1994A&A...285..300G

Millimetre emission line observations of ¹²CO J=1-0 are presented for a sample of translucent molecular clouds previously studied by optical absorption lines towards background stars. The CO maps provide information on the extent, structure, mass and kinematics of the regions. In some clouds, the emission is characterized by a single component which does not vary much across the region. In other cases, multiple close-lying components or sheets are found, which may vary in a self-similar manner across the cloud. None of the clouds is close to simple virial equilibrium. Sensitive searches for ¹³CO and C¹⁸O J=1-0 emission have been made towards the stellar positions. C¹⁸O has been detected in only 5 clouds, most of which have extinctions well in excess of 2 mag. The inferred ¹³CO/C¹⁸O abundance ratios range from 7-25, and the lower limits from >13 to >35. These values are as much as five times larger than the overall interstellar ([¹³C].[¹⁶O])/([¹²C] .[¹⁸O]) ratio, suggesting that isotope-selective photodissociation plays a role in at least some of the clouds. Searches for other molecules at millimetre wavelengths have been made for a few of the best characterized clouds. Surprisingly, no emission was detected from the C₂H or C₃H₂ molecules, even though the abundances of diatomic C₂ and CH are quite large. On the other hand, the abundance of HCO⁺ appears comparable to that found in denser clouds, and the abundance of HCN may be up to an order of magnitude larger than the predictions of models in two clouds.