A chemical solver to compute molecule and grain abundances and non-ideal MHD resistivities in prestellar core collapse calculations

TL;DR

This paper develops a comprehensive chemical solver for prestellar core collapse, calculating molecule and grain abundances to accurately determine non-ideal MHD resistivities, incorporating new physical processes and providing a community resource.

Contribution

It introduces a detailed chemical network including grain evaporation, thermal ionization, and thermionic emission, enabling more accurate modeling of non-ideal MHD effects in prestellar collapse.

Findings

Resistivities are dominated by ambipolar diffusion and Hall effect at low densities.

The chemical equilibrium timescale is shorter than the free-fall time.

A multi-dimensional equilibrium abundance table is created for use in collapse simulations.

Abstract

We develop a detailed chemical network relevant to the conditions characteristic of prestellar core collapse. We solve the system of time-dependent differential equations to calculate the equilibrium abundances of molecules and dust grains, with a size distribution given by size-bins for these latter. These abundances are used to compute the different non-ideal magneto-hydrodynamics resistivities (ambipolar, Ohmic and Hall), needed to carry out simulations of protostellar collapse. For the first time in this context, we take into account the evaporation of the grains, the thermal ionisation of Potassium, Sodium and Hydrogen at high temperature, and the thermionic emission of grains in the chemical network, and we explore the impact of various cosmic ray ionisation rates. All these processes significantly affect the non-ideal magneto-hydrodynamics resistivities, which will modify the dynamics of the collapse. Ambipolar diffusion and Hall effect dominate at low densities, up to n_H = 10^12 cm^-3, after which Ohmic diffusion takes over. We find that the time-scale needed to reach chemical equilibrium is always shorter than the typical dynamical (free fall) one. This allows us to build a large, multi-dimensional multi-species equilibrium abundance table over a large temperature, density and ionisation rate ranges. This table, which we make accessible to the community, is used during first and second prestellar core collapse calculations to compute the non-ideal magneto-hydrodynamics resistivities, yielding a consistent dynamical-chemical description of this process.