.1 50.1 1.5 130.0 133.0 145.9 1.4×0.6
G019.7407+00.2821⊕ 3.6 0.1 166.4 168.5 167.7 21.3×21.2 0.6 0.1 − − − −
G019.7549−00.1282 40.3 0.4 44.6 44.5 44.8 0.6×0.5 29.8 0.5 40.8 40.1 39.1 0.5×0.4
G020.0809−00.1362 205.7 2.3 645.7 656.2 696.5 3.8×1.6 188.7 1.6 387.0 381.0 400.0 1.0×0.5
G020.3633−00.0136 17.6 0.3 53.8 53.2 52.4 2.8×2.3 3.9 0.1 45.9 46.6 47.5 2.8×2.3
G021.3571−00.1766 16.7 0.3 25.4 25.1 24.7 1.4×1.1 6.9 0.1 22.7 21.1 23.8 1.2×1.1
G021.3855−00.2541 81.2 0.4 122.8 121.4 120.8 1.4×1.0 39.5 0.1 103.9 101.7 108.8 1.1×0.9
G021.4257−00.5417⊕ 2.3 0.2 72.7 75.0 71.1 11.8×10.6 0.4 0.1 − − − −
G023.2654+00.0765⊕ 20.1 0.2 83.3 85.6 82.8 5.3×5.0 5.5 0.2 47.7 43.1 46.7 2.7×1.5
G023.4553−00.2010 13.7 0.5 13.8 12.9 10.9 1.2×0.8 11.0 0.1 13.9 13.5 13.2 0.3×0.3
G024.5065−00.2224⊕ 19.9 0.7 257.0 249.5 258.1 7.7×7.5 4.6 0.2 127.3 119.0 117.5 5.2×5.1
G024.7889+00.0824† 25.6 1.1 33.1 37.8 35.7 1.9×1.1 6.9 0.8 23.7 22.9 20.1 1.4×1.2
G024.7898+00.0833 32.4 1.1 31.6 32.9 30.8 1.4×0.9 65.0 0.8 65.8 72.3 71.2 0.3×0.2
G025.3948+00.0332 35.9 0.9 367.9 358.2 369.9 5.1×4.9 7.4 0.4 318.9 278.8 303.5 4.7×4.4
G025.3970+00.5614 117.8 0.7 158.5 164.8 163.1 1.3×0.7 71.4 0.6 158.5 153.5 161.3 0.8×0.8
G025.3981−00.1411⊕ 180.3 3.8 2444.6 2668.5 2510.9 7.5×7.5 36.0 2.7 711.0 734.0 725.0 4.7×4.7
G025.7157+00.0487 12.4 0.2 20.8 21.3 22.5 1.5×1.4 3.9 0.2 15.8 15.8 18.0 1.4×1.1
G025.8011−00.1568 26.6 0.2 37.1 36.8 36.3 1.1×1.0 10.9 0.1 33.9 32.2 33.3 1.1×1.0
G026.5444+00.4169⊕ 15.3 1.0 532.2 556.4 598.7 17.4×17.3 2.5 0.3 − − − −
G027.2800+00.1447⊕ 46.0 0.6 433.7 466.2 446.7 6.0×5.1 9.6 0.6 357.2 289.2 302.8 5.6×3.7
G027.3644−00.1657 37.0 0.2 58.5 56.7 55.8 1.4×1.1 14.9 0.2 40.1 38.4 40.7 0.9×0.8
G027.9782+00.0789⊕ 5.5 0.2 125.3 127.0 144.8 12.9×12.8 0.7 0.1 − − − −
G028.2003−00.0494 259.6 1.8 298.0 320.0 339.0 0.9×0.6 339.1 2.1 558.0 582.0 627.0 0.7×0.5
G028.2879−00.3641 123.9 1.2 592.7 581.1 601.7 4.0×3.8 42.5 1.3 614.6 577.6 549.7 2.1×1.0
G028.6082+00.0185⊕ 52.2 1.1 226.4 228.7 242.5 3.4×3.1 14.7 0.3 188.0 173.0 190.8 3.1×2.4
