Disk-Regulated Mass Transfer Between Rotating Non-Degenerate Stars: Insights from Be and sdOB Binaries

TL;DR

This work identifies a key discrepancy between observed Be+sdOB binaries and traditional rotationally limited mass-transfer prescriptions, proposing a disk-regulated angular-momentum transport mechanism that allows continued accretion near the critical rotation rate. By deriving an analytic, self-consistent accretion efficiency from mass and angular-momentum conservation in a quasi-steady disk, and implementing it in POSYDON/MESA, the authors show that disk-star coupling yields higher accretion efficiencies (near the theoretical 50% threshold) and Be-star masses more in line with observations. The study finds that Be+sdOB systems favor case A/B mass transfer configurations with reduced overshooting, highlighting the importance of disk dynamics, tidal effects on the disk, and envelope-core structure in determining final binary masses and periods. Overall, the disk-star coupling framework provides a physically motivated improvement over rotationally limited accretion, with meaningful implications for modeling binary evolution and interpreting Be-star binaries.

Abstract

Mass transfer between non-degenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically-motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk-star coupling model can produce more massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk-star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

Figures (10)

• Figure 1: The adopted dependence of $j_{\mathrm{acc}}$ on $\Omega/\Omega_{\mathrm{crit}}$, shown together with an approximate representation of the trend from 1991ApJ...370..604P. The dashed line denotes $j_{\mathrm{acc}}=0$.
• Figure 2: Accretion efficiency as a function of $\Omega/\Omega_{\mathrm{crit}}$ for different accretor radii in units of the Roche-lobe radius in the disk-accretion regime (left) and for different mass ratios $q= M_{\mathrm{a}}/M_{\mathrm{d}}$ in the direct-impact regime (right). The dashed line indicates the threshold $\Omega/\Omega_{\mathrm{crit}} = 0.9$.
• Figure 3: Evolution of an example binary under the two accretion models, showing the component masses, accretion efficiency, angular velocity relative to the critical value, accretor radius, and orbital period during mass transfer. The gray dashed horizontal line in the top middle panel marks the threshold of $\Omega/\Omega_{\mathrm{crit}} = 0.9$.
• Figure 4: Parameter space of component masses and orbital periods for the observed Be+sdOB systems and the simulated Be and helium star binaries under both the rotationally limited (gray) and disk–star coupling (orange) accretion models. Systems that undergo case A mass transfer are indicated with crosses and systems undergo case B mass transfer are shown with dots. The observed systems are primarily drawn from 2023AJ....165..203W and 2024ApJ...962...70K2025AA...694A.208K, and include FY CMa 2008ApJ...686.1280P, 59 Cyg 2013ApJ...765....2P, $\phi$ Per 2015AA...577A..51M, HD 55606 2018ApJ...865...76C, $\kappa$ Dra 2022ApJ...940...86K, and V742 Cas 2025ApJ...995..180G.
• Figure 5: Same as Figure \ref{['fig:grid_default']} but for disk-star coupling models with two different overshooting setups.