A Hidden Population of Massive Stars with Circumstellar Shells
  Discovered with the Spitzer Space Telescope
  We have discovered a large number of circular and elliptical shells at 24
microns around luminous central sources with the MIPS instrument on-board the
Spitzer Space Telescope. Our archival follow-up effort has revealed 90% of
these circumstellar shells to be previously unknown. The majority of the shells
is only visible at 24 microns, but many of the central stars are detected at
multiple wavelengths from the mid- to the near-IR regime. The general lack of
optical counterparts, however, indicates that these sources represent a
population of highly obscured objects. We obtained optical and near-IR
spectroscopic observations of the central stars and find most of these objects
to be massive stars. In particular, we identify a large population of sources
that we argue represents a narrow evolutionary phase, closely related or
identical to the LBV stage of massive stellar evolution.
