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.

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.