Mass Transfer in Eccentric Orbits with Self-consistent Stellar Evolution

Abstract We investigate Roche lobe overflow mass transfer (MT) in eccentric binary systems between stars and compact objects (COs), modeling the coupled evolution of both the star and the orbit due to eccentric MT (eMT) in a self-consistent framework. We implement the analytic expressions for secular rates of change of the orbital semimajor axis and eccentricity, assuming a delta function MT at periapse, in the binary stellar evolution code MESA . Two scenarios are examined: (1) a simplified model isolating the effects of eMT on stellar and orbital evolution, and (2) realistic binary configurations that include angular momentum exchange (e.g., tides, mass loss, spin–orbit coupling, and gravitational-wave (GW) radiation). Unlike the ad hoc approach of instant circularization that is often employed, explicit modeling of eMT reveals that a large fraction of binaries can remain eccentric post-MT. Even binaries that naturally circularize during eMT have different properties (donor mass and orbital size) compared to predictions from instant circularization, with some showing fundamentally different evolutionary outcomes (e.g., stable vs. unstable MT). We demonstrate that a binary’s initial mass ratio and eccentricity are predictive of whether it will remain eccentric or circularize after eMT. These findings underscore the importance of eMT in understanding CO-hosting binary populations, including X-ray binaries, GW sources, and other high-energy transients.