The Discovery of a Massive Cluster of Red Supergiants with GLIMPSE
  We report the discovery of a previously unknown massive Galactic star cluster
at l=29.22, b=-0.20. Identified visually in mid-IR images from the Spitzer
GLIMPSE survey, the cluster contains at least 8 late-type supergiants, based on
followup near-IR spectroscopy, and an additional 3-6 candidate supergiant
embers having IR photometry consistent with a similar distance and reddening.
The cluster lies at a local minimum in the 13-CO column density and 8 micron
emission. We interpret this feature as a hole carved by the energetic winds of
the evolving massive stars. The 13-CO hole seen in molecular maps at V_LSR ~95
km/s corresponds to near/far kinematic distances of 6.1/8.7+/-1 kpc. We
calculate a mean spectrophotometric distance of 7.0^+3.7_-2.4 kpc, broadly
consistent with the kinematic distances inferred. This location places it near
the northern end of the Galactic bar. For the mean extinction of A_V=12.6+/-0.5
mag (A_K=1.5+/-0.1 mag), the color-magnitude diagram of probable cluster
members is well fit by isochrones in the age range 18-24 Myr. The estimated
cluster mass is ~20,000 Msun. With the most massive original cluster stars
likely deceased, no strong radio emission is detected in this vicinity. As
such, this RSG cluster is representative of adolescent massive Galactic
clusters that lie hidden behind many magnitudes of dust obscuration. This
cluster joins two similar red supergiant clusters as residents of the volatile
region where the end of our Galaxy's bar joins the base of the Scutum-Crux
piral arm, suggesting a recent episode of widespread massive star formation
there.
