Monte Carlo radiative transfer for the nebular phase of Type Ia supernovae

TL;DR

This paper extends a 3D radiative transfer code to model the nebular phase of Type Ia supernovae, incorporating non-LTE effects, forbidden transitions, and non-thermal processes, validated through tests and applied to sub-Chandrasekhar models.

Contribution

It introduces a comprehensive non-LTE radiative transfer model for late-phase supernovae, including forbidden transitions and non-thermal effects, with validation and application to Ne22 settling scenarios.

Findings

Ne22 settling modestly affects nebular spectra.

High ionisation leads to negligible [Ni II] emission.

Models overproduce [Fe III] relative to observations.

Abstract

We extend the range of validity of the ARTIS 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium (non-LTE) population and ionisation solver, a new multi-frequency radiation field model, and a new atomic dataset with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionisation, and heating. We validate our method with a variety of tests including comparing our synthetic nebular spectra for the well-known one-dimensional W7 model with the results of other studies. As an illustrative application of the code, we present synthetic nebular spectra for the detonation of a sub-Chandrasekhar white dwarf in which the possible effects of gravitational settling of Ne22 prior to explosion have been explored. Specifically, we compare synthetic nebular spectra for a 1.06 M$_\odot$ white dwarf model obtained when 5.5 Gyr of very-efficient settling is assumed to a similar model without settling. We find that this degree of Ne22 settling has only a modest effect on the resulting nebular spectra due to increased Ni58 abundance. Due to the high ionisation in sub-Chandrasekhar models, the nebular [Ni II] emission remains negligible, while the [Ni III] line strengths are increased and the overall ionisation balance is slightly lowered in the model with Ne22 settling. In common with previous studies of sub-Chandrasekhar models at nebular epochs, these models overproduce [Fe III] emission relative to [Fe II] in comparison to observations of normal Type Ia supernovae.