A multi-ion non-equilibrium solver for ionised astrophysical plasmas with arbitrary elemental abundances

¹ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin 15, Ireland
² Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 0HA, UK
³ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁴ Department of Astronomy and Oskar Klein Centre, AlbaNova, Stockholm University, SE-10691 Stockholm, Sweden

^⋆ Corresponding author; arun@cp.dias.ie

Received: 26 September 2024
Accepted: 31 January 2025

Abstract

Context. While many astrophysical plasmas can be modelled successfully assuming ionisation and thermal equilibrium, in some cases this is not appropriate and a non-equilibrium approach is required. In nebulae around evolved stars, the local elemental abundances may also strongly vary in space and time.

Aims. Here we present a non-equilibrium multi-ion module developed for the fluid-dynamics code PION, describing the physical processes included and demonstrating its capabilities with some test calculations.

Methods. A non-equilibrium ionisation solver is developed that allows arbitrary elemental abundances for neutral and ionised (but not molecular) gas, for the elements H, He, C, N, O, Ne, Si, S, and Fe. Collisional ionisation and recombination, photoionisation and charge-exchange reactions are included, and ion-by-ion non-equilibrium radiative cooling is calculated based on the instantaneous ion fractions of each element. Element and ion mass-fractions are advected using passive scalars, operator-split from the microphysical processes.

Results. The module is validated by comparing with equilibrium and non-equilibrium calculations in the literature. Effects of charge exchange on ion abundances in cooling plasmas are discussed. Application to modelling shocks and photo-ionised H II regions is demonstrated. The time-dependent expansion of a WR nebula is studied, including photoionisation and collisional processes, and spectral-line luminosities calculated for non-equilibrium and equilibrium plasma states.

Conclusions. The multi-ion module enables simulation of ionised plasmas with spatially varying elemental abundances using self-consistent ion abundances and thermal evolution. This allows prediction of spectral lines in UV, optical, IR, and X-ray even in cases where the plasma is out of ionisation equilibrium.