Formation of classical Be-stars of the early spectral subclass in the case of nonconservative mass transfer in close binary systems

TL;DR

This study models how nonconservative mass transfer in close binaries can spin up stars to form early Be-stars, emphasizing the role of angular momentum transfer and mass accretion thresholds.

Contribution

It provides a detailed analysis of angular momentum transfer mechanisms and identifies a critical mass accretion threshold for Be-star formation in binary systems.

Findings

Stars accreting more than 30% of their mass reach Be-star rotation speeds.

The process is insensitive to initial rotation, boundary layer angular momentum, disk velocity, and turbulence efficiency.

Mass transfer during binary evolution can naturally produce early Be-stars.

Abstract

Spin-up of a mass gaining component in a binary system is considered taking into account the mass loss from the system during the mass transfer between components in the Hertzsprung gap. The angular momentum that the accreting component gains during mass transfer depends on the increase in the mass of the component at this stage. The increase in the mass was considered over a broad range, from 5% to 100%. The case is considered when, after mass transfer, the mass of the accreting component has a value of 16 M_solar, typical for early Be stars. The transfer of angular momentum within the accreting component occurs due to meridional circulation and shear turbulence. If the accreted mass accounts for more than 30%, the accretor obtains a rotation typical of early Be-stars. This conclusion does not depend on: a) the rotation of the accreting component before mass transfer, b) the amount of angular momentum coming from the boundary layer located between the star and the accretion disk, c) a possible decrease in the angular velocity of the disk below the Keplerian value, d) the efficiency of turbulence in the interior of the accretor.