G029.9559−00.0168⊕ 357.2 7.5 2667.2 2826.8 2951.5 5.9×4.2 103.2 2.4 1519.0 1353.6 1502.1 4.4×1.8
G030.0096−00.2734 5.6 0.1 5.7 5.8 5.8 0.3×0.2 4.9 0.1 5.8 5.5 5.3 0.2×0.2
G030.5353+00.0204⊕ 103.9 1.3 684.7 676.5 682.7 6.7×6.7 25.1 0.8 379.7 380.5 391.2 5.0×5.0
G031.0495+00.4697 10.4 0.1 13.5 13.4 13.1 1.2×0.7 3.4 0.1 12.5 11.7 13.1 1.3×0.8
G031.1596+00.0448 18.6 0.1 24.9 24.7 24.4 1.1×0.9 7.1 0.1 20.6 19.1 20.4 0.9×0.8
G031.2801+00.0632⊕ 14.3 0.6 353.0 400.1 416.5 10.9×10.2 2.1 0.3 − − − −
Article number, page 23 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
Table A.2. –continued Observation results of 112 young H ii regions at X-band (8–12 GHz) and K-band (18–26 GHz).
Name S Peak(X) σ(X) S 9 GHz S 10 GHz S 11 GHz θs(X) S Peak(K) σ(K) S 20 GHz S 22 GHz S 24 GHz θs(K)
(mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′) (mJy/beam) (mJy) (mJy) (mJy) (mJy) (′′×′′)
±10% ±10% ±10% ±10% ±10% ±10% ±10%
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
G032.4727+00.2036 60.1 0.2 102.4 101.1 100.1 1.5×1.2 24.2 0.2 86.7 82.2 85.7 1.4×0.9
G032.7441−00.0755 15.1 0.1 16.2 17.3 18.8 1.0×0.7 31.4 0.1 29.8 31.8 34.7 0.1×0.1
G032.7966+00.1909⊕ 623.5 6.2 3768.0 3813.0 3925.0 11.5×11.4 238.8 6.1 − − − −
G032.9273+00.6060⊕ 54.3 0.5 278.5 272.8 284.5 7.6×7.6 25.1 0.2 − − − −
G033.4163−00.0036⊕ 7.0 0.3 111.5 98.4 101.8 10.1×9.9 1.1 0.1 30.5 18.5 20.9 3.8×2.2
G033.9145+00.1105⊕ 121.2 1.9 732.4 737.6 740.7 6.0×6.0 31.5 0.7 270.9 263.9 288.9 2.8×1.4
G034.2572+00.1535⊕ 956.0 11.0 2710.0 2970.0 3050.0 2.9×1.9 648.0 5.5 3750.0 3760.0 4200.0 1.8×1.2
G034.2573+00.1523† 117.0 10.8 145.6 141.5 180.0 1.7×0.6 60.7 7.5 57.0 45.4 33.1 0.6×0.6
G034.2581+00.1533 182.0 11.4 101.7 110.5 116.9 1.8×1.4 88.0 10.0 120.0 120.0 169.0 0.4×0.2
G034.4032+00.2277 7.7 0.1 9.0 8.9 8.7 0.8×0.5 5.1 0.1 6.9 7.0 6.9 0.4×0.4
G035.0242+00.3502 12.0 0.1 13.1 13.0 13.0 0.8×0.6 9.6 0.1 11.8 11.9 12.2 0.3×0.3
G035.4669+00.1394⊕ 34.3 0.3 309.0 288.0 301.0 6.3×6.1 6.9 0.4 163.0 148.0 173.5 4.6×3.5
