, R. Lucas
National Radio Astronomy Observatory,
520 Edgemont Road, Charlottesville, VA, USA 22903-2475
Institut de Radioastronomie Millimétrique,
300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France
6cm and 
2mm ortho- and
para-H
CO absorption along lines
of sight toward and around compact extragalactic mm-wave continuum sources
(Fig. 6).
From comparison of on and off-source 
6cm profiles it is possible
to derive the anomalous excitation temperature, which is surprisingly close to
the temperature of the cosmic microwave background in several cases. The
2mm absorption profiles show that the ortho/para ratio is 3-4,
which (taken alone) favors formation either on grains at 
K or
at comparable temperatures in the gas phase. However the abundance of H
CO ,
as with other species, is probably too high at low extinction
to be explained by any known process; we find
for 
.
Figure: 
2mm para (
) and ortho absorption
line profiles toward two sources.
