The Initiation and Propagation of Helium Detonations in White Dwarf Envelopes

TL;DR

This study investigates how including C/O pollution and a full nuclear reaction network affects helium detonation ignition in white dwarf envelopes, revealing that detonations are more feasible than previously thought and could explain certain supernova features.

Contribution

It introduces the effects of C/O pollution and a comprehensive reaction network into helium detonation models, showing these factors significantly lower the ignition threshold.

Findings

Decreased minimum hotspot size for helium detonation ignition.

Lower minimum shell mass needed for detonation propagation.

Shell ashes mainly composed of silicon, calcium, and unburned helium.

Abstract

Detonations in helium-rich envelopes surrounding white dwarfs have garnered attention as triggers of faint thermonuclear ".Ia" supernovae and double detonation Type Ia supernovae. However, recent studies have found that the minimum size of a hotspot that can lead to a helium detonation is comparable to, or even larger than, the white dwarf's pressure scale height, casting doubt on the successful ignition of helium detonations in these systems. In this paper, we examine the previously neglected effects of C/O pollution and a full nuclear reaction network, and we consider hotspots with spatially constant pressure in addition to constant density hotspots. We find that the inclusion of these effects significantly decreases the minimum hotspot size for helium-rich detonation ignition, making detonations far more plausible during turbulent shell convection or during double white dwarf mergers. The increase in burning rate also decreases the minimum shell mass in which a helium detonation can successfully propagate and alters the composition of the shell's burning products. The ashes of these low mass shells consist primarily of silicon, calcium, and unburned helium and metals and may explain the high-velocity spectral features observed in most Type Ia supernovae.