Reverse mass transfer and rejuvenation in the massive contact system

Context. Numerous studies have established that in main-sequence binary systems, mass transfer generally proceeds from the initially more massive star to its less massive companion, thereby inducing rejuvenation in the latter. However, in certain massive close binary systems with orbital periods on the order of a few days, a reversed mass transfer scenario emerges: mass flows from the initially less massive component to the more massive one, during which the two stars simultaneously overflow their Roche lobes. This configuration potentially facilitates stellar rejuvenation in both members of the binary system. Aims. The phenomenon of reverse mass transfer in binary systems is closely linked to the efficiency of stellar rejuvenation in the mass accretor. However, the physical mechanism driving the rejuvenation of the accretor remains poorly understood. In this work, we employ the Schwarzschild criterion to define the boundaries of convective regions at the solar metallicity, as opposed to the Ledoux criterion typically used for convective conditions at low metallicities. Our study aims to investigate how mass transfer significantly influences the rejuvenation process of the mass-accreting star. Furthermore, we aim to systematically investigate how initial binary parameters (particularly the mass ratio and orbital period) regulate the initiation of reverse mass transfer from the secondary to the primary component. Methods. We constructed a new set of detailed, grid-based binary evolution models, systematically varying initial orbital periods and mass ratios, q , within a parameter space. Results. Our results show that for systems with initial primary masses of 16 M ⊙ and initial mass ratios of q ini = M 2 M 1 ≥ 0.4, those with shorter initial orbital periods, P orb < 1.8 days, are statistically more prone to evolving into contact binaries that exhibit reverse mass transfer. The results further demonstrate that stellar rejuvenation significantly influences the overall evolution of binary systems, notably extending the main-sequence lifetime of the mass-gaining star. Specifically, rejuvenation can induce increases in both the radius and luminosity of the accretor, potentially triggering reverse mass transfer. The transfer causes the mass of accretor’s convective core to increase, facilitating the mixing of fresh fuel from outer layers into the central nuclear-burning region. Following mass transfer, the evolutionary state of the accretor is closer to the zero age main sequence. Due to the high efficiency of rejuvenation, the mass-gaining star can even outpace the mass-losing star in terms of evolution. Meanwhile, the initially more massive star can also undergo rejuvenation via reverse mass transfer, as it must rapidly adapt to its newly increased mass.