Supernovae from direct collisions of white dwarfs and the role of helium shell ignition

TL;DR

This paper investigates how helium shells on white dwarfs influence supernova explosions resulting from direct collisions, revealing that helium detonation can alter explosion outcomes and elemental yields, with implications for observations and galactic chemical enrichment.

Contribution

It extends previous models by analyzing the effects of low and high mass helium shells on white dwarf collision-induced supernovae, highlighting new explosion dynamics and elemental production mechanisms.

Findings

High mass helium shells cause early detonation affecting explosion evolution.

Low mass helium shells do not detonate but influence element synthesis.

Helium shells alter supernova kinematics and potential observational signatures.

Abstract

Models for supernovae (SNe) arising from thermonuclear explosions of white dwarfs (WDs) have been extensively studied over the last few decades, mostly focusing on the single degenerate (accretion of material of a WD) and double degenerate (WD-WD merger) scenarios. In recent years it was suggested that WD-WD direct collisions provide an additional channel for such explosions. Here we extend the studies of such explosions, and explore the role of Helium-shells in affecting the thermonuclear explosions. We study both the impact of low-mass helium ($\sim0.01$ M$_{\odot})$ shells, as well as high mass shells ($\ge0.1$ M$_{\odot}$). We find that detonation of the massive helium layers precede the detonation of the WD Carbon-Oxygen (CO) bulk during the collision and can change the explosive evolution and outcomes for the cases of high mass He-shells. In particular, the He-shell detonation propagates on the WD surface and inefficiently burns material prior to the CO detonation that later follows in the central parts of the WD. Such evolution leads to larger production of intermediate elements, producing large yields of $^{44}{\rm Ti}$ and $^{48}{\rm Cr}$ relative to the pure CO-CO WD collisions. Collisions of WDs with a low-mass He-shell do not give rise to helium detonation, but helium burning does precede the CO bulk detonation. Such collisions produce a high velocity, low-mass of ejected burned material enriched with intermediate elements, with smaller changes to the overall explosion outcomes. The various effects arising from the contribution of low/high mass He layers change the kinematics and the morphological structure of collision-induced SNe and may thereby provide unique observational signatures for such SNe, and play a role in the chemical enrichment of galaxies and the production of intermediate elements and positrons from their longer-term decay.