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

Interstellar isotope ratios from mm-wave molecular absorption spectra

R. Lucas ¹ and H. Liszt <sup>2

1</sup> 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

Abstract

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

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

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

SIMBAD Objects

Contents

• 1. Introduction
• 2. Our data
  • 2.1. Absorption line profiles
  • 2.2. Common conventions
• 3. Column densities and isotopic abundance ratios
  • 3.1. The ratio
  • 3.2. The ratio
  • 3.3. The ratio
  • 3.4. The ratio
• 4. Discussion
  • 4.1. CO fractionation and anomalies in molecular isotope ratios
  • 4.2. Conditions in the molecular gas toward 3C111
• 5. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
helpdesk.link@springer.de