Astron. Astrophys. 358, 1069-1076 (2000)

Comparative chemistry of diffuse clouds

I. C₂H and C₃H₂

R. Lucas ¹ and H.S. Liszt <sup>2

1</sup> Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 8406 Saint Martin d'Hères, France
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Received 8 March 2000 / Accepted 17 April 2000

Abstract

Using the Plateau de Bure interferometer, we searched for 3mm absorption lines of C₂H, C₃H₂, C₃H, and C₄H from the diffuse and very marginally translucent clouds which lie toward a sample of compact extragalactic mm-wave continuum sources. The C₂H survey in particular is nearly the equivalent of our earlier, exhaustive study of HCO⁺ absorption, albeit with lower signal/noise.

C₂H lines are found corresponding to all components detected in HCO⁺ which places C₂H in a small group of the most ubiquitous molecules we have studied - OH, HCO⁺ and, from this work, C₂H and C₃H₂. But the relationship between N(C₂H) and N(HCO⁺) is highly non-linear, as seems to be the case for all species beside OH. N(C₂H)/N(HCO⁺) is actually relatively high at small N(HCO⁺), 20-40, and declines at higher column density despite a typically steep increase of N(C₂H) around N(HCO⁺) . On the whole we find a mean over all features of N(C₂H)/N(HCO⁺) or N(C₂H)/N() .

By contrast, C₂H and ortho cyclic-C₃H₂ vary in nearly fixed proportion, N(C₂H)/N(C₃H₂- (o)), leading to a total C₃H₂ abundance N(C₃H₂)/N() and (N(C₃H₂)/N(HCO⁺). Our observations of the high-lying C₄H lines at 95 GHz do not place sensitive limits on N(C₄H)/N(HCO⁺). The C₃H data are somewhat more restrictive, and we find N(C₃H)/N(HCO⁺) 0.065 in one direction but more generally we have only that N(C₃H)/N(HCO⁺) 4. The linewidths of C₂H, C₃H₂, and HCO⁺ are sensibly identical.

In comparing the abundances of many simple species in diffuse, translucent, and dark gas, we see that the relative abundances of OH, HCO⁺, CH, and C₂H are relatively constant, but those of C₃H₂ and C₃H are noticeably larger in TMC-1.

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

Send offprint requests to: H.S. Liszt (hliszt@nrao.edu)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. C₂H
  • 2.2. Cyclic C₃H₂
  • 2.3. Linear C₃H
  • 2.4. C₄H
• 3. Systematics
  • 3.1. C₂H
  • 3.2. C₃H₂
    • 3.2.1. The ortho/para ratio in C₃H₂
  • 3.3. C₄H and linear C₃H
  • 3.4. Linewidths and kinematics
• 4. Comparative chemistry of simple carbon chains and hydrocarbons
  • 4.1. Comparison with some prior results for C₃H₂
  • 4.2. Relative abundances in diffuse, translucent, and dense clouds
  • 4.3. The relative abundance of the linear and cyclic forms of C₃H₂
• 5. Diffuse cloud chemistry
• Acknowledgements
• Appendices
  • A: products of gaussian fitting
• References

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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