The Western Rim of the VELA Shell
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A 3.3 1.2 pc segment of the western boundary of the Vela Shell has been mapped in CS (2-1) with a spatial resolution of 51 (0.11 pc) and a spectral resolution of 0.11 km s-1. The cloud has moderate CS column densities of slightly less than 1013 cm-2 and a total mass of 97 M. The kinetic energy of the cloud was found to be about an order of magnitude greater than its gravitational potential energy, indicating that it is not gravitationally bound and would disperse on very short timescales if it were not for the ram pressure of the expanding Vela Shell which continually sweeps up new interstellar matter into the CS "cloud." CN (N = 1-0) was observed at the peaks of the CS emission and found to have excitation temperatures of 3 K and column densities of 4-11 1013 cm-2. The low excitation of CN puts an upper limit on the gas density of 104 cm-3 and the CS emission requires densities of a few times 103-104 cm-3. We find that only 1%-10% of the line of sight is filled with gas in the above density range, from which we infer that the cloud is clumpy on scale sizes smaller than our resolution. The CN/CS column density ratios are 7 or greater in this cloud. Among the chemical models of Herbst and Leung, the only ones that are consistent with our observed CN/CS ratios, as well as those of CO/CS and HCO+/CS, are the "low metal" abundances for clouds that have not reached chemical equilibrium (i.e., ages less than 3 105 yr). 1996. The American Astronomical Society. All rights reserved.