G035.5781−00.0305 141.1 0.7 218.2 202.8 212.1 1.3×0.9 147.7 1.1 209.2 202.0 207.6 0.6×0.2
G036.4057+00.0226 15.4 0.45 23.5 23.6 24.1 2.3×1.6 4.4 0.32 18.9 18.9 19.6 1.4×1.3
G036.4062+00.0221† 10.4 0.45 11.4 11.3 10.5 2.3×1.7 7.8 0.32 8.1 7.7 7.9 1.9×1.7
G037.5457−00.1120⊕ 38.7 1.0 450.0 499.0 550.0 7.0×5.5 8.4 0.4 167.8 142.3 168.8 3.5×2.8
G037.7347−00.1128 12.9 0.2 16.3 16.3 16.7 1.1×0.9 6.1 0.1 14.6 13.9 14.4 0.8×0.7
G037.8731−00.3996⊕ 552.8 7.0 2588.0 2465.0 2720.0 4.1×3.5 264.1 3.5 1320.0 1201.0 1153.0 1.2×1.1
G037.9723−00.0965 9.2 0.1 21.0 21.5 20.5 1.9×1.9 2.5 0.1 18.9 17.0 20.6 1.9×1.7
G038.8756+00.3080 89.4 0.7 305.2 295.2 318.1 2.8×2.5 22.8 0.8 231.9 210.5 238.2 2.2×1.9
G039.1956+00.2255 48.4 0.2 62.0 60.6 60.6 0.8×0.8 22.6 0.3 58.3 56.7 55.8 0.8×0.8
G039.8824−00.3460⊕ 65.8 0.4 367.4 361.7 373.8 3.8×3.5 14.7 0.6 214.5 212.0 226.4 3.2×3.2
G042.4345−00.2605⊕ 23.4 0.4 66.2 62.7 61.3 3.1×2.7 6.0 1.2 42.3 40.0 41.4 2.7×1.4
G043.1651−00.0283⊕ 544.9 13.5 3809.0 3607.1 4020.7 7.6×7.5 215.6 4.5 2023.9 1941.5 − 3.4×3.3
G043.1652+00.0129 286.0 17.1 310.0 326.0 338.2 1.7×1.4 345.0 10.5 529.0 545.0 516.0 0.6×0.4
G043.1657+00.0116 271.0 16.4 370.6 381.7 416.0 2.4×1.8 132.0 13.1 − − − −
G043.1665+00.0106⊕ 524.4 18.0 2740.0 2766.6 2900.0 3.6×3.2 292.9 11.4 2257.4 2068.0 2293.9 2.9×2.8
G043.1778−00.5181 12.9 0.5 150.2 146.4 160.0 6.9×6.9 1.8 0.3 39.5 40.3 41.0 4.3×2.8
G045.0694+00.1323 35.4 1.7 71.0 78.0 80.0 1.8×1.8 448.1 4.2 − − − −
G045.0712+00.1321 − 1.3 388.0 424.0 461.2 0.8×0.6 436.5 4.2 750.0 745.0 806.5 0.6×0.5
G045.1223+00.1321⊕ 831.8 5.4 4191.0 4036.4 4197.0 12.8×12.7 517.5 5.7 2860.1 2847.0 2888.9 2.7×2.6
G045.4545+00.0591⊕ 79.2 6.6 940.0 1057.0 1020.0 8.8×8.7 12.5 1.0 194.0 177.0 162.0 6.2×6.1
G045.4656+00.0452 109.7 2.5 135.6 139.2 134.0 1.2×0.5 93.2 0.1 139.4 155.5 143.0 0.8×0.4
G045.4790+00.1294⊕ 17.0 2.6 326.0 304.0 349.3 8.6×8.4 0.4 0.1 − − − −
G048.6057+00.0228 28.1 0.3 44.5 47.6 44.3 1.8×1.3 9.6 0.3 31.6 29.8 32.0 0.9×0.9
