Influence of radiative pumping on the HD rotational level populations in diffuse molecular clouds of the interstellar medium

TL;DR

This paper models how ultraviolet radiative pumping influences HD molecule rotational levels in diffuse interstellar clouds, highlighting its significance over collisions in certain conditions and its potential impact on primordial gas cooling.

Contribution

It provides a theoretical framework for understanding radiative pumping effects on HD excitation, incorporating self-shielding and UV field models, which was not comprehensively addressed before.

Findings

Radiative pumping dominates HD excitation at low gas pressures and column densities.

HD rotational levels are more sensitive indicators of UV radiation than atomic carbon levels.

Radiative pumping may enhance HD cooling in primordial gas influenced by first stars.

Abstract

We present a theoretical calculation of the influence of ultraviolet radiative pumping on the excitation of the rotational levels of the ground vibrational state for HD molecules under conditions of the cold diffuse interstellar medium (ISM). Two main excitation mechanisms have been taken into account in our analysis: (i) collisions with atoms and molecules and (ii) radiative pumping by the interstellar ultraviolet (UV) radiation field. The calculation of the radiative pumping rate coefficients $\Gamma_{\rm ij}$ corresponding to Drane's model of the field of interstellar UV radiation, taking into account the self-shielding of HD molecules, is performed. We found that the population of the first HD rotational level ($J = 1$) is determined mainly by radiative pumping rather than by collisions if the thermal gas pressure $p_{\rm th}\le10^4\left(\frac{I_{\rm{UV}}}{1}\right)\,\mbox{K\,cm}^{-3}$ and the column density of HD is lower than $\log N({\rm{HD}})<15$. Under this constraint the populations of rotational levels of HD turns out to be as well a more sensitive indicator of the UV radiation intensity than the fine-structure levels of atomic carbon. We suggest that taking into account radiative pumping of HD rotational levels may be important for the problem of the cooling of primordial gas at high redshift: ultraviolet radiation from first stars can increase the rate of HD cooling of the primordial gas in the early Universe.