Abstract
The 6-cm transition of H$sub 2$CO was mapped in absorption over a region approx.1degreetimesapprox.1degree with 6' resolution toward the rho Oph dark cloud. The distribution of H$sub 2$CO column density agrees well with the extinction due to dust. At least three regions of high H$sub 2$CO concentration can be identifed. Two regions of reduced H$sub 2$CO column density coincide with regions of carbon recombination-line emission, reflection nebulosity, and concentration of infrared sources. The most likely mechanisms of line intensity reduction are increased excitation temperature, due to electron collisions, and reduced column density, due to destructive C$sup +$ collisions. Both effects are associated with the observed C ii regions (s). A north-south velocity gradient of approx.0.5 km s$sup -1$ pc$sup -1$ is observed, which is probably not sufficient to stabilize the cloud against large-scale gravitational collapse. (AIP)
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