Pulsational Stability of Rotating Main‐Sequence B Stars

Abstract

We revisit the problem of the pulsational stability of rotating main-sequence B stars. We examine the effects of the rotational deformation of the equilibrium structure on the pulsational stability against axisymmetric acoustic modes. The rotational deformation of a uniformly rotating star is calculated by applying the Chandrasekhar-Milne expansion to the hydrostatic and Poisson equations. Using the mean radius of the equipotential surface as an independent variable, the pulsation equations are formulated to be consistent to the order of the square of the angular frequency Ω of rotation. We have found that the axisymmetric nonradial modes of early B main-sequence stars are stabilized by the effect of the rotational deformation in the outer envelope at rapid rotation rates, but the axisymmetric quasi-radial modes are not necessarily stabilized by the deformation effect. The axisymmetric nonradial modes are stabilized when the contribution from the damping region above the main driving zone becomes significant at rapid rotation. We argue that this damping contribution appears when the equatorial transition zone separating the nonadiabatic exterior from the quasi-adiabatic interior shifts outward as Ω increases. We also suggest that the rotational deformation is not effective to stabilize the low-frequency g-modes excited in the middle and late B-type main-sequence stars.