Chlorine in Dense Interstellar Clouds: the Abundance

We report the first detection of a chlorine-bearing molecular species in the interstellar medium via emission from the J = 1----> 0 transition of HCl at 625.9 GHz toward OMC-1. The relative strengths, widths, and velocities of the resolved hyperfine components are consistent with moderate optical depth emission originating from dense, quiescent molecular cloud material (VLsR = 9 km s1). The overall emission strength implies a fractional abundance off(HCl/H2) "'(0.5-5.0) x 108 , depending on the density of the emitting region. This is approximately an order of magnitude below previous theoretical estimates and a factor of 3-30 below the cosmic abundance of Cl. Recent laboratory work suggests that the lowered fractional abundance of HCl is caused by a combination of depletion onto grains with gas-phase loss processes such as the reaction of HCl with c+. Subject headings: interstellar: molecules molecular processes