Carbon gas in high galactic latitude molecular clouds

The high Galactic latitude molecular clouds (HLCs) are the simplest molecular objects in the interstellar medium. HLCs are nearby (distance 100 parsecs) translucent (visual extinction 1-5 magnitudes) clouds which are exposed to the average interstellar radiation field (ISRF). Models of the photochemistry in translucent clouds predict that they are intermediate between clouds dominated by gas-phase carbon in molecular form and those dominated by gas-phase carbon in atomic form. The structure of translucent HLCs is investigated using observations of the three most abundant forms of carbon gas: carbon monoxide (CO), neutral atomic carbon (C I), and singly-ionized carbon (C II ). The newly commissioned Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) was used to measure two rotational transitions of CO, as well as the (³P₁ --> ³P ₀) transition of [C I], in eight HLCs. As predicted by chemical models, the C I/CO column density ratio in the clouds is approximately unity, and varies inversely with total gas column density. Most of the CO emission originates in material with density greater than 10 ⁴ molecules cm^-3. The HLC gas seems to exist in the form of clumps with mean size 0.001 parsecs, which is much smaller than the resolution limit of the AST/RO telescope. The Infrared Space Observatory (ISO) was used to observe the ²P_3/2 --> ²P_1/2 transition of [C II] towards four HLCs. The C II is part of a uniformly-distributed interclump medium (ICM) which envelops the clouds, shielding the C I and CO from the ISRF. The CO clumps are not in pressure equilibrium with the C II, and are predicted to be expanding into it. In order to test the conclusions of the AST/RO and ISO surveys of HLCs, and to constrain further the properties of the gas, a detailed case study of the nearest molecular cloud, HLC MBM-12, was made using the Caltech Submillimeter Observatory. Observations of CO and C I at 0.01 parsec scales confirm that the CO is located primarily in dense clumps, but that approximately 70% of the C I exists in the ICM. Direct kinematic evidence is found for the expansion of clumps into the ICM.