Ionisation by non-thermal particles in thermonuclear supernovae

Ionisation by non-thermal particles in thermonuclear supernovae

Luke Shingles, Stuart Sim, Christine Collins
Astrophysics Research Centre, Queen’s University Belfast, Northern Ireland

Type Ia supernovae (SNe Ia) originate from the thermonuclear explosions of white dwarf stars. Understanding these events is important to astronomy since their brightness means they can be used to study cosmological expansion, and they are important contributors to the chemical evolution of galaxies. Currently, however, our theoretical understanding of these events is limited and the mechanisms responsible for these events are hotly debated. Our DiRAC project involves using computer simulations of radiative transfer to predict the spectrum of light that would be emitted by theoretical models of these supernovae. By comparing these simulations to observations, we aim to better understand the physical conditions in these supernovae and ultimately understand their origins.

By 100 days after explosion, the ejecta of SNe Ia have expanded sufficiently that light can escape even from the inner most regions of the explosion. This makes it possible to study the most centrally located material, which is likely where the explosion initiated. Thus these “nebular” spectra place important observational constraints on candidate SNe Ia explosion scenarios.

The nebular spectra of SNe Ia consist mainly of emission lines from singly- and doubly-ionised Fe-group nuclei. However, theoretical models for many scenarios predict that non-thermal ionisation leads to multiply-ionised species whose recombination photons ionise and deplete Fe+, resulting in negligible [Fe II] emission.

In our project, we recently investigated the scope for how the treatment of non-thermal energy deposition in the simulations may reconcile over-ionised theoretical models with observations. To quantify the magnitude of additional heating processes that would be required to sufficiently reduce ionisation from fast leptons, we investigated increased rates of energy loss to collisions with free electrons. Our Figure shows the comparison of the resulting spectra from these studies to the observed nebular spectrum of a real Type Ia supernova. We find that the equivalent of as much as an eight-times increase to the plasma loss rate would be needed to reconcile the model simulations with observed spectra, which places an important constraint on the effective densities that must be reached in the ejecta. This work will soon be published (Shingles et al. 2022, submitted) and further studies might be able to distinguish between reductions in the non-thermal ionisation rates and increased recombination rates, such as by clumping (as suggested by Wilk et al. 2020).

This work also used the Cambridge Service for Data Driven Discovery (CSD3), part of which is operated by the University of Cambridge Research Computing on behalf of the STFC DiRAC HPC Facility (

Figure caption: An observed infrared spectrum of a Type Ia supernova at 247 days after explosion (SN 2013ct, Maguire et al. 2016), versus synthetic spectra calculated with several variations to the treatment of ionisation by high-energy leptons.


  • Maguire et al. 2016, MNRAS, 457, 3254
  • Shingles et al., 2020, MNRAS, 492, 2029
  • Wilk K. D., Hillier D. J., Dessart L., 2020, MNRAS, 494, 2221