G048.6099+00.0270⊕ 27.8 0.6 98.2 97.1 102.0 6.9×6.8 0.4 0.6 − − − −
G048.9296−00.2793⊕ 15.0 1.6 316.0 283.0 311.2 9.2×8.1 1.2 1.6 32.1 29.9 30.2 3.6×3.6
G049.2679−00.3374⊕ 6.5 0.4 72.2 71.0 81.3 5.1×5.0 1.1 0.2 32.7 28.6 35.6 3.2×3.2
G049.3666−00.3010† 54.0 4.9 387.0 367.3 406.7 6.4×6.2 1.0 1.3 − − − −
G049.3704−00.3012 54.8 4.9 652.4 646.0 667.0 9.1×9.0 5.5 1.3 269.0 177.0 152.5 4.8×3.8
G049.4905−00.3688⊕ 667.2 9.9 3900.0 3770.0 4020.0 3.9×3.7 186.7 8.2 2591.4 2336.9 2470.8 2.8×2.8
G050.3152+00.6762 47.1 0.4 79.6 76.8 76.1 1.3×1.2 15.9 0.2 62.7 59.4 63.9 1.3×1.0
G051.6785+00.7193 27.4 0.1 30.7 31.2 32.4 1.1×0.5 33.9 0.2 35.0 35.8 38.2 0.2×0.1
G052.7533+00.3340⊕ 17.1 0.2 370.2 368.6 371.6 8.9×8.0 2.1 0.3 − − − −
G053.9589+00.0320 20.5 0.1 45.3 44.6 43.5 1.8×1.6 5.7 0.2 35.0 31.8 38.2 1.7×1.2
G060.8842−00.1286 24.1 0.8 34.1 36.2 32.6 1.7×1.5 11.1 0.2 24.9 23.3 24.2 0.7×0.7
G061.4763+00.0892⊕ 129.2 3.5 580.4 550.0 592.1 4.5×4.3 56.8 1.5 334.0 324.0 400.0 3.5×2.0
G061.4770+00.0892† 125.0 3.5 153.0 155.0 161.0 1.61×1.0 29.0 1.5 137.5 127.9 157.0 0.9×0.6
Notes. This table will be available in electronic form at the CDS. The ‘-’ symbol means no measurement is available. † refers to the five added
UC H ii regions in the observed fields with rising spectra between C and X band; see Sect 3.1. ⊕ indicates that the sources are extended and their
K-band flux densities should be considered as lower limits. Columns: (1) Source name; (2) and (3) peak flux density and local RMS at X-band;
(4-6) flux density at 9 GHz, 10 GHz and 11 GHz, respectively; (7) deconvolved source size at X-band; (8) and (9) peak flux density and RMS at
K-band; (10-12) flux density at 20 GHz, 22 GHz and 24 GHz, respectively; (13) deconvolved source size at K-band. The symbol of ±10% refers
to an overall estimation of the percentage error at each frequency.
Article number, page 24 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Table A.3. Derived physical properties of 116 young H ii regions.
