C-type shock modelling -- the effect of new H$_2$-H collisional rate coefficients

TL;DR

This paper models C-type shocks in dense molecular clouds using new H$_2$-H collisional data, revealing how cosmic ray ionization influences H$_2$ excitation, ortho-to-para ratio, and emission features.

Contribution

It introduces state-of-the-art H$_2$-H collisional rate coefficients into shock models, improving understanding of molecular excitation and conversion processes in astrophysical shocks.

Findings

H/H$_2$ ratio is mainly determined by cosmic ray ionization rate.

H$_2$-H collisions dominate para-to-ortho conversion.

H$_2$ excitation and OPR depend strongly on cosmic ray ionization.

Abstract

We consider collisional excitation of H$_2$ molecules in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for (de-)excitation of H$_2$ molecule in collisions with H atoms and new H$_2$ dissociation rates are used. The new H$_2$-H collisional data are state of the art and are based on the most accurate H$_3$ potential energy surface. We re-examine the excitation of rotational levels of H$_2$ molecule, the para-to-ortho-H$_2$ conversion, and H$_2$ dissociation by H$_2$-H collisions. At cosmic ray ionization rates $\zeta \geq 10^{-16}$ s$^{-1}$ and at moderate shock speeds, the H/H$_2$ ratio at the shock front is mainly determined by the cosmic ray ionization rate. The H$_2$-H collisions play the main role in the para-to-ortho-H$_2$ conversion and, at $\zeta \geq 10^{-15}$ s$^{-1}$, in the excitation of vibrationally excited states of H$_2$ molecule in the shock. The H$_2$ ortho-to-para ratio (OPR) of the shocked gas and column densities of rotational levels of vibrationally excited states of H$_2$ are found to depend strongly on the cosmic ray ionization rate. We discuss the applicability of the presented results to interpretation of observations of H$_2$ emission in supernova remnants.