Dense Gas in High-Latitude Molecular Clouds?

Details Dense Gas in High-Latitude Molecular Clouds? Dense Gas in High-Latitude Molecular Clouds?

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4801r&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #48.01; Bulletin of the American Astronomical Society, Vol. 24, p.1198

Astronomy and Astrophysics

Astronomy

Scientific paper

We have surveyed high-latitude molecular clouds (MBM 12, 7, 55, 40) in spectral lines that are believed to be `dense-gas' tracers due to the high H_2 volume density required for collisional excitation. An extensive CS(2--1) line map of MBM 12 reveals emission that is not confined to clumps. Less than 20% of the integrated line emission from the cloud originates in clearly identified clumps with size between 0.2 pc and 0.02 pc (resolution) in the integrated line map. The bulk of the emission originates from a relatively smooth, continuous `horseshoe' structure (about 0.1 pc wide and 1 pc long). Our CS(2--1) map correlates positively with the published, Bell Labs (13) CO map, with significant (13) CO emission even where the CS emission is undetectable. Within the central core, the C(18) O(1--0) and CS(2--1) lines are positively correlated with significant scatter. There is some indication of higher CS/(13) CO in the cores than the `horseshoe'. The observed correlations suggest that both the diffuse CS and (13) CO originate from either (1) numerous, unresolved clumps, or (2) the diffuse parts of the cloud. High-spatial-resolution observations of HCO(+) from MBM 12 obtained with the BIMA Hat Creek array demonstrate that the main core emission is primarily on spatial scales greater than 0.004 pc (800 AU). Thus it appears that we have resolved most of the spatial structure of the `dense-gas' tracers, and we have found that the emission is primarily diffuse, with a few clumps. In order to understand the excitation mechanism of the CS rotational levels, we are performing a multitransitional study of the 1--0, 2--1, and 3--2 lines. The CS excitation may be governed by electron collisions in regions with H_2 column densities an order of magnitude lower than the `critical density' of ~ gt 2times 10(4) cm(-3) , as has been previously found for very weak CS lines from diffuse clouds. If electron collisions are populating the CS levels, then the CS and (13) CO lines can both be produced in the outer parts of the cloud, explaining their positive correlation at low levels.

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