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

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 $\zeta \ge 10^{-16}$ 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 $\zeta \ge 10^{-15}$ 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.

arXiv comment10 pages, 4 figures, accepted for publication in MNRAS

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