2009ApJ...703.1635M

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 (VLT) 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 µm to 5.1 µm. The use of a slit width of 0".6 implied a spectral resolution of R = λ/Δλ = 800 (Δv = 375 km/s). These observations resulted in the M-band spectra of 15 bright sources in the central stellar cluster of the Milky Way. In addition to gaseous ¹²CO (4.666 µm) and ¹³CO (4.77 µm) vibration-rotational absorptions, we detect a strong absorption due to a mixture of polar and apolar CO ice (centered at 4.675 µm). In the shorter wavelength absorption wing there is an absorption feature due to XCN at 4.62 µm. The XCN absorption is strongest toward the M2 supergiant IRS 7. 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 ¹³CO gas. In combination with published hydrogen number density estimates from sub-millimeter CO(7-6) and FIR [O I] line data, we obtain gas masses of the circumstellar shells of the order of 10^–3 and 10^–2 M_☉. 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.