Formation of Be stars in binary systems with conservative mass exchange

In the case of conservative mass exchange in a binary system in the Hertzsprung gap, the mass of the accreting star can increase by up to two times. The accreted matter brings Keplerian angular momentum with it. The meridional circulation transfers 80–85% of the incoming angular momentum to the surface of the accretor. The increase in the mass and angular momentum of the accretor occurs due to the loss of this part of the angular momentum. After the mass exchange is completed, most of the angular momentum is concentrated in the accreted layers. The accretor has a large enough angular momentum to maintain its status as a Be star during its subsequent evolution on the main sequence.