Comfort zones of stars: A fundamental limit on orbital tightening via stable mass transfer

Description

This dataset accompanies the paper
“Comfort zones of stars: A limit on orbital tightening via stable mass transfer shapes the properties of binary black hole mergers”
(Klencki et al. 2025, Astronomy & Astrophysics, link), and provides the numerical results and MESA setups used in that study. 

Contents

The repository contains, for each model variation:

1. Mass transfer stability datasets summarizing the outcomes of detailed binary evolution calculations from the boundary between stable and unstable interaction.

2. MESA working directories (incl. inlists), allowing the models to be reproduced or modified.

These are provided for three variations of the mass-transfer treatment explored in the paper:

• Fiducial model ("default"): Eddington-limitted accretion, with all non-accreted mass leaving the system carrying the specific angular momentum of the accretor.

• Enhanced angular-momentum loss model ("L2"): identical to the fiducial case, but with 25% of the non-accreted mass lost through the outer Lagrangian point.

• Low-semiconvection model ("sc001"): identical to the fiducial angular-momentum treatment, but with a reduced efficiency of semiconvective mixing.

MESA model setup

The MESA simulations follow binaries consisting of a black hole accretor and a zero-age main-sequence (ZAMS) massive-star donor. Each system is evolved through Roche-lobe overflow and resulting mass transfer until either:

• mass transfer becomes dynamically unstable (leading to a stellar merger/common envelope), or

• the donor reaches central carbon depletion, after which further evolution toward a binary black hole (BBH) system is inferred.

The calculations self-consistently track the response of the donor star, orbital evolution, and mass-transfer stability. The provided MESA setups are intended as reference implementations of the methodology described in the paper; readers are referred to the paper for the full motivation and parameter choices.

Mass transfer stability datasets

The output text files include critical mass-ratio and minimum orbital separation stability datasets derived from grids of binary models. For each donor mass and radius, these data provide:

• the critical mass ratio separating stable from unstable mass transfer,

• the minimum post–stable-mass-transfer orbital separation achievable without triggering instability.

A key result highlighted in the paper is that stable mass transfer in BH–star systems exhibits a fundamental lower limit on the post-interaction orbital separation,
largely independent of uncertainties in angular-momentum loss. Systems attempting to shrink below this limit become dynamically unstable and merge. The critical mass ratios reported here will depend on the assumed angular momentum budget of the mass exchange (e.g. accretion efficiency, L2 outflows), whereas the minimum post-SMT separations do not. 

New treatment of mass transfer stability for rapid binary codes

As discussed in Sec 4.5 of the paper, the minimum post–stable-mass-transfer separation is a more robust and physically motivated criterion for mass-transfer stability than traditional prescriptions based solely on critical mass ratios. These results are therefore directly applicable to the calibration of rapid binary population synthesis codes, particularly for modeling the formation and properties of merging black hole systems.

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