Pulsational stability of g-modes in slowly pulsating B stars

Low-frequency g-modes excited by the opacity bump mechanism in main-sequence stars with the masses M ∼ 4 M⊙ are believed to be responsible for the periodic variability observed in slowly pulsating B (SPB) stars. We calculate fully nonadiabatic g-modes with a low-degree l in an SPB star model to examine the pulsational stability by taking into account the effects of the Coriolis force and assuming a uniform rotation. We find that the slow rotation of the star contributes to the excitation of the high radial order retrograde g-modes with low l. This contribution to the excitation is most significant for the g-modes with l = 1. At rapid rotation, many g-modes with low l in the model are found to be pulsationally unstable even in the inertial regime in which | ω | < 2Ω, where ω denotes the oscillation frequency observed in the corotating frame and Ω is the angular rotation frequency. As the rotation rate increases, mode coupling caused by the Coriolis force between g-modes with different values of l becomes influential on the pulsational stability. We find that some of the g-modes in the inertial regime are stabilized by mode coupling at rapid rotation and that this stabilization effect is most significant for the retrograde g-modes with l > |m|.