Rotational excitation of interstellar OH by para- And ortho-H2

Abstract

In warm and dense interstellar clouds, the amount of ortho-H2 is comparable to that of para-H2. Yet most models of the excitation of interstellar molecules have considered only para-H2. We present here calculations of the rate coefficients for rotational excitation of OH by both para- and ortho-H2 in their ground states within the coupled-states formulation. The coupled states results are shown to reproduce satisfactorily the more accurate close-coupling calculations. The importance of inclusion of ortho-H2 (j = l) in the OH excitation is explored using statistical equilibrium models. In general, the presence of ortho-H2 reduces the asymmetry between the strengths of the two A-doubling components, and affects the results by factors of a few. The conditions under which the OH far-infrared lines will be observable with the Infrared Space Observatory are investigated.