Angular Momentum Transfer by Low Frequency Modes in Rapidly Rotating Slowly Pulsating B Stars

We discuss angular momentum transfer by non-axisymmetric low frequency modes in rapidly rotating slowly pulsating B (SPB) stars, in which low frequency g- and r-modes are excited by an opacity bump mechanism associated with the iron abundance. We use a mean flow equation, which is derived under the Lagrangian mean formalism, to discuss the forcing on the zonal mean flow (rotation), caused by pulsationally unstable low frequency modes in these stars. For rapidly rotating SPB stars, only prograde g-modes may become pulsationally unstable because retrograde g- modes are likely to be stabilized by the effects of rapid rotation. Since weak differential rotation in the outer envelope tends to stabilize r-modes of vertical bar m vertical bar >= 2 with m being the azimuthal wave number, only retrograde r-modes of vertical bar m vertical bar = 1 and prograde g-modes may play a role in angular momentum transfer in these stars. Although low frequency modes work for both acceleration and deceleration of the mean flow, assuming contributions to acceleration of the mean flow are dominant, we construct a disc-star model, in which a rapidly rotating star steadily expels gas material from the equatorial region to form a viscous Keplerian decretion disc around it.