The Unification of Asymmetry Signatures of Type Ia Supernovae

TL;DR

This study combines optical and IR spectroscopic and spectropolarimetric data to probe the 3D explosion geometry of Type Ia supernovae, revealing consistent asymmetry patterns and supporting an off-centre delayed detonation model.

Contribution

It uniquely integrates polarization and nebular spectral line profiles to comprehensively analyze the 3D structures of Type Ia supernovae, providing new insights into their explosion asymmetries.

Findings

Polarization correlates with velocity evolution, indicating asymmetry influence.

Weak correlation between polarization and nebular line velocities.

Evidence supports an off-centre delayed detonation explosion model.

Abstract

We present a compilation of the geometry measures acquired using optical and IR spectroscopy and optical spectropolarimetry to probe the explosion geometry of Type Ia SNe. Polarization measurements are sensitive to asymmetries in the plane of the sky, whereas line profiles in nebular phase spectra are expected to trace asymmetries perpendicular to the plane of the sky. The combination of these two measures can overcome their respective projection effects, completely probing the 3D structures of these events. For 9 normal Type Ia SNe, we find that the polarization of \ion{Si}{2} $\lambda 6355$ at 5 days before maximum ($p_{Si\,II}$) is well correlated with its velocity evolution ($\dot{\rm v}_{Si\,II}$), implying $\dot{\rm v}_{Si\,II}$ is predominantly due to the asymmetry of the SNe. We find only a weak correlation between the polarization of \ion{Si}{2} and the reported velocities (${\rm v}_{neb}$) for peak emission of optical \ion{Fe}{2} and \ion{Ni}{2} lines in nebular spectra. Our sample is biased, with polarization measurements being only available for normal SNe which subsequently exhibited positive (i.e. redshifted) ${\rm v}_{neb}$. In unison these indicators are consistent with an off-centre delayed detonation, in which the outer layers are dominated by a spherical oxygen layer, mixed with an asymmetric distribution of intermediate mass elements. The combination of spectroscopic and spectropolarimetric indicators suggests a single geometric configuration for normal Type Ia SNe, with some of the diversity of observed properties arising from orientation effects.