Low-frequency oscillations of rotating massive stars

Abstract

Stellar rotation significantly modifies the property of nonradial oscillations when the frequencies (in the co-rotating frame) are comparable to or less than the frequency of rotation. The angular dependence of the amplitude of such an oscillation cannot be expressed by a single spherical harmonic, Y _l ^m (θ, φ). The amplitude of a g-mode tends to be confined to a narrow equatorial region compared to the non-rotating case. In addition to g-modes, which exist in a radiative equilibrium region, inertial (oscillatory convective) modes exist in a convective region. Half of the inertial modes have negative energy, while all the g-modes have positive energy. When the oscillation frequency of a positive energy mode is close to that of a negative energy mode, resonance coupling between the two modes occurs and energy flows from the negative energy mode to the positive one to increase the amplitude of both modes; i.e., to lead to the overstability of the oscillations. Therefore, overstable g-mode oscillations are possible for all rotating stars with inner convective and outer radiative regions. The resonance excitation of g-modes in the radiative envelope by inertial oscillations in the rotating convective core give natural explanation for rapid variations of early type stars. From observed periods of a variable early type star we can obtain information on the superadiabatic temperature gradient and the angular rotation frequency of the convective core.