The Rotational Excitation of OH in Orion

Abstract

The Orion–KL region has been studied extensively by observations of lines originating from highly–excited rotational levels of CO (see Watson et al. 1985). The lines arise in the shocked gas that lies at the interface of the hypersonic mass outflow of a newly–formed star and the quiescent material in Orion. The CO line intensities are sensitive to the physical conditions in the shocked region, and have been interpreted with a kinetic temperature T ≃ 600 K and density n (H 2 )≃ 2 × 10 6 cm −3 . Recently, pure rotational lines of OH have been detected as well in the far-infrared spectrum of Orion. Since the OH and CO molecules differ substantially in their energy level structure and in other properties, it is of interest to investigate whether the OH excitation probes the same physical conditions as the CO excitation or whether the two molecules reside in different parts of the shock.