Post-AGB discs from common-envelope evolution

TL;DR

This paper models circumbinary discs around post-AGB stars formed through common-envelope evolution, exploring their impact on binary star properties and evolution.

Contribution

It introduces a combined analytic and binary population synthesis approach to study disc formation and effects in post-AGB binary systems.

Findings

Discs can prolong post-AGB star lifetimes.

Discs can increase binary orbital eccentricity.

Formation depends on disc mass, angular momentum, and proximity to the binary.

Abstract

Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stellar populations and hence explore the rather uncertain parameter space involved with disc formation. We find that, provided the discs form with sufficient mass and angular momentum, and have an inner edge that is relatively close to the binary, they can both prolong the life of their parent post-AGB star and pump the eccentricity of orbits of their inner binaries.