The Response of Giant Stars To Dynamical-Timescale Mass Loss

TL;DR

This study uses simulations to show that giant stars do not significantly expand during rapid mass loss events, challenging previous assumptions based on adiabatic models.

Contribution

The paper demonstrates that during dynamical-timescale mass loss, giant stars do not undergo significant radius expansion, highlighting the importance of thermal and dynamical readjustments.

Findings

Giant stars show minimal radius increase during rapid mass loss.

Outer stellar layers readjust thermally on a timescale comparable to dynamical times.

Conditions for unstable mass transfer may need re-evaluation based on these results.

Abstract

We study the response of giant stars to mass loss. One-dimensional simulations of red and asymptotic giant branch stars with mass loss rates from $10^{-3}$ up to a few \msun/yr show in no case any significant radius increase. The largest radius increase of 0.2% was found in the case with the lowest mass loss rate. For dynamical-timescale mass loss rates, that may be encountered during a common envelope phase, the evolution is not adiabatic. The superadiabatic outer layer of the giant's envelope has a local thermal timescale comparable to the dynamical timescale. Therefore, this layer has enough time to readjust thermally. Moreover, the giant star is driven out of hydrostatic equilibrium and evolves dynamically. In these cases no increase of the stellar radius with respect to its initial value is found. If the mass loss rate is high enough, the superadiabaticity of the outer layer is lost progressively and a radiative zone forms due to a combination of thermal and dynamical readjustment. Conditions for unstable mass transfer based on adiabatic mass loss models that predict a significant radius increase, may need to be re-evaluated.