Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means ofinvestigating the density and velocity structure of the molecular interstellarmedium. Here we study the lowest four rotational transitions of CO towardshigh-latitude translucent molecular clouds (HLCs). We report new observationsof the J = (4-3), (2-1), and (1-0) transitions of CO towards eighthigh-latitude clouds. The new observations are combined with data from theliterature to show that the emission from all observed CO transitions islinearly correlated. This implies that the excitation conditions which lead toemission in these transitions are uniform throughout the clouds. Observed13CO/12CO (1-0) integrated intensity ratios are generally much greater than theexpected abundance ratio of the two species, indicating that the regions whichemit 12CO (1-0) radiation are optically thick. We develop a statistical methodto compare the observed line ratios with models of CO excitation and radiativetransfer. This enables us to determine the most likely portion of the physicalparameter space which is compatible with the observations. The model enables usto rule out CO gas temperatures greater than 30K since the most likelyhigh-temperature configurations are 1 pc-sized structures aligned along theline of sight. The most probable solution is a high density and low temperature(HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). Thesecells are thus tiny fragments within the 100 times larger CO-emitting extent ofa typical high-latitude cloud. We discuss the physical implications of HDLTcells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The Astrophysical Journa