May 1999 • 1999PhDT.........1I
Abstract • 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 2P3/2 --> 2P1/2 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.