Enhanced OH in C-Type Shock Waves in Molecular Clouds

Cosmic-ray and X-ray ionizations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically, a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50 K. The resulting column density of warm OH rises from 1015 to 1016 cm-2 as the ionization rate is increased from 10-17 s-1 (typical of dark clouds) to 10-15 s-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH and are consistent with the OH/H2O ratios inferred from Infrared Space Observatory observations of emission from molecular shocks. For high ionization rates, the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground-state transitions of OH at 18 cm and heterodyne measurements of emission in the far-infrared (e.g., from the Stratospheric Observatory for Infrared Astronomy).