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

TL;DR

This paper models the vibrational kinetics of H₂ and H₂⁺ in post-shock primordial gas, revealing detailed non-equilibrium effects on cooling processes relevant to early universe structure formation.

Contribution

It provides the first detailed vibrationally resolved kinetic model for H₂ and H₂⁺ in post-shock primordial gas, improving understanding of cooling and chemical evolution.

Findings

Non-equilibrium vibrational distributions significantly affect cooling rates.

Cooling times depend on initial shock velocity and redshift.

Enhanced accuracy in modeling early universe gas cooling.

Abstract

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