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Millimeter-wave Observations of Diffuse Clouds

R. Lucas tex2html_wrap_inline738 , H. S. Liszt tex2html_wrap_inline776
tex2html_wrap_inline738 IRAM, Grenoble (France)
tex2html_wrap_inline776 NRAO, Charlottesville (USA)
Abstract: 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 tex2html_wrap_inline748 , 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 tex2html_wrap_inline748 , H tex2html_wrap_inline930 CO, CN, HCN, HNC, CS, SO, H tex2html_wrap_inline930 S, C tex2html_wrap_inline930 H, and C tex2html_wrap_inline934 H tex2html_wrap_inline930 in several lines of sight, intersecting about 20 individual clouds with CO column densities in the range tex2html_wrap_inline760 to tex2html_wrap_inline762  cm tex2html_wrap_inline764 . These results confirm that complex molecules achieve dark-cloud abundances at low extinctions, either by formation in the gas phase or on grains.
paper presented at IAU Symp. 178, Molecules in Astrophysics: Probes and Processes, ed. E. van Dishoek, Leiden, July 1996