2000ApJ...535..211I

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO toward high-latitude translucent molecular clouds (HLCs). We report new observations of the J=(4-3), (2-1), and (1-0) transitions of CO toward eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions that lead to emission in these transitions are uniform throughout the clouds. Observed ¹³CO/¹²CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions that emit ¹²CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space that is compatible with the observations. The model enables us to rule out CO gas temperatures greater than ∼30 K, since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high-density and low-temperature (HDLT) solution, with volume density, n=10^4.5±0.5 cm^–3, kinetic temperature, T_k~8 K, and CO column density per velocity interval N_CO/ΔV=10^16.6±0.3 cm^–2/( km.s^–1). The CO cell size is L∼0.01 pc (∼2000 AU). These cells are thus tiny fragments within the ∼100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.