Hydroxyl emission in translucent molecular clouds

Abstract

Observations of the 18 cm ground-state hyperfine transitions of OH in translucent molecular clouds at high Galactic latitudes are presented. Fifteen lines of sight in nine high-latitude clouds were observed with the Arecibo radio telescope. In all instances, the column density obtained from the 1665 and 1667 MHz transitions is less than 2.0 x 10 to the 14th/sq cm. If the visual extinction and gas-to-dust ratio along the observed lines of sight are typical of translucent clouds, then the OH abundance with respect to H is of order 10 to the -8th to 10 to the -7th. This value is normal for low-extinction dust clouds and contradicts an earlier study of the OH abundance in high-latitude clouds. Without an enhanced OH molecular abundance, the case for a shock-front origin for the high-latitude molecular clouds is weakened. 33 refs.