Heterodyne spectroscopy of C II in molecular clouds

Abstract

High-resolution spectra of the 158 μm fine-structure line of ionized carbon have been obtained with a heterodyne spectrometer toward a number of galactic molecular clouds. Strongly reversed C II line profiles are seen in sources noted for similar behavior in low-J ¹²CO. The C⁺ producing the optically thick (τ≥1) absorption component is cool or subthermally excited, with excitation temperatures typically less than 25–50 K. Column densities for the absorption component are ≥10¹⁸ cm⁻², similar to values predicted by UV-photodissociation models for C II emitting gas. Thus the total C⁺ column density in the vicinity of these molecular clouds is significantly greater than previously estimated from integrated intensity measurements.

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