Critically-rotating accretors and non-conservative evolution in Algols

Abstract

Context. During the mass-transfer phase in Algol systems, a large amount of mass and angular momentum are accreted by the gainer star, which can be accelerated up to its critical Keplerian velocity. The fate of the gainer once it reaches this critical value is unclear. Aims. We investigate the orbital and stellar spin evolution in semi-detached binary systems, specifically for systems with rapidly rotating accretors. Our aims are to better distinguish between the different spin-down mechanisms proposed that can consistently explain the slow rotation observed in Algols' final states and to assess the degree of non-conservatism due to the formation of a hotspot. Methods. We use our state-of-the-art binary evolution code, Binstar, which incorporates a detailed treatment of the orbital and stellar spin and includes all torques due to mass transfer, the interactions between a star and its accretion disc, tidal effects, and magnetic braking. We also present a new prescription for mass loss due to the formation of a hotspot based on energy conservation. Results. The coupling between the star and the disc via the boundary layer prevents the gainer from exceeding the critical rotation. Magnetic-field effects, although operating, are not the dominant spin-down mechanism for sensible field strengths. Spin down owing to tides is 2-4 orders of magnitudes too weak to compensate the spinning-up torque due to mass accretion. Moreover, we find that the final separation strongly depends on the spin-down mechanism. The formation of a hotspot leads to a large event of mass loss during the rapid phase of mass transfer. The degree of conservatism strongly depends on the opacity of the impacted material. Conclusions. A statistical study and new observational constraints are needed to find the optimal set of parameters (magnetic-field strength, hotspot geometry, etc.) to reproduce Algol evolutions.

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