Influence of the Outer Boundary Condition on models of AGB stars

TL;DR

This study examines how different outer boundary conditions in stellar models affect the evolution of AGB stars, revealing minor impacts on observable properties but uncovering a new phenomenon that could influence carbon-star lifetimes and core masses.

Contribution

It introduces a detailed analysis of boundary condition treatments using MARCS/COMARCS atmospheres and reports a new physical phenomenon affecting AGB star evolution.

Findings

Observable quantities like final core mass are not significantly affected.

A new phenomenon may increase carbon-star lifetime and core mass in specific initial-mass ranges.

Alterations in boundary conditions influence the complex processes of TP-AGB evolution.

Abstract

Current implementations of the stellar atmosphere typically derive boundary conditions for the interior model from either grey plane-parallel atmospheres or scaled solar atmospheres, neither of which can be considered to have appropriate underlying assumptions for the Thermally Pulsing Asymptotic Giant Branch (TP-AGB). This paper discusses the treatment and influence of the outer boundary condition within stellar evolution codes, and the resulting effects on the AGB evolution. The complex interaction of processes, such as the third dredge up and mass loss, governing the TP-AGB can be affected by varying the treatment of this boundary condition. Presented here are the results from altering the geometry, opacities and the implementation of a grid of MARCS/COMARCS model atmospheres in order to improve this treatment. Although there are changes in the TP-AGB evolution, observable quantities, such as the final core mass, are not significantly altered as a result of the change of atmospheric treatment. During the course of the investigation, a previously unseen phenomena in the AGB models was observed and further investigated. This is believed to be physical, although arising from specific conditions which make its presence unlikely. If it were present in stars, this phenomenon would increase the carbon-star lifetime above 10Myr and increase the final core mass by $\sim0.1M_{\odot}$ in the narrow initial-mass range where it was observed ($\sim2-2.3M_{\odot}$).