Adiabatic mass loss in binary stars. III. From the base of the red giant branch to the tip of asymptotic giant branch

TL;DR

This paper investigates the stability of mass transfer in close binary stars across various evolutionary stages using adiabatic models, revealing that red giant and asymptotic giant branch stars are more stable than previously thought, impacting binary evolution understanding.

Contribution

It provides a systematic survey of dynamical mass transfer thresholds for binary stars at different evolutionary stages using adiabatic and isentropic envelope models.

Findings

Red giant and asymptotic giant branch stars are more stable than previously believed.

Criteria for dynamical instability are presented in tabular and graphical forms.

Results help explain the observed abundance of post-AGB binary stars with ~1000-day periods.

Abstract

The distinguishing feature of the evolution of close binary stars is the role played by the mass exchange between the component stars. Whether the mass transfer is dynamically stable is one of the essential questions in binary evolution. In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We use the adiabatic mass loss model to systematically survey the thresholds for dynamical timescale mass transfer over the entire span of possible donor star evolutionary states. We also simulate mass loss process with isentropic envelopes, the specific entropy of which is fixed to be that at the base of the convective envelope, to artificially mimic the effect of such mass loss in superadiabatic surface convection regions, where the adiabatic approximation fails. We illustrate the general adiabatic response of 3.2 Msun donor stars at different evolutionary stages. We extend our study to a grid of donor stars with different masses (from 0.1 to 100 Msun with Z = 0.02) and at different evolutionary stages. We proceed to present our criteria for dynamically unstable mass transfer in both tabular and graphical forms. For red giant branch and asymptotic giant branch donors in systems with such mass ratios, they may have convective envelopes deep enough to evolve into common envelopes on a thermal timescale, if the donor star overfills its outer Lagrangian radius. Our results show that the red giant branch and asymptotic giant branch stars tend to be more stable than previously believed, and this may be helpful to explain the abundance of observed post-AGB binary stars with an orbital period of around 1000 days.