Temperatures of galactic molecular clouds showing CO self-absorption

Abstract

The CO J=2- l line has been observed and, in most cases, mapped in 10 star-forming molecular clouds (W3, NGC 1333, NGC 2071, Mon R2, CRL 961, p Oph, W49N, W51A, DR 21, and Cep A). The CO J = 3-2 line has been observed in W3 and DR 21. The CO lines from all of these sources are strongly self-absorbed. By comparing our results with published CO (1-0) line profiles, we find that large corrections to the temperatures of the cloud cores, as measured by the CO (1-0) lines, are required. The corrections for self-absorption bring the CO brightness temperatures into closer agreement with the grain temperatures inferred from far-infrared photometry. The absorbing gas appears to have a variety of geometrical relationships with the absorbed 'background' gas for this group of sources; in one or two cases there may be no physical connection between the absorbed and absorbing regions. For some CO sources, the combined CO data indicate the presence of velocity gradients; however, whether these are due to expansion or collapse cannot be decided on the basis of the presently available data.