C-type shock modelling – the effect of new H2–H collisional rate coefficients

Abstract

We consider the collisional excitation of H$_2$ molecule in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for the (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. \n \nAt cosmic ray ionization rates $\\zeta \\gtrsim 10^{-16}$~s$^{-1}$ and at moderate shock speeds, the H/H$_2$ ratio in the shock 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 $\\zeta \\gtrsim 10^{-15}$~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 in interpretation of observations of H$_2$ emission in supernova remnants.