Physical state of molecular gas in high galactic latitude translucent clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interste llar medium. Here we study the lowest four rotational transitions of CO tow ard high-latitude translucent molecular clouds (HLCs). We report new observ ations of the J = (4-3), (2-1), and (1-0) transitions of CO toward eight hi gh-latitude clouds. The new observations are combined with data from the li terature to show that the emission from all observed CO transitions is line arly correlated. This implies that the excitation conditions that lead to e mission in these transitions are uniform throughout the clouds. Observed (C O)-C-13/(CO)-C-12 (1_0) integrated intensity ratios are generally much grea ter than the expected abundance ratio of the two species, indicating that t he regions that emit (CO)-C-12 (1-0) radiation are optically thick. We deve lop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the mos t likely portion of the physical parameter space that is compatible with th e observations. The model enables us to rule out CO gas temperatures greate r than similar to 30 K, since the most likely high-temperature configuratio ns are 1 pc-sized structures aligned along the line of sight. The most prob able solution is a high-density and low-temperature (HDLT) solution, with v olume density, n=10(4.5+/-0.5) cm(-3), kinetic temperature, T-k approximate to 8 K, and CO column density per velocity interval N-CO/Delta V. = 10(16. 6+/-0.3) cm(-2)/(km s(-1)). The CO cell size is L similar to 0.01 pc (simil ar to 2000 AU). These cells are thus tiny fragments within the similar to 1 00 times larger GO-emitting extent of a typical high-latitude cloud. We dis cuss the physical implications of HDLT cells, and we suggest ways to test f or their existence.