Pre-Explosion Properties of Helium Star Donors to Thermonuclear Supernovae

TL;DR

This study refines models of helium star and white dwarf binaries as progenitors of thermonuclear supernovae, characterizing pre-explosion properties and implications for observations, especially in relation to SN 2012Z.

Contribution

It updates binary evolution predictions using MESA, explores the properties of helium donors at explosion, and discusses observational signatures and constraints for supernova progenitors.

Findings

Systems with initial CO WD masses up to 1.2 M_sun undergo off-center carbon ignition.

Pre-explosion magnitudes can be modeled for comparison with observations.

High-mass WD systems may not produce typical thermonuclear supernovae.

Abstract

Helium star - carbon-oxygen white dwarf (CO WD) binaries are potential single-degenerate progenitor systems of thermonuclear supernovae. Revisiting a set of binary evolution calculations using the stellar evolution code $\texttt{MESA}$, we refine our previous predictions about which systems can lead to a thermonuclear supernova and then characterize the properties of the helium star donor at the time of explosion. We convert these model properties to NUV/optical magnitudes assuming a blackbody spectrum and support this approach using a matched stellar atmosphere model. These models will be valuable to compare with pre-explosion imaging for future supernovae, though we emphasize the observational difficulty of detecting extremely blue companions. The pre-explosion source detected in association with SN 2012Z has been interpreted as a helium star binary containing an initially ultra-massive WD in a multi-day orbit. However, extending our binary models to initial CO WD masses of up to $1.2\,M_{\odot}$, we find that these systems undergo off-center carbon ignitions and thus are not expected to produce thermonuclear supernovae. This tension suggests that, if SN 2012Z is associated with a helium star - WD binary, then the pre-explosion optical light from the system must be significantly modified by the binary environment and/or the WD does not have a carbon-rich interior composition.