Radio Surveys of Star Formation in the Milky Way with the Next Generation of Interferometers

Abstract

This thesis embodies work that contributes towards building our knowledge on the birth of massive stars. It presents three projects that study young HII regions which, being a direct product of the birth of massive stars, offer a unique window into this process. I have mapped a 2.3°×2.3° region around W49A with the first Galactic Plane survey to explore the subarcsecond and sub-mJy regime at 14 GHz. I have detected a total of 1238 compact sources from which I have identified 82 as young HII regions. I have derived SEDs for a subsample of the sources using 4 KuGARS sub-bands and CORNISH at 5 GHz. The derived spectrum shows that the HII regions are optically thick at 5 GHz and optically thin at 14 GHz. Their sizes imply they are in transition between the ultracompact (UC) and hypercompact (HC) stages. This process is contiguous with no sharp transition and no visible boundaries between the evolutionary stages, putting into question the need for multiple classifications. The observed trends suggest that HC and UC HII regions are in fact the same object. I have estimated the time they spend in high emission measure (EM) as an analogue to the lifetime of an HII region spent in the HC phase of its evolution. Using 108 pc cm−6 as an arbitrary boundary between high and low EM, I estimate that HII regions spend ∼41% of their lifetime in high EM, something of the order 104 years. I carried out targeted observations toward 78 molecular clumps with star forming properties in the Outer Galaxy using the C-band of the VLA. This was in order to search for embedded UC HII regions, which would indicate ongoing star formation. Only 9 clumps have an embedded radio source, none of which are detected for the first time, implying massive star formation in the Outer Galaxy is not as prevalent as in the Inner Galaxy. I present a catalogue of compact sources extracted from the MeerKAT-GPS, an L-band survey of the southern Galactic Plane covering around half the GP from 252° < l < 358°, 2° < l < 60° and |b| < 1.5° down to a nominal depth of ∼ 10-30 μJy. The catalogue contains 409790 compact sources above 5σ . I have extracted a subsample of 350 UC HII regions identified as counterparts to the RMS UC HII region samples in a search for extended emission associated with UC HII regions - this would conclusively explain the lifetime problem. However, I have found extended emission around ∼ 1/3 of the sources, a lower ratio than found in previous studies (∼ 8/10). The lower ratio seen in this complete sample means we cannot rely entirely on extended components to explain the large number of observed UC HII regions.