Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs: polarisation signatures

TL;DR

This study examines the polarization signatures of a violent white dwarf merger model for Type Ia supernovae, finding modest polarization in some views but challenges in matching the low polarization observed in typical events.

Contribution

It provides the first detailed spectropolarimetric analysis of a specific white dwarf merger model, highlighting its potential and limitations in explaining observed supernova polarization.

Findings

Polarization varies with viewing angle, being modest in the equatorial plane.

The model matches highly polarized supernovae like SN 2004dt.

It struggles to reproduce the low polarization levels of normal Type Ia supernovae.

Abstract

The violent merger of two carbon-oxygen white dwarfs has been proposed as a viable progenitor for some Type Ia supernovae. However, it has been argued that the strong ejecta asymmetries produced by this model might be inconsistent with the low degree of polarisation typically observed in Type Ia supernova explosions. Here, we test this claim by carrying out a spectropolarimetric analysis for the model proposed by Pakmor et al. (2012) for an explosion triggered during the merger of a 1.1 M$_{\odot}$ and 0.9 M$_{\odot}$ carbon-oxygen white dwarf binary system. Owing to the asymmetries of the ejecta, the polarisation signal varies significantly with viewing angle. We find that polarisation levels for observers in the equatorial plane are modest ($\lesssim$ 1 per cent) and show clear evidence for a dominant axis, as a consequence of the ejecta symmetry about the orbital plane. In contrast, orientations out of the plane are associated with higher degrees of polarisation and departures from a dominant axis. While the particular model studied here gives a good match to highly-polarised events such as SN 2004dt, it has difficulties in reproducing the low polarisation levels commonly observed in normal Type Ia supernovae. Specifically, we find that significant asymmetries in the element distribution result in a wealth of strong polarisation features that are not observed in the majority of currently available spectropolarimetric data of Type Ia supernovae. Future studies will map out the parameter space of the merger scenario to investigate if alternative models can provide better agreement with observations.