Detection and significance of the interstellar OH line lambda 3078.

The interstellar OH line near 3078 A has been observed, and the first accurate abundance of OH in diffuse clouds has been obtained. The measured equivalent widths and column densities are W/sub lambda/=3.5 +- 1.1 mA and log N (OH) =14.0 for o Per and W/sub lambda/=1.1 +- 0.7 mA and log N (OH) =13.5 for zeta Oph. Based on the known oscillator strength of OH lambda3078 and the probable detection of OH lambda1222.07 toward o Per with the Copernicus satellite, the oscillator strength of the latter transition is 0.004. In current ''ion-molecule'' schemes for gas reactions in diffuse interstellar clouds, OH is a product of the neutralization of hydrogen and is thus of particular interest. Assuming that the OH is produced by these processes, we have derived the rate coefficient for the H/sup +/+O charge-transfer from the astronomical data in a manner that is relatively insensitive to uncertainties in the physical conditions. (AIP)