Excitation of OH toward interstellar dust clouds

Abstract

Observations of the 18 cm OH lines have been performed toward and just off the continuum sources W40 and 3C 123. W40 is an H II region, and 3C 123 is an extragalactic radio source, both of which lie beyond optically detectable dust clouds. The excitation temperature and optical depth of each of the four transitions have been derived from these data as functions of radial velocity, without assuming thermalization of the main lines. Toward both clouds each of the two satellite lines is inverted in one wing of the line profile and anti-inverted in the other wing. Pumping by infrared photons with overlapping of the infrared lines of OH probably explains the satellite-line data. Definite departures from thermal equilibrium are found for the OH main lines toward both clouds. Such departures could lead to large errors in estimates of OH column densities if thermalization is assumed.CO observations of both clouds have also been performed. These data suggest that the W40 H II region is embedded in the far side of the molecular cloud and is heating the dust near it. Although a small collapse velocity or perhaps compression of the cloud by the H II region is present, themore » dominant kinematics must be turbulence. No evidence for small-scale clumping of the dust clouds was found.« less