The Astrophysical Journal · 1973 · 22 citations · 0 references
Observations of the four 18-cm lines of OH have been made at 20 positions in 11 dark dust clouds. They show that anomalous excitation of the two satellite lines occurs in almost every case, and that anomalies also exist in the main lines in 7 cases. Collisions of H or H2 with OH are shown to pump the 18-cm A-doublet even at the low temperatures encountered in dust clouds. Ultraviolet pumping may also produce these effects in principle, provided suitable sources of ultraviolet radiation exist. The satellite-line anomalies are explained in terms of pumping by infrared radiation. Such radiation can be released by the primary pumping process for the A-doublet, or by other processes (electron or noninverting H2 collisions) that excite the OH to the 11312, j = 5/2 levels; it can also be generated by infrared stars within the dust clouds. Subject headings: interstellar matter - masers - molecules, interstellar - radio lines