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Astron. Astrophys. 337, 246-252 (1998)

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Interstellar isotope ratios from mm-wave molecular absorption spectra

R. Lucas 1 and H. Liszt 2

1 Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France
2 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Received 18 March 1998 / Accepted 22 June 1998


We have measured galactic [FORMULA]mm absorption spectra of HCO+, HCN, HNC, and CS toward compact extragalactic continuum sources in order to derive the isotopic abundance ratios [FORMULA], [FORMULA], [FORMULA], and [FORMULA] in local diffuse clouds. For carbon, our data confirm recent results for the local ISM: we find [FORMULA] = [FORMULA]. For nitrogen, we find [FORMULA] = 237(-21,+27) consistent with the Solar value of 270, but substantially smaller than the values inferred from HCN emission in dense clouds. For sulfur we find [FORMULA] [FORMULA] consistent with the Solar value of 23. We also find one striking individual anomaly: toward 3C111 (B0415+379), [FORMULA] = 170[FORMULA] in one kinematic component. We attribute this to the fractionation of [FORMULA] into [FORMULA] which may be so great that HCN is starved for [FORMULA].

Key words: ISM: abundances – ISM: clouds – ISM: molecules – ISM: structure – radio lines: interstellar

Send offprint requests to: R. Lucas
Correspondence to: lucas@iram.fr

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998