Name ne diam EM νt logNLy Spectral fd
(105 cm−3) (pc) (107 pc cm−6) (GHz) (s−1) Type
(1) (2) (3) (4) (5) (6) (7) (8)
G010.3009−00.1477 0.09 0.119 0.92 1.69 47.94 O9.5 0.86
G010.4724+00.0275 1.43 0.022 45.2 10.77 48.11 O9 0.94
G010.6234−00.3837 0.16 0.166 4.39 3.55 48.9 O6.5 0.81
G010.9584+00.0221 0.36 0.029 3.78 3.31 47.35 B0 −
G011.0328+00.0274 0.13 0.014 0.24 0.89 45.57 B1 −
G011.1104−00.3985 0.07 0.145 0.62 1.4 47.94 O9.5 0.27
G011.1712−00.0662 0.09 0.053 0.45 1.21 46.91 B0 −
G011.9368−00.6158 0.07 0.155 0.86 1.63 48.12 O9 0.19
G011.9446−00.0369 0.17 0.075 2.2 2.56 47.84 O9.5 −
G012.1988−00.0345 0.07 0.148 0.65 1.43 47.98 O9 0.77
G012.2081−00.1019 0.05 0.268 0.72 1.5 48.59 O7 0.83
G012.4294−00.0479 0.26 0.02 1.38 2.05 46.55 B0.5 −
G012.8050−00.2007 0.1 0.248 2.68 2.81 49.05 O6 −
G012.9995−00.3583 0.28 0.008 0.6 1.38 45.41 B1 −
G013.2099−00.1428 0.14 0.082 1.67 2.24 47.88 O9.5 −
G013.3850+00.0684 0.37 0.032 4.43 3.57 47.49 B0 −
G014.7785−00.3328 0.27 0.016 1.21 1.92 46.4 B0.5 −
G016.1448+00.0088 0.14 0.058 1.11 1.85 47.42 B0 −
G016.3913−00.1383 0.16 0.023 0.59 1.36 46.3 B0.5 −
G016.9445−00.0738 0.06 0.389 1.48 2.12 49.16 O6 −
G017.0299−00.0696 0.2 0.024 0.94 1.7 46.61 B0.5 −
G017.1141−00.1124 0.03 0.145 0.1 0.59 47.23 B0 0.75
G018.3024−00.3910 0.07 0.158 0.83 1.61 48.15 O8.5 0.14
G018.4433−00.0056 0.07 0.16 0.68 1.46 48.04 O9 −
G018.4614−00.0038 0.11 0.196 2.23 2.57 48.75 O6.5 0.54
G018.6654+00.0294 0.17 0.034 1.01 1.76 46.92 B0 0.88
G018.7106+00.0002 0.3 0.023 2.03 2.46 46.84 B0 −
G018.7612+00.2630 0.11 0.111 1.36 2.03 48.05 O9 0.83
G018.8250−00.4675 0.05 0.051 0.13 0.66 46.41 B0.5 −
G018.8338−00.3002 0.05 0.251 0.55 1.32 48.36 O8 0.29
G019.0754−00.2874 0.06 0.205 0.33 1.04 47.99 O9 0.85
G019.4912+00.1352 0.04 0.388 0.77 1.55 48.87 O6.5 −
G019.6087−00.2351 0.09 0.416 3.06 2.99 49.5 O5 0.63
G019.6090−00.2313 0.08 0.219 1.34 2.02 48.63 O6.5 0.95
G019.7407+00.2821 0.15 0.138 3.13 3.02 48.54 O7.5 −
G019.7549−00.1282 0.29 0.037 3.17 3.04 47.45 B0 0.59
G020.0809−00.1362 0.14 0.191 3.49 3.18 49.04 O6 0.52
G020.3633−00.0136 0.17 0.032 0.96 1.73 46.81 B0 −
G021.3571−00.1766 0.13 0.062 1.02 1.77 47.42 B0 0.62
G021.3855−00.2541 0.12 0.108 1.61 2.21 48.1 O9 0.61
G021.4257−00.5417 0.04 0.091 0.17 0.77 46.98 B0 0.58
G023.2654+00.0765 0.11 0.073 0.87 1.65 47.49 B0 −
G023.4553−00.2010 0.49 0.015 3.55 3.21 46.72 B0.5 −
G024.5065−00.2224 0.08 0.118 0.91 1.68 47.94 O9.5 0.46
G024.7889+00.0824 0.25 0.031 1.94 2.41 47.21 B0 0.92
G024.7898+00.0833 3.38 0.008 91.84 15.1 47.52 B0 0.92
G025.3948+00.0332 0.05 0.36 1.01 1.76 48.96 O6 0.24
G025.3970+00.5614 0.07 0.211 1.08 1.82 48.52 O7.5 0.20
G025.3981−00.1411 0.04 0.589 1.17 1.89 49.46 O5.5 0.66
G025.7157+00.0487 0.09 0.071 0.54 1.31 47.29 B0 0.72
G025.8011−00.1568 0.13 0.07 1.23 1.94 47.6 O9.5 0.87
G026.5444+00.4169 0.06 0.271 1.04 1.79 48.75 O6.5 0.15
G027.2800+00.1447 0.03 0.505 0.49 1.26 48.94 O6.5 −
G027.3644−00.1657 0.05 0.12 0.35 1.06 47.58 B0 0.83
G027.9782+00.0789 0.1 0.078 0.78 1.56 47.49 B0 −
G028.2003−00.0494 1.41 0.027 53.54 11.68 48.37 O8 0.43
G028.2879−00.3641 0.05 0.398 0.81 1.59 48.91 O6.5 0.80
G028.6082+00.0185 0.06 0.17 0.68 1.46 48.12 O9 0.60
G029.9559−00.0168 0.06 0.331 1.14 1.87 48.9 O6.5 0.65
G030.0096−00.2734 1.91 0.004 15.57 6.49 46.21 B0.5 −
Article number, page 25 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
Table A.3. –continuue Derived physical properties of 116 young H ii regions.
