# Using late-time optical and near-infrared spectra to constrain Type Ia supernova explosion properties

## TL;DR

This study analyzes late-time optical and near-infrared spectra of seven Type Ia supernovae to investigate their explosion physics, focusing on emission line ratios, velocities, and asymmetries to compare with explosion models.

## Contribution

It provides new insights into the explosion mechanisms of SNe Ia by analyzing spectral line ratios and velocities at late times, supporting specific explosion models and ejecta geometries.

## Key findings

- The Ni/Fe ratio aligns with Chandrasekhar-mass and sub-Chandrasekhar models.
- [Co III] lines are more centrally located and broader than [Fe II] and [Ni II].
- Higher Si II velocities correlate with blueshifted [Fe II] lines.

## Abstract

The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes of the underlying physics of their explosions. We investigate the late-time optical and near-infrared spectra of seven SNe Ia obtained at the VLT with XShooter at $>$200 d after explosion. At these epochs, the inner Fe-rich ejecta can be studied. We use a line-fitting analysis to determine the relative line fluxes, velocity shifts, and line widths of prominent features contributing to the spectra ([Fe II], [Ni II], and [Co III]). By focussing on [Fe II] and [Ni II] emission lines in the ~7000-7500 \AA\ region of the spectrum, we find that the ratio of stable [Ni II] to mainly radioactively-produced [Fe II] for most SNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonation explosion models, as well as sub-Chandrasekhar mass explosions that have metallicity values above solar. The mean measured Ni/Fe abundance of our sample is consistent with the solar value. The more highly ionised [Co III] emission lines are found to be more centrally located in the ejecta and have broader lines than the [Fe II] and [Ni II] features. Our analysis also strengthens previous results that SNe Ia with higher Si II velocities at maximum light preferentially display blueshifted [Fe II] 7155 \AA\ lines at late times. Our combined results lead us to speculate that the majority of normal SN Ia explosions produce ejecta distributions that deviate significantly from spherical symmetry.