The Fate of a WD Accreting H-Rich Material at High Rates

TL;DR

This study investigates high-rate accretion onto white dwarfs, revealing that instead of steady burning, the system undergoes cyclic thermonuclear flashes that prevent net mass gain, challenging previous assumptions about their evolution.

Contribution

It provides the first long-term simulation showing that high-rate accretion leads to cyclic flashes and no net mass accumulation, contradicting earlier steady-state models.

Findings

Accretion causes cyclic thermonuclear runaways.

Large helium flashes expel the accreted envelope.

White dwarfs do not gain mass to trigger supernovae.

Abstract

We study C/O white dwarfs with masses of 1.0 to 1.4 Msun accreting solar-composition material at very high accretion rates. We address the secular changes in the WDs, and in particular, the question whether accretion and the thermonuclear runaways result is net accretion or erosion. The present calculation is unique in that it follows a large number of cycles, thus revealing the secular evolution of the WD system. We find that counter to previous studies, accretion does not give rise to steady state burning. Instead, it produces cyclic thermonuclear runaways of two types. During most of the evolution, many small cycles of hydrogen ignition and burning build a helium layer over the surface of the white dwarf. This He layer gradually thickens and progressively becomes more degenerate. Once a sufficient amount of He has accumulated, several very large helium burning flashes take place and expel the accreted envelope, leaving no net mass accumulation. The results imply that such a system will not undergo an accretion induced collapse, nor will it lead to a SN Type Ia, unless a major new physical process is found.