Name ne diam EM νt logNLy Spectral fd
(105 cm−3) (pc) (107 pc cm−6) (GHz) (s−1) Type
(1) (2) (3) (4) (5) (6) (7) (8)
G030.5353+00.0204 0.07 0.12 0.55 1.32 47.73 O9.5 −
G030.5887−00.0428 2.08 0.009 39.1 10.06 47.28 B0 −
G030.7197−00.0829 0.22 0.093 4.54 3.61 48.44 O7.5 −
G030.8662+00.1143 0.37 0.031 4.25 3.5 47.46 O9.5 −
G031.0495+00.4697 0.21 0.012 0.56 1.34 45.75 B1 −
G031.1596+00.0448 0.12 0.026 0.39 1.13 46.27 B0.5 −
G031.2801+00.0632 0.14 0.092 1.78 2.31 48.05 O9 0.49
G032.4727+00.2036 0.19 0.033 1.25 1.96 46.98 B0 −
G032.7441−00.0755 2.79 0.011 82.77 14.37 47.69 O9.5 0.88
G032.7966+00.1909 0.04 0.81 1.5 2.13 49.83 O4 0.21
G032.9273+00.6060 0.03 0.44 0.52 1.29 48.83 O6.5 −
G033.1328−00.0923 0.21 0.1 4.56 3.61 48.54 O7.5 0.16
G033.4163−00.0036 0.12 0.066 1.01 1.76 47.48 B0 −
G033.9145+00.1105 0.07 0.229 0.98 1.74 48.57 O8 0.44
G034.2572+00.1535 0.52 0.038 10.12 5.28 48.03 O9 0.51
G034.2573+00.1523 3.55 0.005 58.23 12.16 46.58 B0.5 0.98
G034.2581+00.1533 3.01 0.004 37.26 9.83 46.54 B0.5 0.98
G034.4032+00.2277 0.38 0.006 0.87 1.65 45.38 B1 −
G035.0242+00.3502 0.47 0.008 1.74 2.28 45.86 B0.5 0.58
G035.4669+00.1394 0.05 0.24 0.6 1.37 48.33 O8 0.82
G035.5781−00.0305 0.22 0.093 4.46 3.58 48.36 O8 0.81
G036.4057+00.0226 0.33 0.016 1.74 2.29 46.48 B0.5 −
G036.4062+00.0221 0.13 0.022 0.4 1.13 46.16 B0.5 −
G037.5457−00.1120 0.07 0.242 1.23 1.94 48.7 O7 0.24
G037.7347−00.1128 0.1 0.062 0.62 1.4 47.21 B0 0.77
G037.8731−00.3996 0.05 0.532 1.41 2.07 49.38 O5.5 0.42
G037.9723−00.0965 0.13 0.082 1.36 2.03 47.79 O9.5 −
G038.8756+00.3080 0.05 0.344 0.74 1.52 48.79 O6.5 −
G039.1956+00.2255 0.2 0.081 3.26 3.08 48.12 O9 −
G039.8824−00.3460 0.05 0.256 0.7 1.48 48.51 O7.5 −
G042.4345−00.2605 0.15 0.044 1.06 1.8 47.1 B0 −
G043.1651−00.0283 0.12 0.387 5.33 3.9 49.69 O5 0.42
G043.1652+00.0129 0.88 0.053 41.47 10.34 48.91 O6.5 0.90
G043.1657+00.0116 1.57 0.046 113.18 16.68 48.69 O7 0.94
