The stratified evolution of a cool star

TL;DR

This paper investigates the stratification and abundance anomalies in a 0.95 solar mass star throughout its evolution from the main sequence to the Horizontal Branch, exploring how internal processes affect its surface composition and final state.

Contribution

It provides a detailed evolutionary model of a low-mass star, highlighting stratification effects and their potential impact on surface abundances and stellar evolution outcomes.

Findings

Stratification occurs around the hydrogen burning shell during giant branch evolution.

Surface abundance anomalies, including iron overabundance, may result from internal stratification processes.

The internal hydrodynamic properties influence the star's evolution and final state on the Horizontal Branch.

Abstract

A low mass star usually experiences stratification and abundance anomalies during its evolution. A 0.95 solar mass star with a metallicity Z = 0.004 is followed from the main-sequence to the Horizontal Branch (HB). On the main-sequence the larger effects of stratification may come from accretion as was suggested in relation to metallicity and planet formation. As it evolves through the giant branch, stratification appears around the hydrogen burning shell. It may create hydrodynamic instabilities and be related to abundance anomalies on the giant branch. After the He flash the star evolves to the HB. If it loses enough mass, it ends up a hot HB star (or in the field an sdB star) with effective temperatures larger than 11000 K. All sdB stars are observed to have an approximately solar iron abundance whatever their original metallicity, implying overabundances by factors of up to 100. So should the 0.95 solar mass star. How its internal hydrodynamic properties on the main sequence may influence its fate on the HB is currently uncertain.