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

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.

💡 Research Summary

This paper presents a pioneering investigation into the chemical evolution of low-mass, metal-poor stars in binary systems with extremely massive companions, such as stellar-mass black holes. Motivated by recent discoveries of quiescent black hole binaries by Gaia (e.g., Gaia BH3), the study addresses a critical question: can the observed surface metallicity of these companion stars be altered from their birth composition by accreting metal-enriched gas from the interstellar medium (ISM) over cosmic time, and how is this process affected by the presence of a massive gravitating companion?

The core of the research lies in extending the classical Bondi-Hoyle-Lyttleton (BHL) accretion theory to binary systems with high mass ratios (M_BH / M_* » 1). The authors develop an analytical framework predicting that when a low-mass star orbits within the accretion radius of its massive black hole companion, it travels through a region of gas that has been gravitationally focused and densified by the black hole’s potential. This leads to a significant enhancement in the star’s accretion rate compared to if it were isolated. The analytical model suggests the enhancement factor scales linearly with the mass ratio (M_BH/M_*) and inversely with the binary separation normalized to the black hole’s accretion radius (a/R_a).

To test and refine this model, the team conducts a suite of three-dimensional hydrodynamical simulations. These simulations model a binary system moving supersonically through a uniform-density ISM, varying the mass ratio and orbital separation. The results robustly confirm the analytical predictions, providing numerical values for the scaling relations. For instance, in a system with a mass ratio of 100 and a separation of a = 10 R_a, the low-mass star’s accretion rate can be enhanced by a factor of ~1000.

The study then places this physical mechanism into a cosmological context. Using stellar trajectory data from the “Eris” zoom-in cosmological simulation—which tracks the motion of stars and the chemical evolution of the ISM in a Milky Way-like galaxy—the authors calculate the total mass of metals a star like the Gaia BH3 companion could accumulate over its ~13 Gyr lifetime. They find that accretion from the ISM alone, significantly boosted by the black hole companion, could potentially increase the star’s surface iron abundance