Astronomy and Astrophysics, volume 358, 1069-1076 (2000/6-3)
Comparative chemistry of diffuse clouds. I. C2H and C3H2.
LUCAS R. and LISZT H.S.
Abstract (from CDS):
Using the Plateau de Bure interferometer, we searched for λ3mm absorption lines of C2H, 3h2, 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 3h2. 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+)≃1012cm–2. On the whole we find a mean over all features of <N(C2H)/N(HCO+)≥14.5±6.7 or <N(C2H)/N(H2)≥∼2.9±1.3x10–8. By contrast, C2H and ortho cyclic-3h2 vary in nearly fixed proportion, <N(C2H)/N(3h2-(o))≥27.7±8.0, leading to a total 3h2 abundance N(3h2)/N(H2)≃1.4±0.7x10–9 and (<N(3h2)/N(HCO+)≥∼0.7. Our observations of the high-lying C4H lines at 95GHz 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, 3h2, 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 3h2 and C3H are noticeably larger in TMC-1.
ISM: abundances - ISM: clouds - ISM: molecules - ISM: structure - radio lines: ISM
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