Comparative chemistry of diffuse clouds
I. C2H and C3H2
R. Lucas 1 and
H.S. Liszt 2
Received 8 March 2000 / Accepted 17 April 2000
Using the Plateau de Bure interferometer, we searched for 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+) . On the whole we find a mean over all features of N(C2H)/N(HCO+) or N(C2H)/N() .
By contrast, C2H and ortho cyclic-C3H2 vary in nearly fixed proportion, N(C2H)/N(C3H2- (o)), leading to a total C3H2 abundance N(C3H2)/N() and (N(C3H2)/N(HCO+). 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+) 0.065 in one direction but more generally we have only that N(C3H)/N(HCO+) 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 (firstname.lastname@example.org)
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000