Background Infrared Sources for Studying the Galactic Center's
  Interstellar Gas
  We briefy describe the results of a K-band spectroscopic survey of over 500
highly reddened point-like objects on sightlines toward the Central Molecular
Zone (CMZ) of the Galaxy. The goal was to find stars with featureless or nearly
featureless spectra suitable for near- and mid-infrared absorption spectroscopy
of the Galactic center's interstellar gas on sightlines spread across the CMZ.
Until recently only a few such stars have been known outside of very localized
sightlines in the vicinity of the Quintuplet and Central clusters. We have used
Spitzer Space Telescope (GLIMPSE) and 2MASS photometry to select promising
candidates, and over the last ten years have been acquiring low-resolution
K-band spectra of them. As expected, the vast majority are cool and/or highly
reddened red giants with complex photospheric spectra unsuitable for measuring
faint interstellar lines. Approximately ten percent of them, whose observations
are reported here, have featureless or nearly featureless spectra. Although not
evenly distributed in Galactic longitude, these stars are scattered across the
CMZ. Many of them are luminous stars that are deeply embedded in warm dust
cocoons, and have K-band continua rising steeply to longer wavelengths. A
significant fraction of them are hot stars of a variety of spectral types,
including at least five newly discovered Wolf-Rayet stars. All of them should
be suitable for spectroscopy of interstellar absorption lines at infrared
wavelengths greater than 3 microns and many are also suitable at shorter
wavelengths.
