Comparative chemistry of diffuse clouds. I. C_2H and C_3H_2

Details Comparative chemistry of diffuse clouds. I. C_2H and C_3H_2 Comparative chemistry of diffuse clouds. I. C_2H and C_3H_2

Jun 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...358.1069l&link_type=abstract

Astronomy and Astrophysics, v.358, p.1069-1076 (2000)

Astronomy and Astrophysics

Astrophysics

58

Ism: Abundances, Ism: Clouds, Ism: Molecules, Ism: Structure, Radio Lines: Ism

Scientific paper

Using the Plateau de Bure interferometer, we searched for lambda 3mm absorption lines of C_2H, C_3H_2, C_3H, and C_4H from the diffuse and very marginally translucent clouds which lie toward a sample of compact extragalactic mm-wave continuum sources. The C_2H survey in particular is nearly the equivalent of our earlier, exhaustive study of HCO+ absorption, albeit with lower signal/noise. C_2H lines are found corresponding to all components detected in HCO+ which places C_2H in a small group of the most ubiquitous molecules we have studied - OH, HCO+ and, from this work, C_2H and C_3H_2. But the relationship between N(C_2H) and N(HCO+) is highly non-linear, as seems to be the case for all species beside OH. N(C_2H)/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(C_2H) around N(HCO+) ~ 1012 pcc . On the whole we find a mean over all features of = 14.5+/-6.7 or ~ 2.9+/-1.3x 10-8. By contrast, C_2H and ortho cyclic-C_3H_2 vary in nearly fixed proportion, = 27.7+/-8.0, leading to a total C_3H_2 abundance N(C_3H_2)/N(H_2) ~ 1.4+/-0.7x 10-9 and ( ~ 0.7. Our observations of the high-lying C_4H lines at 95 GHz do not place sensitive limits on N(C_4H)/N(HCO+). The C_3H data are somewhat more restrictive, and we find N(C_3H)/N(HCO+) < 0.065 in one direction but more generally we have only that N(C_3H)/N(HCO+) < 4. The linewidths of C_2H, C_3H_2, 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 C_2H are relatively constant, but those of C_3H_2 and C_3H are noticeably larger in TMC-1.

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