WARM NEUTRAL HALOS AROUND MOLECULAR CLOUDS .5. OH (1665 AND 1667 MHZ)OBSERVATIONS

We have made 10 strip maps of 1665 and 1667 MHz OH emission, traversin g the outer boundaries of five molecular clouds. The OH emission is fo und to be significantly extended relative to CO, implying that OH is t o be found in abundance in the partly atomic, partly molecular gas sur rounding dense neutral clouds. The fractional OH abundance is calculat ed using existing H I and CO observations, and detailed source models which include a complete chemistry network and a radiative transfer co de. The largest OH abundances are found outside of the CO clouds, espe cially in those regions thought to be suffused by intense UV radiation . We conclude that the extended OH is formed, not by the exothermic re action of O with H-3+, but by the endothermic reaction, H+ + O --> H O+. The increased importance of the endothermic channel, relative to that assumed in earlier work, results from: (1) larger temperatures an d (2) a lower efficiency of H-2 production, resulting in a larger H I column density. This is consistent with, and is in fact predicted by, earlier models based on H I and CO observations alone.