Millimetre observations of southern translucent clouds

Details Millimetre observations of southern translucent clouds Millimetre observations of southern translucent clouds

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285..300g&link_type=abstract

Astronomy and Astrophysics 285, 300-321 (1994)

Astronomy and Astrophysics

Astrophysics

28

Ism: Abundances, Ism: Molecules, Radio Lines: Molecular, Ism: Clouds, Ism: Structure

Scientific paper

Millimetre emission line observations of ^12^CO 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 ^13^CO and C^18^O J=1-0 emission have been made towards the stellar positions. C^18^O has been detected in only 5 clouds, most of which have extinctions well in excess of 2 mag. The inferred ^13^CO/C^18^O 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 ([^13^C].[^16^O])/([^12^C] .[^18^O]) 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 C_2_H or C_3_H_2_ molecules, even though the abundances of diatomic C_2_ 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.

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