State-to-state vibrational kinetics of H₂and H$_2^+$ in a post-shock cooling gas with primordial composition

Abstract

The radiative cooling of shocked gas with primordial chemical composition is\nan important process relevant to the formation of the first stars and\nstructures, as well as taking place also in high velocity cloud collisions and\nsupernovae explosions. Among the different processes that need to be\nconsidered, the formation kinetics and cooling of molecular hydrogen are of\nprime interest, since they provide the only way to lower the gas temperature to\nvalues well below $\\sim$10$^4$~K. In previous works, the internal energy level\nstructure of H$_2$ and its cation has been treated in the approximation of\nrovibrational ground state at low densities, or trying to describe the dynamics\nusing some arbitrary $v>0$ H$_2$ level that is considered representative of the\nexcited vibrational manifold. In this study, we compute the vibrationally\nresolved kinetics for the time-dependent chemical and thermal evolution of the\npost-shock gas in a medium of primordial composition. The calculated\nnon-equilibrium distributions are used to evaluate effects on the cooling\nfunction of the gas and on the cooling time. Finally, we discuss the dependence\nof the results to different initial values of the shock velocity and redshift.\n