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P.F. Goldsmith (NAIC, Department of Astronomy, Cornell University)
Depletion of molecules from the gas phase onto surfaces of dust grains is becoming increasingly well-established observationally. The reduction in the abundance of key gas phase coolants will inevitably reduce the cooling rate and tend to increase the gas temperature in the central, well-shielded regions of dark clouds. The increased temperature would have the effect of limiting the depletion. We investigate this effect of depletion by calculating the gas phase cooling rate as a function of depletion factor. The SWAS determination of the surprisingly low abundances of molecular oxygen and water help define the important coolants. The cooling is only weakly dependent on the molecular depletion, due to the large optical depths of the key cooling lines, particularly the lower rotational transitions of carbon monoxide. The coupling with dust grains is included in a calculation of thermal balance, and we find that in well-shielded regions, at densities above 104 cm-3, the coupling of gas and relatively cool dust limits the temperature rise of the gaseous component, and the dust temperature is relatively unaffected as well. It appears that depletion factors as large as a factor of 100 can occur without significantly increasing the temperature in these regions. Cold dense cores can thus remain cold even as the gas phase becomes signficantly depleted.
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