| dc.description.abstract | I present low-resolution (Dl 6 A° ) follow-up spectroscopy of 370 Ha emitters (12 . r .
17) identified with IPHAS, in a 100 deg2 wide section of the Galactic plane that is located
between ` = (120 ; 140 ) and b = (1 ; +4 ). Classical Be stars are found to be the most
numerous group of the observed targets ( 60%). Sixty-eight classical Be stars have also
been observed at higher spectral resolution (Dl 24 A° ) and S/N ratio, which allows
spectral typing to an estimated precision of 1 sub-type. Colour excesses were measured
via spectral energy distribution fitting of flux-calibrated data. I took care to remove the
circumstellar contribution to the measured colour excess, using an established scaling to
the Ha equivalent widths. In doing so, this method of correction was re-evaluated and
modified to better suit the data at hand. Spectroscopic parallaxes were measured constraining
the luminosity class via estimates of distances to main sequence A/F stars, which
are found within a few arcminutes of each classical Be star on the sky.
In order to probe the structure of the outer Galactic disc, I studied the spatial distribution
of 63 out of 248 classical Be stars identified. Their cumulative distribution function
with respect to the distance is statistically compatible both with a smooth exponential density
profile and with a simple spiral arms representation. The distribution of reddenings
of classical Be stars is compared with estimates of the total Galactic reddening along their
sightlines. It is expected that the measured reddenings match the integrated Galactic values,
for distant stars located outside the Galactic dust layer, or they are smaller than the
asymptotic values if the stars are less distant. The outcome meets expectations, and lends
support to the conclusion that the measured reddenings are determined to a precision of
10%.
The sample of 248 objects doubles the number of known classical Be stars in this part
of the Galactic plane. Unlike the pre-existing bright sample, the new objects are seen
at large distances, between 2 – 8 kpc with typical E(BV) 0:9. Only four stars are
members of known clusters. Ten classical Be stars are proposed to be well beyond the
putative Outer Arm, at distances larger than 8 kpc. The large sample of stars, which has
been identified here, is the result of a successful selection and analysis of classical Be stars
that is offered for more exploitation in future. The proposition is that GAIA observations
will use the present sample of classical Be stars as a new tracer of the Galactic disc. | en_US |
