Radio observations of interstellar CH. I

Abstract

Results are presented for some eight months of almost continuous high-sensitivity observations of the three hyperfine transitions of the CH ground state in the directions to or in extended areas around more than 30 H I and H II regions, dark dust clouds, OH/IR stars, and other sources. The observations were made with a traveling-wave maser preamplifier on a 25.6-m telescope, giving a zenith-system noise temperature of about 34 K. Detailed discussions of the CH spectra are given in source descriptions and tables; radiative-transfer considerations and possible CH excitation mechanisms are included. Some major findings are that: (1) CH is widespread in the Galaxy; (2) the three CH transitions are observed in emission almost everywhere and exhibit weak maser characteristics; (3) satellite transitions are generally more inverted than the main line; (4) CH, OH, and H2CO seem to be well mixed in Orion-arm and Perseus-arm cloud configurations; (5) the average main-line excitation temperature is about -15 K; (6) the lower satellite transition seems to be a very sensitive far-IR indicator; (7) significant departures from equilibrium line-intensity ratios appear toward H I and H II regions; and (8) the relative abundance of CH with respect to other molecular species seems to diminish systematically from dilute H I clouds to the densest molecular clouds.