2000A&A...358.1069L

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.