The Spin-up of a Star Gaining Mass in a Close Binary System on the Thermal Time Scale

Abstract

We investigate the exchange of mass in a binary system as a channel through which a Be star can receive a rapid rotation. The mass-transfer phase in a massive close binary system in the Hertzsprung-gap is accompanied by the spinning up of the accreting component. We consider a case when the mass of the accreting component increases by 1.5 times. The component acquires mass and angular momentum while in a state of critical rotation. The angular momentum of the component increases by 50 times. Meridional circulation effectively transports angular momentum inside the component during the mass-transfer phase and during the thermal time scale after the end of the mass-transfer phase. As a result of mass transfer, the component acquires the rotation typical of classical Be stars.

Formation of a Rapidly Rotating Classical Be-star in a Massive Close Binary System

“…The star surface gains critical rotation shortly after the start of accretion. Later, at the accretion stage in a state of accretor critical rotation, the circulation carries part of the angular momentum brought along with the accreted mass from subsurface layers to the star's surface (Staritsin 2022). Thus, accreted layers can shrink, as usually happens during accretion.…”

“…In Staritsin (2022), the transfer of angular momentum in the accretor interior was carried out only by meridional circulation. The turbulence was artificially suppressed.…”

“…In an earlier study (Staritsin 2022), we considered the following: when at the stage of subcritical rotation, angular momentum enters the accretor through two channels, namely together with matter having the same rotation velocity as the accretor's surface and due to turbulence within the rate (1). This is a case of high efficiency of angular momentum transfer from the boundary layer to the accretor's upper part.…”

“…In the first variant, to clarify the influence of angular momentum transport by turbulence in the accretor's interior on the spinning up of the accretor, we calculated accretion with the same rate of arrival of angular momentum into the accretor as in Staritsin (2022). At the stage of subcritical rotation, the parameter W max in the angular momentum source (1) is equal to α Ω Kep , where α = 0.8; here, Ω Kep is the Keplerian velocity of the star's surface at the equator.…”

“…We consider mass exchange in a binary system with the component masses of 13.4 and 10.7 M e and the period P = 35 d as in Staritsin (2022). By the beginning of mass exchange, star rotation with a mass of 10.7 M e is synchronized with orbital motion.…”