Implications for the Explosion Mechanism of Type Ia Supernovae from their Late-time Spectra

TL;DR

This study analyzes late-time spectra of 36 Type Ia supernovae to estimate their Ni to Fe ratios, revealing that most are consistent with sub-Chandrasekhar-mass explosion models and suggesting progenitor metallicity influences spectral velocity diversity.

Contribution

It provides the first comprehensive analysis of late-time spectra for a sizable sample of SNe Ia, linking spectral features to explosion models and progenitor metallicity effects.

Findings

67% of SNe Ia are consistent with sub-Chandrasekhar-mass models.

Ni to Fe ratio correlates with Si II velocity, indicating metallicity influence.

Late-time spectra reveal inner ejecta composition and explosion physics.

Abstract

Late-time spectra of Type Ia supernovae (SNe Ia) are important in clarifying the physics of their explosions, as they provide key clues to the inner structure of the exploding white dwarfs. We examined late-time optical spectra of 36 SNe Ia, including five from our own project (SNe 2019np, 2019ein, 2021hpr, 2021wuf, and 2022hrs), with phase coverage of $\sim 200$ to $\sim 400$ days after maximum light. At this late phase, the outer ejecta have become transparent and the features of inner iron-group elements emerge in the spectra. Based on multicomponent Gaussian fits and reasonable choices for the pseudocontinuum around Ni and Fe emission features, we get reliable estimates of the Ni to Fe ratio, which is sensitive to the explosion models of SNe Ia. Our results show that the majority (about 67%) of our SNe Ia are more consistent with the sub-Chandrasekhar-mass (i.e., double-detonation) model, although they could be affected by evolutionary or ionisation effects. Moreover, we find that the Si II $\lambda$6355 velocity measured around the time of maximum light tends to increase with the Ni to Fe ratio for the subsample with either redshifted or blueshifted nebular velocities, suggesting that progenitor metallicity might play an important role in accounting for the observed velocity diversity of SNe Ia.