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The $4.67\mu$m absorption feature of solid CO is an important signature of grain mantles. We have observed the solid CO feature towards a number of sources in the Serpens Dark Cloud. Towards one source, CK2, the feature is substantially deeper than any previously reported in other lines of sight. The average ratio of column densities $N$(CO)/$N$(H$_2$O) $\simeq 40\%$, whereas the average ratio in other clouds is typically less than 30\%. In the line of sight towards CK2, our results imply that 40\% of all CO is frozen onto grains, comparable to the depletion for the Taurus cloud. We therefore conclude that the strength of solid CO absorption in Serpens does not appear to imply an exceptionally high degree of freeze-out from the gas. We propose that the water-ice is, on average, underabundant in the Serpens cloud relative to other regions.
We compare our observations with models of percentage mantle composition, specifically, the time-dependent model of Breukers (1991) and the time-independent model of Tielens \& Allamandola (1987). Breukers' model predicts that $N$(CO)/$N$(H$_2$O) $\simeq 40\%$ at $10^6$\,years, fitting comfortably within the mean timescale for cloud dispersal. The time-independent model of Tielens \& Allamandola predicts that in regions of high density, there will be few hydrogenated species and therefore H$_2$O will be present in only trace amounts. The high abundance of CO relative to H$_2$O in Serpens, compared with other clouds, can be explained by assuming that a greater proportion of the dust within the line of sight resides in dense regions where the CO--rich (and H$_2$O--poor) component of the mantles dominates.
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