, H. Liszt
Institut de Radioastronomie Millimétrique,
300 Rue de la Piscine,
F-38406 Saint Martin d'Hères,
France
National Radio Astronomy Observatory,
520 Edgemont Road,
Charlottesville, VA,
USA 22903-2475 
3mm HCO
absorption along 30 lines of
sight toward compact extragalactic mm-wave continuum sources, using the
Plateau de Bure Interferometer. Such absorption is surprisingly common,
occurring approximately 30% as often as 
21cm absorption in H I.
In fact, the mean free path for detection of absorption with optical depth
of unity is the same in both species. The slope of the HCO
optical
depth and column density distribution functions is very similar to that of
H I but the mean HCO
linewidth is much smaller, 0.95 km s
. A search for
accompanying HCO
emission finds only an occasional weak (0.05 K) line,
even when 
(HCO
) 
, so that quite accurate column densities
may be derived from observations of only the lowest line.
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 H
formation, 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).
