The spin-up of a star gaining mass in a close binary system on the thermal time scale

TL;DR

This study models how mass transfer in close binary systems can spin up a star to rapid rotation, resembling classical Be stars, through angular momentum transfer during the thermal timescale.

Contribution

It provides a detailed simulation of angular momentum transfer during mass accretion in binary systems, highlighting the role of meridional circulation in star spin-up.

Findings

Angular momentum increases by 50 times during accretion.

The star reaches rotation rates typical of classical Be stars.

Meridional circulation effectively transports angular momentum during and after mass transfer.

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.