Single massive stars at the critical rotational velocity: possible links with Be and B[e] stars

Abstract

Using single star models including the effects of shellular rotation with and without magnetic fields, we show that massive stars at solar metallicity with initial masses lower than about 20-25 M$_\odot$ and with an initial rotation above $\sim 350$ km s$^{-1}$ likely reach the critical velocity during their Main-Sequence phase. This results from the efficient outwards transport of angular momentum by the meridional circulation. This could be a scenario for explaining the Be stars. After the Main-Sequence phase, single star in this mass range can again reach the critical limit when they are on a blue loop after a red supergiant phase \citep{HL98}. This might be a scenario for the formation of B[e] stars, however as discussed by Langer & Heger (1998), this scenario would predict a short B[e] phase (only some 10$^4$ years) with correspondingly small amounts of mass lost.