Modelling the ionisation state of Type Ia supernovae in the nebular-phase

TL;DR

This study examines the ionisation states in nebular-phase Type Ia supernovae spectra, testing models against observations and exploring how non-thermal energy deposition affects ionisation predictions.

Contribution

It introduces a method to determine excitation conditions from [Fe II] line ratios and evaluates different treatments of non-thermal energy deposition to improve model-observation agreement.

Findings

Simple work function approximation aligns better with data than detailed Spencer-Fano treatment.

Significant additional heating, up to eight times the current energy loss rate, is needed to match observations.

Line ratios alone cannot reliably distinguish between different explosion scenarios.

Abstract

The nebular spectra of Type Ia supernovae ($\gtrapprox$ 100 days after explosion) 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. We investigate a method to determine the collisional excitation conditions from [Fe II] line ratios independently from the ionisation state and find that it cannot be applied to highly-ionised models due to the influence of recombination cascades on Fe$^{+}$ level populations. When the ionisation state is artificially lowered, the line ratios (and excitation conditions) are too similar to distinguish between explosion scenarios. We investigate changes to the treatment of non-thermal energy deposition as a way to reconcile over-ionised theoretical models with observations and find that a simple work function approximation provides closer agreement with the data for sub-Mch models than a detailed Spencer-Fano treatment with widely-used cross section data. To quantify the magnitude of additional heating processes that would be required to sufficiently reduce ionisation from fast leptons, we artificially boost the rate of energy loss to free electrons. We find that the equivalent of as much as an eight times increase to the plasma loss rate would be needed to reconcile the sub-Mch model with observed spectra. Future studies could distinguish between reductions in the non-thermal ionisation rates and increased recombination rates, such as by clumping.