Dissecting the microphysics behind the metallicity-dependence of massive stars radii

TL;DR

This study investigates how different microphysical processes influenced by metallicity affect the radii of massive stars throughout their evolution, using numerical experiments with varied microphysics inputs.

Contribution

It introduces a methodology to isolate and quantify the impact of microphysical processes on stellar radii, revealing dominant factors at different metallicities and evolutionary stages.

Findings

Opacity dominates at solar metallicity, contributing 60-90% to radius changes.

Nuclear reactions impact more at subsolar metallicity, contributing 50-70%.

Microphysics uncertainties can be propagated to stellar observable uncertainties.

Abstract

Understanding the radii of massive stars throughout their evolution is important to answering numerous questions about stellar physics, from binary interactions on the main sequence to the pre-supernova radii. One important factor determining a star's radius is the fraction of its mass in elements heavier than Helium (metallicity, $Z$). However, the metallicity enters stellar evolution through several distinct microphysical processes, and which dominates can change throughout stellar evolution and with the overall magnitude of $Z$. We perform a series of numerical experiments with 15M$_{\odot}$ MESA models computed doubling separately the metallicity entering the radiative opacity, the equation of state, and the nuclear reaction network to isolate the impact of each on stellar radii. We explore separately models centered around two metallicity values: one near solar $Z=0.02$ and another sub-solar $Z\sim10^{-3}$, and consider several key epochs from the end of the main sequence to core carbon depletion. We find that the metallicity entering the opacity dominates at most epochs for the solar metallicity models, contributing to on average $\sim$60 - 90% of the total change in stellar radius. Nuclear reactions have a larger impact ($\sim$50 - 70%) during most epochs in the subsolar $Z$ models. The methodology introduced here can be employed more generally to propagate known microphysics errors into uncertainties on macrophysical observables including stellar radii.