An obscured cluster associated with the HII region RCW173
  The discovery of several clusters of red supergiants towards l=24-30 deg has
triggered interest in this area of the Galactic plane, where lines of sight are
very complex and previous explorations of the stellar content were very
preliminary. We attempt to characterise the stellar population associated with
the HII region RCW173 (Sh2-60), located at l=25.3 deg, as previous studies have
suggested that this population could be beyond the Sagittarius arm. We obtained
UBV photometry of a stellar field to the south of the brightest part of RCW173,
as well as spectroscopy of about twenty stars in the area. We combined our new
data with archival 2MASS near-infrared photometry and Spitzer/GLIMPSE imaging
and photometry. We find a significant population of early-type stars located at
d=3.0 kpc, in good agreement with the near dynamical distance to the HII
region. This population should be located at the near intersection of the
Scutum-Crux arm. A luminous O7II star is likely to be the main source of
ionisation. Many stars are concentrated around the bright nebulosity, where
GLIMPSE images in the mid infrared show the presence of a bubble of excited
material surrounding a cavity that coincides spatially with a number of B0-1V
stars. We interpret this as an emerging cluster, perhaps triggered by the
nearby O7II star. We also find a number of B-type giants. Some of them are
located at approximately the same distance, and may be part of an older
population in the same area, characterised by much lower reddening. A few have
shorter distance moduli and are likely to be located in the Sagittarius arm.
The line of sight in this direction is very complex. Optically visible tracers
delineate two spiral arms, but seem to be absent beyond d~3 kpc. Several HII
regions in this area suggest that the Scutum-Crux arm contains thick clouds
actively forming stars.
