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

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Comparative chemistry of diffuse clouds

I. C2H and C3H2

R. Lucas 1 and H.S. Liszt 2

1 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


Using the Plateau de Bure interferometer, we searched for [FORMULA]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+) [FORMULA]. On the whole we find a mean over all features of [FORMULA]N(C2H)/N(HCO+)[FORMULA] or [FORMULA]N(C2H)/N([FORMULA])[FORMULA] [FORMULA].

By contrast, C2H and ortho cyclic-C3H2 vary in nearly fixed proportion, [FORMULA]N(C2H)/N(C3H2- (o))[FORMULA], leading to a total C3H2 abundance N(C3H2)/N([FORMULA]) [FORMULA] and ([FORMULA]N(C3H2)/N(HCO+)[FORMULA]. 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+) [FORMULA] 0.065 in one direction but more generally we have only that N(C3H)/N(HCO+) [FORMULA] 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)

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000