Three-fluid plasmas in star formation II. Momentum transfer rate coefficients

TL;DR

This paper provides comprehensive and accurate momentum transfer rate coefficients for multi-component interstellar plasmas, crucial for magnetohydrodynamical modeling of star formation regions, improving upon previous approximations.

Contribution

It derives and fits new momentum transfer rate coefficients from experimental and theoretical data for a wide range of conditions, replacing less accurate approximations.

Findings

Accurate numerical momentum transfer rates are provided for various species.

The polarization approximation is only valid for specific H2 and molecular ion collisions.

New analytical formulas are proposed for better modeling of interstellar medium dynamics.

Abstract

The charged component of the insterstellar medium consists of atomic and molecular ions, electrons, and charged dust grains, coupled to the local Galactic magnetic field. Collisions between neutral particles (mostly atomic or molecular hydrogen) and charged species, and between the charged species themselves, affect the magnetohydrodynamical behaviour of the medium and the dissipation of electric currents. The friction force due to elastic collisions between particles of different species in the multi-component interstellar plasma is a nonlinear function of the temperature of each species and the Mach number of the relative drift velocity. The aim of this paper is to provide an accurate and, as far as possible, complete set of momentum transfer rate coefficients for magnetohydrodynamical studies of the interstellar medium. Momentum transfer rates are derived from available experimental data and theoretical calculations of cross sections within the classic approach developed by Boltzmann and Langevin for a wide range of values of the temperature and the drift velocity. Accurate numerical values for momentum transfer rates are obtained and fitted to simple analytical formulae expressing the dependence of the results on the gas temperature and the relative drift velocity. The often used polarization approximation is in satisfactory agreement with our results only for collisions between H2and molecular ions (HCO+, H3+). For other kinds of collisions, the polarization approximation fails by large factors, and must be replaced by more accurate expressions.