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 4.4 \mu m to 5.1 \mu m. The use of a slit width of 0.6 \arcsec implied a spectral resolution of R = \lambda / \Delta \lambda =800 ( \Delta v = 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 \mu m ) and ^ { 13 } CO ( 4.77 \mu m ) vibration-rotational absorptions , we detect a strong absorption due to a mixture of polar and apolar CO ice ( centered at 4.675 \mu m ) .
In the shorter wavelength absorption wing there is an absorption feature due to XCN at 4.62 \mu m. 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 IRS 2L and IRS 16C 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 } M _ { \odot } .
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 .