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Session 24 - General Interstellar Medium.
Oral session, Monday, June 10
Historical Society,

[24.01] Atomic Carbon in Southern Hemisphere High Latitude Clouds

J. G. Ingalls, R. A. Chamberlin, T. M. Bania, J. M. Jackson (Boston U.), A. A. Stark, A. P. Lane (CfA)

We report observations of the 492\ghz\ (^3P_1 - ^3P_0) transition of neutral atomic carbon [\ci ] toward eight Southern Hemisphere high galactic latitude molecular clouds (HLCs) using the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). Due to the extremely low atmospheric opacity above the Antarctic plateau, AST/RO allows us to observe submillimeter-wavelength lines of atomic and molecular species in the interstellar medium with unprecedented sensitivity. In general, the HLCs are nearby translucent clouds, regions in which molecules exist in abundance and photoprocesses dominate the chemistry. As such, they are the simplest regions in which to study molecule formation and destruction in the interstellar medium. Atomic carbon is one of the best tracers of translucent gas: carbon is a product of the photodissociation of CO, and it occurs in regions of clouds in which \htwo\ is the most abundant species, but in which there is little or no CO.

We observed the central regions of eight molecular clouds above b=15\deg and detected all eight of them in [\ci ]. The C/CO column density ratio in these objects ranges from 0.4 to 2.5. These observations have significantly increased the number of HLCs with measured carbon abundances. The measured abundances of C and CO in the HLCs can be explained by a simple model of a homogeneous cloud with a diffuse far-ultraviolet radiation field incident on its surface. The observations of [\ci ] in these objects confirm the assertion that the HLCs are translucent objects, in which the atomic-to-molecular transition dominates the chemistry and structure.

This research was supported in part by the National Science Foundation under a cooperative agreement with the Center for Astrophysical Research in Antarctica (CARA), grant number NSF DPP 89-20223. CARA is a National Science Foundation Science and Technology Center.

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