Influence of Radiative Pumping on the HD Rotational Level Populations in Diffuse Molecular Clouds of the Interstellar Medium

Abstract

We have performed a theoretical calculation of the influence of radiative pumping on the populations of the rotational levels of the ground vibrational state for HD molecules under conditions of the cold phase of the interstellar medium. Two main excitation mechanisms have been taken into account in our analysis: (i) collisions with atoms and molecules of the interstellar medium and (ii) radiative pumping by the ultraviolet interstellar background. The radiative pumping rate coefficients $\Gamma_{ij}$ corresponding to the average galactic ultraviolet background in Draine’s model have been determined. The influence of HD self-shielding on the radiative pumping rate coefficients has been studied. The population of the first HD rotational level ($J=1$) is shown to be determined mainly by radiative pumping rather than by collisions if the thermal gas pressure $p_{\rm th}\leq 10^{4}\left(\frac{I_{\textrm{UV}}}{1}\right)\textrm{ K cm}^{-3}$ and the column density $\log N({\textrm{HD}})<15$. In such clouds the relative population of the HD levels $N(J=1)/N(J=0)$ turns out to be more sensitive to the ultraviolet background intensity than the relative population of the C I fine-structure levels. Thus, an analysis of the relative HD level population can become an important additional source of information about the physical conditions in the interstellar medium both in our Galaxy and in the forming galaxies of the early Universe.