2D hydrodynamical simulations of Be star decretion disc formation through boundary layer effects

Abstract Be stars are massive main-sequence stars rotating close to their breakup rate. They possess a decretion disc of material built up due to mass loss from the star, however, there is not a consensus to the mechanism responsible for the formation of the disc because of their sub-breakup spin rates. We present the first 2D hydrodynamical simulations of the formation of a Be star decretion disc from a rapidly rotating star due to boundary layer effects that reduce the rotation rate of the disc close to the star. In our simulations with a disc aspect ratio of h/r = 0.1, a decretion disc forms around a star rotating with $80~{{\ \rm per\ cent}}$ of the breakup rate, but fails when rotating at $70~{{\ \rm per\ cent}}$ of the breakup rate. For a thinner disc, a faster stellar spin may be needed to form a dynamically important decretion disc. We also demonstrate good agreement between 1D and 2D models. Although this work does not consider the presence of magnetic fields and the angular momentum transport is through viscosity, our results robustly show a Be star disc may be built up hydrodynamically through boundary layer effects, and may play an essential role in regulating the stellar spin.