Slow Accretion of Helium Rich Matter onto C-O White Dwarf: Does Composition of WD Matter?

Abstract

Background: In binary systems, the helium accretion onto carbon-oxygen (CO) white dwarfs (WDs) plays a vital role in many astrophysical scenarios, especially in supernovae type Ia. Moreover, ignition density for accretion rate M ≲ 10-9 M yr-1 in helium accreting CO white dwarfs decides the triggering mechanism of a supernova explosion which could be either off-centre helium flash or central carbon flash. Objectives: We aim to study the accretion of helium with a slow accretion rate 510-10 M yr-1 onto relatively cool and hot white dwarfs of different abundances of carbon and oxygen. Methods: The simulation code “Modules for Experiments in Stellar Astrophysics” (MESA) has been used for our study. We analyze the variation in several properties like surface gravity (g), helium luminosity (LHe), and effective temperature (Te f f ) during the accretion phase of the white dwarfs. We also calculate the ignition density (rHe) and ignition temperature (THe) of helium burning. Findings: As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Novelty: This is the first-ever study of the dependence of helium accreting WD evolution on its composition. We find that white dwarfs of the lower abundance of carbon accrete slightly longer before the onset of helium ignition. Keywords: Supernovae Ia; White Dwarfs; Accretion; Helium fusion; Stellar evolution