M-Band Spectra of Dust Embedded Sources at the Galactic Center
  The goal of the present paper is to investigate the circumstellar material
around the brightest dust-enshrouded sources in the central stellar cluster of
the Milky Way. Observations have been carried out at the European Southern
Observatory's Very Large Telescope on Paranal, Chile. We have used the long
wavelength (LWS3) low resolution (LR) spectroscopic mode of the ISAAC camera at
the VLT in the spectral range of the M filter from 4.4micron to 5.1micron. The
use of a slit width of 0.6" implied a spectral resolution of R=l/Dl=800 (Dv=375
km/s). These observations resulted in M-band spectra of 15 bright sources in
the central stellar cluster of the Milky Way. In addition to gaseous 12^CO
(4.666 micron) and 13^CO (4.77 micron) vibration-rotational absorptions, we
detect a strong absorption due to a mixture of polar and apolar CO ice
(centered at 4.675 micron). In the shorter wavelength absorption wing there is
an absorption feature due to XCN at 4.62 micron. The XCN absorption is
strongest toward the M2 supergiant IRS7. We find that the extinctions due to
material traced by the CO ice and the CO gas absorptions may be of comparable
importance. Using the spectra of IRS2L and IRS16C we perform a first order
correction of the line of sight absorption due to CO-ice and 13^CO gas. In
combination with published hydrogen number density estimates from sub-mm
CO(7-6) and FIR [OI] line data we obtain gas masses of the circumstellar shells
of the order of 10^-3 and 10^-2 solar masses. This implies that in future
spectra taken at high spectral and angular resolution the bright and dust
embedded Galactic Center sources should show a substantial line absorption due
to source intrinsic absorption.
