The Accretion of Solar Material onto White Dwarfs: No Mixing with Core Material Implies that the Mass of the White Dwarf is Increasing

TL;DR

This study shows that when accreted material does not mix with a white dwarf's core, the white dwarf can grow in mass, potentially leading to a Type Ia supernova, challenging previous assumptions about nova explosions.

Contribution

It introduces new calculations demonstrating that non-mixing accretion allows white dwarfs to gain mass, unlike previous models assuming mixing which led to mass loss.

Findings

White dwarfs can grow in mass without mixing of accreted material.

Mass ejected during nova events is less than the accreted mass.

White dwarfs may reach the Chandrasekhar limit, enabling supernova explosions.

Abstract

Cataclysmic Variables (CVs) are close binary star systems with one component an accreting white dwarf (WD) and the other a larger cooler star that fills its Roche Lobe. One consequence of the WDs accreting material, is the possibility that they are growing in mass and will eventually reach the Chandrasekhar Limit. This evolution could result in a Supernova Ia (SN Ia) explosion and is designated the Single Degenerate Progenitor (SD) scenario. One problem with the single degenerate scenario is that it is generally assumed that the accreting material mixes with WD core material at some time during the accretion phase of evolution and, since the typical WD has a carbon-oxygen (CO) core, the mixing results in large amounts of carbon and oxygen being brought up into the accreted layers. The presence of enriched carbon causes enhanced nuclear fusion and a Classical Nova (CN)explosion. Thus, the WD in a Classical Nova system is decreasing in mass and cannot be a SN Ia progenitor. In new calculations reported here, the consequences to the WD of no mixing of accreted material with core material have been investigated and the material involved in the explosion has only a Solar composition. I find that once sufficient material has been accreted, nuclear burning continues until a thermonuclear runaway (TNR) occurs and the WD either ejects a small amount of material or its radius grows to about $10^{12}$ cm and the calculations are stopped. In all cases where mass ejection occurs, the mass of the ejecta is far less than the mass of the accreted material. Therefore, all the WDs are growing in mass. (Abridged)