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H. S. Liszt (NRAO), R. Lucas (IRAM)
Compact extragalactic mm-wave continuum sources are occasionally occulted by nearby galactic diffuse clouds and we have used the Plateau de Bure Interferometer to survey mm-wave absorption from many species toward them. Typically we see 1014 < N(CO)< 1016 cm-2, n(H2)*Tk ~ 103 -- 104 cm-3K, as did the Copernicus group a generation ago. 13CO is very heavily fractionated for larger values of N(CO), so the CO must form in a cool medium where nearly all the gas-phase carbon is in CII. For the very thinnest clouds, CO emission counterparts of absorption features are absent.
These properties of CO make the diffuse cloud connection very simply, but such clouds are actually more interesting for all the other species which we observe. Perhaps like CH+, though without its occasional kinematic peculiarities,we find that many species are overabundant, relative to standard diffuse cloud models, by about 2 orders of magnitude. Examples include HCO+, HCN, HNC, C2H, H2CO (seen earlier at cm-wavelengths, of course) C3H2 (ditto) as well as CS and CN. That all these species should have relative abundances typical of dark cloud cores while largely unshielded from the interstellar uv field is something of a happy (for observers, at least) mystery. Beside CO, only HCO+ is occasionally found in emission and it is very, very weak (0.05 K or less).
The abundances of most species increase abruptly when N(HCO+) ~ 1012 cm-2 but N(HCO+)/N(OH) is constant at 0.03 over an enormous range, from 5 times below that of Zeta Oph up to such N(OH) as are seen in TMC-1. We surmise that both OH and HCO+ have nearly fixed abundances relative to hydrogen. Chemically-related species like HCN, HNC, and CN or C2H and C3H2 vary smoothly together even when their behaviour relative to HCO+ is abrupt.
These high abundances have not yet been explained, but may contribute to our understanding of other problems. For instance, one may drop the observed abundance of HCO+ into an otherwise standard model, and recover the correct amount of CO.
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