On the formation of over-ionized plasma in evolved supernova remnants

TL;DR

This study introduces a new simulation framework to model the ionization evolution in supernova remnants, explaining the formation of over-ionized plasma through various environmental scenarios and matching observations.

Contribution

Developed a comprehensive 1D hydrodynamics and non-equilibrium ionization simulation platform for SNRs, exploring environmental effects on over-ionized plasma formation.

Findings

Recombining plasma can form in young SNRs expanding in low-density winds.

Old SNRs in dense ISM can also develop over-ionized plasma.

Model results align well with most observed evolved SNRs.

Abstract

One of the outstanding mysteries surrounding the rich diversity found in supernova remnants (SNRs) is the recent discovery of over-ionized or recombining plasma from a number of dynamically evolved objects. To help decipher its formation mechanism, we have developed a new simulation framework capable of modeling the time evolution of the ionization state of the plasma in an SNR. The platform is based on a one-dimensional hydrodynamics code coupled to a fully time-dependent non-equilibrium ionization calculation, accompanied by a spectral synthesis code to generate space-resolved broadband X-ray spectra for SNRs at arbitrary ages. We perform a comprehensive parametric survey to investigate the effects of different circumstellar environments on the ionization state evolution in SNRs up to a few 10,000 years. A two-dimensional parameter space, spanned by arrays of interstellar medium (ISM) densities and mass-loss rates of the progenitor, is used to create a grid of models for the surrounding environment, in which a core-collapse explosion is triggered. Our results show that a recombining plasma can be successfully reproduced in the case of a young SNR (a few 100 to 1,000 years old) expanding fast in a spatially extended low-density wind, an old SNR (> a few 1,000 years) expanding in a dense ISM, or an old SNR broken out from a confined dense wind region into a tenuous ISM. Finally, our models are confronted with observations of evolved SNRs, and an overall good agreement is found except for a couple of outliers.