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Millimeter-wave observations of diffuse clouds

Published online by Cambridge University Press:  25 May 2016

R. Lucas  and
H. S. Liszt
R. Lucas
Affiliation:
IRAM, Domaine Universitaire de Grenoble, 38406 St Martin D'Hérès, France
H. S. Liszt
Affiliation:
NRAO, Charlottesville, VA 22903, USA

Abstract

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Using the IRAM instruments (interferometer on Plateau de Bure and 30-m telescope on Pico Veleta) we have made numerous observations of molecular absorption lines in front of extragalactic millimeter wavelength radio sources. Observations of HCO+, CO and OH show that the lines of sight studied in this way sample the outer edges of molecular clouds or the diffuse clouds with highest column densities. Collisional excitation of the rotational levels is not significant in this density range for most molecular species, and accurate column densities may be derived by assuming radiative equilibrium with the cosmic microwave background. Using this technique we have measured column densities of CO, HCO+, H2CO, CN, HCN, HNC, CS, SO, H2S, C2H, and C3H2 in several lines of sight, intersecting about 20 individual clouds with CO column densities in the range 2 1014 to ∼ 1016 cm−2. These results confirm that complex molecules achieve dark-cloud abundances at low extinctions, either by formation in the gas phase or on grains.

Type
Diffuse and Translucent Clouds
Information
Symposium - International Astronomical Union , Volume 178: Molecules in Astrophysics: Probes and Processes , 1997 , pp. 421 - 430
Copyright
Copyright © Kluwer 1997 

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