G043.1665+00.0106 0.24 0.22 12.21 5.78 49.55 O5 0.58
G043.1778−00.5181 0.06 0.165 0.58 1.36 48.11 O9 0.71
G045.0694+00.1323 0.74 0.026 13.97 6.16 47.76 O9.5 0.97
G045.0712+00.1321 1.22 0.04 58.98 12.23 48.73 O6.5 0.76
G045.1223+00.1321 0.07 0.429 2.08 2.49 49.43 O5.5 0.68
G045.4545+00.0591 0.08 0.235 1.4 2.06 48.72 O6.5 0.77
G045.4656+00.0452 1.02 0.023 23.56 7.89 47.88 O9.5 0.81
G045.4790+00.1294 0.01 0.462 0.09 0.56 48.06 O9 −
G048.6057+00.0228 0.31 0.042 4.1 3.44 47.76 O9.5 0.97
G048.6099+00.0270 0.1 0.116 1.19 1.91 47.99 O9 0.85
G048.9296−00.2793 0.22 0.065 3.26 3.08 47.97 O9 −
G049.2679−00.3374 0.07 0.089 0.49 1.25 47.46 B0 −
G049.3666−00.3010 0.94 0.031 26.89 8.41 48.05 O9 0.83
G049.3704−00.3012 0.11 0.128 1.61 2.2 48.34 O8 0.67
G049.4905−00.3688 0.1 0.27 2.57 2.75 49.04 O6 0.85
G050.3152+00.6762 0.26 0.018 1.2 1.92 46.42 B0.5 −
G051.6785+00.7193 0.64 0.026 10.74 5.44 47.68 O9.5 0.88
G052.7533+00.3340 0.05 0.254 0.75 1.53 48.51 O7.5 −
G053.9589+00.0320 0.06 0.066 0.24 0.89 46.87 B0 −
G060.8842−00.1286 0.93 0.012 10.8 5.35 46.4 B0.5 −
G061.4763+00.0892 0.13 0.09 1.72 2.27 48.1 O9 0.81
G061.4770+00.0892 4.45 0.004 78.76 14.04 46.81 B0 0.99
Notes. This table will be available in electronic form at the CDS. The fraction of Lyman continuum photons absorbed by dust within H ii regions fd
should be taken as upper limits. The ‘-’ symbol means no measurement is available. Columns: (1) source name; (2) electron density; (3) physical
diameter; (4) emission measure; (5) turnover frequency; (6) Lyman continuum flux; (7) spectral type; (8) dust absorption fraction.
Article number, page 26 of 28
A.Y. Yang, J.S. Urquhart, M.A. Thompson: hypercompact H ii regions identified from young H ii regions
Appendix B: Additional images
Article number, page 27 of 28
A&A proofs: manuscript no. ms_AA_revision_correction_v03
Fig. B.1. Examples of the best-fitting SEDs and the radio images in C-band, X-band, and K-band observations.
