On the relationship of OH and formaldehyde with interstellar extinction

We have mapped the distribution of OH and of H2CO in two regions in Taurus where the photographic extinction varies from 2 to 7 magnitudes. A high degree of correlation is found between the extinction and both the excitation temperature of OH and the strength of absorption in the 6-cm line of H2CO. No correlation is found between OH abundance and extinction. From these observations, and similar ones by Knapp showing that the strength of self-absorption of atomic hydrogen is also correlated with extinction, we deduce that (a) OH abundance is independent of extinction; (b) the OH is probably subthermally excited in these regions, although this implies an ionization fraction <4 x 10-6; (c) the anomalous excitation of the 6-cm H2CO transition is probably collisional rather than radiative. To explain the independence of OH abundance and extinction, we conclude that OH formation probably occurs via ion-molecule reactions rather than by neutral gas phase reactions or by reactions on grain surfaces. Subject headings: interstellar matter - molecules, interstellar