Molecular Depletion and Thermal Balance in Dark Cloud Cores

We analyze the effects of molecular depletion on the thermal balance of well-shielded, quiescent dark cloud cores. Recent observations of the significant depletion of molecules from the gas phase onto grain surfaces in dark clouds suggest the possibility that the gas-phase cooling in these regions is greatly reduced and consequently that gas kinetic temperatures might be increased. We reexamine cooling and heating processes in light of possible molecular depletion, including the effect of coupling between the gas and the grains. At densities <=10^3.5 cm^-3, the gas temperature can be significantly increased by the depletion of coolant species without significantly affecting the dust temperature because of the relatively weak gas-dust coupling. At higher densities, this coupling becomes sufficiently rapid to overwhelm the effect of the reduced gas-phase cooling, and depletion has little effect on the gas temperature while raising the dust temperature ~=1 K. The result is that depletion at densities >=10^4.5 cm^-3 can proceed without being evident as an enhanced gas temperature or without self-limiting due to an increase in the dust temperature increasing the desorption rate. This is consistent with observations of depletion in cold, dense regions of quiescent molecular clouds. It also suggests that depletion in moderate density regions can increase the thermal gas pressure, effectively enhancing the confinement of denser portions of molecular clouds and possibly accelerating the collapse of cloud cores.