Observations of interstellar CH and a study of its chemistry and excitation

The 140-foot telescope of the National Radio Astronomy Observatory was used to observe the three Adoublet transitions in the 211112, J = il state of interstellar CH. CH is detected mainly in optically dark nebulae and other, more distant, regions that are probably similar types of clouds. However, CH is not observed in many other dark clouds and globules, and is also not abundant in the very dense clouds associated with H II regions. Thus, the CH!(H + H2) abundance ratio varies in molecular clouds in the galaxy from 10-8 to less than 10- . CH is much less abundant than OH, even in regions of apparently low extinction. It is also less abundant than H2CO except in regions of low extinction and in some dark nebulae. Improved rest frequencies for the three CH hyperfine components are obtained. These are discussed in terms of the structure of the molecule. The three hyperfine lines are always observed in emission with relative intensities often far from those expected in local thermodynamic equilibrium. The CH ground state A-doublet population is generally inverted in the Galaxy, probably via special collisions with hydrogen molecules. The nonequilibrium hyperfine ratios probably result from radiative processes. The formation rate of CH is inadequate to explain the anomalous excitation in terms of selection of particular states during formation. Comparison is made between observed abundances and those predicted by some ion-molecule reaction schemes. Subject heading: molecules, interstellar