Post-common envelope binary systems experiencing helium-shell driven stable mass transfer

TL;DR

This study models the evolution of binary star systems through a common envelope phase, revealing that helium-shell burning can lead to additional mass transfer and the formation of a white dwarf without fine-tuning initial conditions.

Contribution

It demonstrates that helium-shell driven stable mass transfer can occur naturally after a common envelope phase, influencing the final binary configuration.

Findings

Post-CE RLOF allows helium-shell burning and further mass transfer.

The final remnant is a carbon-oxygen white dwarf of about 0.8 solar masses.

Double mass transfer can strip hydrogen and helium layers without fine tuning.

Abstract

We evolve stellar models to study the common envelope (CE) interaction of an early asymptotic giant branch star of initial mass $5\,\rm M_{\odot}$ with a companion star of mass ranging from $0.1$ to $2\,\rm M_{\odot}$. We model the CE as a fast stripping phase in which the primary experiences rapid mass loss and loses about 80 per cent of its mass. The post-CE remnant is then allowed to thermally readjust during a Roche-lobe overflow (RLOF) phase and the final binary system and its orbital period are investigated. We find that the post-CE RLOF phase is long enough to allow nuclear burning to proceed in the helium shell. By the end of this phase, the donor is stripped of both its hydrogen and helium and ends up as carbon-oxygen white dwarf of mass about $0.8\,\rm M_{\odot}$. We study the sensitivity of our results to initial conditions of different companion masses and orbital separations at which the stripping phase begins. We find that the companion mass affects the final binary separation and that helium-shell burning causes the star to refill its Roche lobe leading to post-CE RLOF. Our results show that double mass transfer in such a binary interaction is able to strip the helium and hydrogen layers from the donor star without the need for any special conditions or fine tuning of the binary parameters.