Chemical enrichment of metal-poor stars orbiting massive black hole companions

TL;DR

This study uses hydrodynamical simulations to show how metal accretion from the interstellar medium onto stars in binary systems with black holes can alter surface abundances, especially during the main sequence phase.

Contribution

First simulation-based analysis of metal enrichment in low-mass stars orbiting black holes, highlighting the impact on surface abundances and observational implications.

Findings

Metal accretion can significantly alter stellar surface metallicity.

Effects are most observable in stars still on the main sequence.

Initial surface enrichment may be erased after stellar evolution.

Abstract

There are millions of undetected black holes wandering through our galaxy. Observatories like {\it Chandra}, LIGO, and more recently, {\it Gaia}, have provided valuable insights into the configurations of these elusive objects when residing in binary systems. Motivated by these advances, we study, for the first time, the enhanced accretion of metals from the interstellar medium (ISM) onto low-mass companions in binary systems with highly unequal mass ratios, utilizing a series of hydrodynamical simulations. Our study demonstrates that a stellar companion's metal accretion history from the ISM alone, from its formation to the present, can significantly influence its surface abundances, especially when enhanced by a massive black hole companion. However, this effect is likely only measurable in stars that are still in the main sequence. Once a stellar companion evolves off the main sequence, similar to what has been observed with the {\it Gaia} BH3 companion, the initial dredge-up process are likely to erase any excess surface abundance resulting from the metals that were accreted. As we discover more unequal mass ratio binary systems, it is crucial to understand how the observed metallicity of sun-like companions may differ from their birth metallicity, especially if they are not yet evolved.