Interacting binaries on the Main Sequence as in-situ tracers of mass transfer efficiency and stability

Understanding the transfer of mass and angular momentum in binary interactions is crucial for modelling the evolution of any interacting binary after the first mass transfer phase. Mass transfer physics assumptions shape the predictions for later stages of binary evolution, such as the immediate progenitors of stripped-envelope supernovae and gravitational wave mergers. We constrain the efficiency and stability of thermal timescale mass transfer in massive binary evolution using the observed population of 62 massive interacting binaries on the Main Sequence (`Algols’) in the Milky Way, Large and Small Magellanic Clouds. We find that purely conservative or non-conservative mass transfer cannot explain the current mass ratio and orbital period of all massive Algols. Angular momentum conservation rules out conservative mass transfer in $\sim$28,% of massive Algols in the SMC. About three-quarters of all massive Algols are consistent with having undergone inefficient mass transfer ($\lesssim$,50,%), while the remaining systems, mostly residing in the LMC and Milky Way, require mass transfer to have been more efficient than 25%. For our fiducial assumption on the extent of envelope stripping, the current sample of massive Algols does not require mass transfer to be efficient at the shortest orbital periods ($\sim$2,d) at any metallicity. We find evidence that mass transfer on the Main Sequence needs to be stable for initial accretor-to-donor mass ratios as unequal as $\sim 0.6$. Unless biased by observational selection effects, the massive Algols in the SMC seem to have undergone less efficient mass transfer than those in the LMC and Milky Way.

💡 Research Summary

This paper investigates the efficiency and stability of mass transfer during the thermal‑timescale (Case A) phase of massive binary evolution by using a sample of 62 massive Algol‑type systems—semi‑detached binaries in which both components are still on the main sequence. The sample spans three different metallicity environments: the Milky Way (MW), the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The authors develop an analytical framework that links the observable present‑day quantities (orbital period, donor mass, accretor mass) to the binary’s initial parameters (initial donor mass, initial mass ratio, initial orbital period). By imposing physically motivated limits on the initial configuration, they infer the range of possible mass‑transfer efficiencies (ε) and specific angular‑momentum loss factors (γ) that can reproduce each observed system.

Key methodological steps:

1. Observational data – The SMC sample (29 systems) is taken from de Mink et al. (2007); the LMC and MW samples (33 systems) are compiled from Sen et al. (2022). All systems have well‑determined current donor and accretor masses (≈ 8–40 M⊙) and orbital periods between ~1 and 10 days (SMC limited to ≤ 5 days).
2. Initial donor mass estimation – The authors assume that the present donor mass equals the initial convective‑core mass (including overshoot). This is motivated by detailed binary‑evolution models that show the fast thermal‑timescale mass‑transfer episode strips the donor down to its zero‑age main‑sequence (ZAMS) core. Using a grid of binary models (covering the three metallicities) they map each observed donor mass to an initial donor mass M_d,i.
3. Initial accretor mass bounds – The maximum possible accretor mass at ZAMS is limited by the donor’s initial mass (to avoid reversing the direction of mass transfer). The minimum possible accretor mass corresponds to the case where the accretor captures all mass lost by the donor (conservative transfer). The actual initial accretor mass therefore lies in a range