100 101
Frequency (GHz)
102
103
Fl
ux
 (m
Jy
)
ne = 8.81e+03
diam(pc) = 0.119
G010.3009-00.1477
18h08m55.68s56.16s56.64s
RA (J2000)
06'00.0"
52.8"
-20°05'45.6"
D
ec
 (J
20
00
)
G010.3009-00.1477
Cband(0.34'×0.34')
15
30
45
m
Jy
/b
ea
m
18h08m55.68s56.16s56.64s
RA (J2000)
06'00.0"
52.8"
-20°05'45.6"
D
ec
 (J
20
00
)
G010.3009-00.1477
Xband(0.34'×0.34')
9
34
59
84
m
Jy
/b
ea
m
18h08m55.68s56.16s56.64s
RA (J2000)
06'00.0"
52.8"
-20°05'45.6"
D
ec
 (J
20
00
)
G010.3009-00.1477
Kband(0.34'×0.34')
4
9
14
m
Jy
/b
ea
m
100 101 102
Frequency (GHz)
101
102
Fl
ux
 (m
Jy
)
ne = 1.43e+05
diam(pc) = 0.022
G010.4724+00.0275
18h08m37.92s38.40s38.88s
RA (J2000)
57.6"
50.4"
-19°51'43.2"
D
ec
 (J
20
00
)
G010.4724+00.0275
Cband(0.34'×0.34')
5
13
21
m
Jy
/b
ea
m
18h08m37.92s38.40s38.88s
RA (J2000)
57.6"
50.4"
-19°51'43.2"
D
ec
 (J
20
00
)
G010.4724+00.0275
Xband(0.34'×0.34')
8
31
54
76
m
Jy
/b
ea
m
18h08m37.92s38.40s38.88s
RA (J2000)
57.6"
50.4"
-19°51'43.2"
D
ec
 (J
20
00
)
G010.4724+00.0275
Kband(0.34'×0.34')
20
40
60
m
Jy
/b
ea
m
100 101
Frequency (GHz)
102
103
Fl
ux
 (m
Jy
)
ne = 2.35e+04
diam(pc) = 0.220
G043.1665+00.0106
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Cband(0.34'×0.34')
59
119
179
m
Jy
/b
ea
m
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Xband(0.34'×0.34')
52
195
339
483
m
Jy
/b
ea
m
19h10m12.96s13.44s13.92s
RA (J2000)
+9°06'07.2"
14.4"
21.6"
D
ec
 (J
20
00
)
G043.1665+00.0106
Kband(0.34'×0.34')
28
108
187
266
m
Jy
/b
ea
m
(a) Radio SED (b) C-band image (c) X-band image (d) K-band image
Panel (a): Radio SED and best-fitting model for each source in the sample. The SED shows the free-free emission fit to flux
density points between 1 and 26 GHz for a single compact source, while the extended source has the best fit to flux density points
between 1 and 11 GHz as their K-band flux measurements are not reliable owing to the shortage of short baseline spacings. The
uncertainties on flux measurements of these points are used to constrain the fitting process and to obtain the best estimate. The
best-fitting results of electron density ne( cm−3) and physical diameter diam (pc) for each source are shown in the upper-left
corner of each figure. Panels (b), (c) and (d): Radio images in C-band, X-band, and K-band marked with the positions of the
young UC H ii regions in each image, including single-component compact sources, extended sources, and cluster sources. The
C-band images are taken from the CORNISH survey and are used to compare with the images at X-band in this work as the
X-band observations have comparable beam sizes to those of the CORNISH survey. For some sources, the K-band images are
not shown because of the poor quality of observational data at K-band. The lime polygons in the C-band images shown for some
sources are similar to the defined region in the CORNISH survey. The lime polygons in the X-band images shown for some
sources refer to the manually drawn emission regions used to measure the observational results following the same strategy in
CORNISH survey. The white contour levels in the images are equally spaced by 5σ and start at a level of 5σ. The image size of
each target is shown in the upper-middle part of each image. The beam sizes for C-band (1.5′′), X-band (∼ 1.7′′), and K-band
(0.7′′) are shown in the lower-left corner of each image. Note: Figures for the full sample are available in electronic form at the
Zenodo via https://doi.org/10.5281/zenodo.4293684.
Article number, page 28 of 28