In comparing CO (emission) and HCO absorption, little consistency is at first apparent. However, we can now show that the strength of CO emission increases abruptly when . This is the same phenomenon observed in uv-absorption studies at and is understandable in terms of the onset of dust- and self-shielding at extinctions below 1 magnitude in diffuse clouds or the outer regions of dark clouds. A similar turn on occurs in at the same values of .
Comparison of CO and HCO emission and absorption can also be used to derive the physical conditions in the clouds; in general we find , if K. This is within but at the high end of the range for diffuse clouds studied optically and well below values appropriate to dark cloud cores. In a few cases, the presence of a relatively large electron fraction () seems indicated, along with temperatures of 20-30K. Thus the excitation analysis is consistent with the existence of a CO turn on. Although it is somewhat surprising that relatively strong CO and (especially) CO emission is present when carbon is only partially recombined to and bound up in carbon monoxide, this is not atypical in the outer regions of dark clouds.
We have also surveyed 18cm OH absorption in six directions using the VLA. Unlike most other species, but in keeping with traditional notions of diffuse cloud chemistry, there is a remarkably uniform relationship between the OH and HCO column densities, even at low values. We find for ; this is within 40% of values quoted for TMC-1, but at 100 times lower column density. Using relevant values of observed in uv absorption spectra of diffuse clouds, at and E(B-V) = 0.3 mag, it follows that across a very broad range of extinction.
Apparently, many diatomics and polyatomics form readily in diffuse clouds as long as there is any appreciable amount of Hformation, and well before the carbon conversion from C to CO is complete or CO emission is strong. Although the strong coupling between OH and HCO is characteristic of conventional diffuse cloud chemistry, such chemistry in general falls far short of reproducing the observed amounts of HCO, and perhaps CO, even when OH can be explained. Although not understood at present, it is clearly the case that many molecules, including those formed on grains (CO), appear with abundances characteristic of dark, cold, cloud cores even when mag.
Figure: Digest of all detected HCO J=1-0 absorption profiles seen at the Plateau de Bure Interferometer. The channel spacing is 78kHz and the resolution is 140 kHz (0.47 km s)
Figure: Left: Column densities of OH and HCO. The OH column densities assume 1 K excitation above the CMB; those for HCO use . Column densities of OH seen in three classical diffuse clouds are shown as bars near . Right: The same data plotted on a log scale, and extended to higher . The symbols marked L and T represent values quoted for L134N and TMC-1 by Ohishi, Kaifu, and Irvine (1992).