Orbital Evolution of Compact White Dwarf Binaries

TL;DR

This paper studies the orbital evolution of compact white dwarf binaries, focusing on extremely low mass helium WDs, their mass transfer behavior, and implications for AM CVn systems, revealing longer inspiral times and larger stable donor mass ranges.

Contribution

It confirms the long-term inspiral period of 1 million years with H-rich mass transfer and highlights the potential for stable mass transfer in a broader range of ELM WD donors.

Findings

ELM WDs can have stable mass transfer over a wider donor mass range.

The inspiral period of 1 million years is confirmed for certain WD binaries.

ELM WDs in AM CVn systems have larger radii due to higher entropy.

Abstract

The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich matter and highlight that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H burning stage.