BPASS stellar evolution models incorporating $\alpha$-enhanced composition -- I. Single star models from 0.1 to 316 M$_\odot$

TL;DR

This study presents a comprehensive grid of stellar evolution models with varying $oldsymbol{ ext{alpha}}$-element enhancements, showing that $oldsymbol{ ext{alpha}}$-enrichment significantly affects stellar properties and evolution, impacting galaxy modeling and supernova predictions.

Contribution

It introduces a dense grid of single-star models with different $oldsymbol{ ext{alpha}}$-to-iron ratios, highlighting the effects of $oldsymbol{ ext{alpha}}$-enhancement on stellar evolution parameters.

Findings

Stellar radii and surface temperatures vary with $oldsymbol{ ext{alpha}}$-enhancement.

Main Sequence lifetimes can differ by up to 0.4 dex due to $oldsymbol{ ext{alpha}}$-enrichment.

Supernova rates are highly uncertain when $oldsymbol{ ext{Fe/H}}$ is unknown.

Abstract

Stellar evolution modelling is fundamental to many areas of astrophysics including stellar populations in both nearby and distant galaxies. It is heavily influenced by chemical composition. Observations of distant galaxies and nucleosynthesis calculations show that $\alpha$-process elements are enriched faster than iron group elements. We present a dense grid of single-star models calculated using the BPASS stellar evolution code and covering masses ($0.1\le\mathrm{M/M}_\odot\le316$), metallicity mass fractions ($10^{-5} \le Z \le 0.04$) and $\alpha$-to-iron abundance ratios ($-0.2\le[\alpha/\mathrm{Fe}]\le+0.6$). By comparing Solar-scaled models to ones enriched in $\alpha$-process elements, we find that stellar radii, surface temperatures, Main Sequence lifetimes, supernova progenitor properties and supernova rates are all sensitive to changes in [$\alpha$/Fe]. Lifetimes of low-mass stars differ by up to 0.4 dex, while surface temperatures of massive stars at the end of the Main Sequence also differ by around 0.4 dex. Inferred supernova rates when [Fe/H] is unknown can be highly uncertain. Models with different [$\alpha$/Fe] but comparable iron abundances show smaller variations, indicating that while iron primarily defines the course of evolution; $\alpha$-enhancement nonetheless has an impact of up to 0.1 dex on stellar properties. Such changes are small for individual stars, but have a large cumulative effect when considering an entire stellar population as demonstrated by isochrone fitting to nearby clusters. Changes in radii and lifetimes have further consequences for a stellar population including binary stars, as they influence the timing, nature and occurrence rate of mass transfer events.