SOLID CARBON-MONOXIDE IN THE SERPENS DARK CLOUD

The 4.67 mum absorption feature of solid CO is an important signature of grain mantles in molecular clouds. This paper presents new observat ions of solid CO absorption in the spectra of four infrared sources in and behind the Serpens dark cloud. We also present supplementary 2.0- 2.5 mum spectra which clarify the nature of selected sources. Solid CO profiles are compared with models based on laboratory data. In common with previous studies of other clouds, we find that the observed prof iles are consistent with the presence of two distinct phases of CO-bea ring ice, one dominated either by CO itself or by CO mixed with nonpol ar molecules such as O2, the other with H2O as the primary constituent . The strength of solid CO absorption relative to that of the water-ic e feature at 3.1 mum is unusually high in Screens: we deduce column de nsities for CO and H2O in solid form and conclude that the mean CO abu ndance in dust is 40% of the H2O abundance, compared with values in th e range 0%-30% typical of other molecular clouds. On the basis of its 2.0-2.5 mum spectrum and near-infrared colors, we deduce the infrared source CK 2 to be a K5-MO III field star situated behind the Serpens c loud and suffering 40-50 mag of visual extinction. Assuming empirical correlations between extinction and total (gas + dust) CO column densi ty, we estimate the CO depletion (i.e., the fraction of interstellar C O in the line of sight tied up in grains) to be approximately 40% in t he intracloud medium of the Serpens cloud, comparable with previous es timates for the Taurus dark cloud, and much higher than that found in the environments of protostellar objects.