Comparative chemistry of diffuse clouds. I. C_2H and C_3H_2

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 <N(C_2H)/N(HCO⁺)> = 14.5+/-6.7 or <N(C_2H)/N(H_2)> ~ 2.9+/-1.3x 10^-8. By contrast, C_2H and ortho cyclic-C_3H_2 vary in nearly fixed proportion, <N(C_2H)/N(C_3H_2-(o))> = 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 (<N(C_3H_2)/N(HCO⁺)> ~ 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.