He-accreting carbon-oxygen white dwarfs and type Ia supernovae

TL;DR

This study uses stellar evolution simulations to explore how helium accretion onto white dwarfs influences whether they produce type Ia supernovae or undergo collapse, revealing critical accretion rates that determine the outcome.

Contribution

It identifies the critical accretion rate for off-centre carbon ignition and revises the initial parameter space for SNe Ia in the He star donor channel.

Findings

Off-centre carbon ignition occurs above a critical accretion rate.

The parameter space for SNe Ia is smaller than previously thought.

Chemical profiles at ignition are provided for explosion modeling.

Abstract

He accretion onto carbon-oxygen white dwarfs (CO WDs) plays a fundamental role when studying the formation of type Ia supernovae (SNe Ia). Employing the MESA stellar evolution code, we calculated the long-term evolution of He-accreting CO WDs. Previous studies usually supposed that a WD can grow in mass to the Chandrasekhar limit in the stable He burning region and finally produce a SN Ia. However, in this study we find that off-centre carbon ignition occurs in the stable He burning region if the accretion rate is above a critical value (~2.05*10^{-6}{M$_\odot$}{yr}^-1), resulting in accretion-induced collapse rather than a SN Ia. If the accretion rate is below the critical value, explosive carbon ignition will eventually happen in the centre producing a SN Ia. Taking into account the possibility of off-centre carbon ignition, we have re-determined the initial parameter space that produces SNe Ia in the He star donor channel, one of the promising channels to produce SNe Ia in young populations. Since this parameter space is smaller than was found in the previous study of Wang et al. (2009), the SN Ia rates are also correspondingly smaller. We also determined the chemical abundance profile of the He-accreting WDs at the moment of explosive carbon ignition, which can be used as initial input for SN Ia explosion models.