Light Curves and Spectra from a Thermonuclear Explosion of a White Dwarf Merger

TL;DR

This paper models the light curves and spectra of white dwarf mergers, showing they can explain slowly declining Type Ia supernovae like SN 2001ay with less viewing angle dependence, advancing understanding of supernova mechanisms.

Contribution

It introduces a spiral instability-driven merger model that produces synthetic light curves and spectra aligning with observations of certain Type Ia supernovae, improving upon previous violent merger models.

Findings

Spiral merger models produce slowly declining light curves similar to SN 2001ay.

Light curves and spectra are less viewing-angle dependent than violent mergers.

The 56Ni distribution follows an hourglass shape affecting gamma-ray observations.

Abstract

Double-degenerate (DD) mergers of carbon-oxygen white dwarfs have recently emerged as a leading candidate for normal Type Ia supernovae (SNe Ia). However, many outstanding questions surround DD mergers, including the characteristics of their light curves and spectra. We have recently identified a spiral instability in the post-merger phase of DD mergers and demonstrated that this instability self-consistently leads to detonation in some cases. We call this the spiral merger SN Ia model. Here, we utilize the SuperNu radiative transfer software to calculate three-dimensional synthetic light curves and spectra of the spiral merger simulation with a system mass of 2.1 $M_\odot$ from Kashyap et al. Because of their large system masses, both violent and spiral merger light curves are slowly declining. The spiral merger resembles very slowly declining SNe Ia, including SN 2001ay, and provides a more natural explanation for its observed properties than other SN Ia explosion models. Previous synthetic light curves and spectra of violent DD mergers demonstrate a strong dependence on viewing angle, which is in conflict with observations. Here, we demonstrate that the light curves and spectra of the spiral merger are less sensitive to the viewing angle than violent mergers, in closer agreement with observation. We find that the spatial distribution of 56Ni and IMEs follows a characteristic hourglass shape. We discuss the implications of the asymmetric distribution of 56Ni for the early-time gamma-ray observations of 56Ni from SN 2014J. We suggest that DD mergers that agree with the light curves and spectra of normal SNe Ia will likely require a lower system mass.