A reexamination of the energetics of molecular clouds

The standard picture of molecular cloud energetics has been that the gas is heated by collisions with warmer dust grains. However, in several molecular clouds, the gas has appeared to be hotter than the dust, based on CO J = 1 tends to O data and airborne far-infrared data. In other clouds, it has been suggested that the densities are not high enough to couple the dust to the gas and thereby to heat the gas to the observed temperatures. For eight regions, gas temperatures have been remeasured with new observations of CO J = 1 tends to O and CO J = 2 tends to 1 and have determined the dust temperature using IRAS data. If the dust temperatures derived from the IRAS flux densities at 100 and 60 microns are adopted, most discrepancies are removed, but one or two may persist. Various possible explanations for the remaining discrepancies and the implications of the generally good agreement between gas and dust temperatures are considered. 58 refs.