Millimeter-Wave Absorption Studies of the Onset of Dark-Cloud Chemistry

Abstract

We have recently observed mm-wave absorption lines of ¹³CO, HCO⁺, HCN, C₂H, HNC, CN in front of distant, compact radio sources (Lucas and Liszt 1993, 1994). Molecules are easily detected, while the extinction is moderate (typically 1 mag.); HCO⁺ spectra are richer, broader, and more complex than ¹²CO emission observed with single dishes in the same directions (BL Lac, NRAO 150, B0212+735). Molecular abundances are similar in magnitude to those in dark clouds, while the UV radiation field should be dominant. These results suggest that a chemically rich component of the interstellar medium could be present, but not well represented in CO emission surveys. The clouds we have detected appear to be very similar to diffuse clouds such as ζ Oph, where Liszt and Lucas (1994) have detected, in emission, a comparable column density of HCO⁺. We briefly summarize here further recent progresses.