Angular momentum transport by stochastically excited oscillations in rapidly rotating massive stars

Abstract

We estimate the amount of angular momentum transferred by the low-frequency oscillations detected in the rapidly rotating hot Be star HD 51452. Here, we assume that the oscillations detected are stochastically excited by convective motions in the convective core of the star, that is, we treat the oscillations as forced oscillations excited by the periodic convective motions of the core fluids having the frequencies observationally determined. With the observational amplitudes of the photometric variations, we determine the oscillation amplitudes, which makes it possible to estimate the net amount of angular momentum transferred by the oscillations using the wave-meanflow interaction theory. Since we do not have any information concerning the azimuthal wavenumber $m$ and spherical harmonic degree $l$ for each of the oscillations, we assume that all the frequencies detected are prograde or retrograde in the observer's frame and they are all associated with a single value of $m$ both for even modes ($l=|m|$) and for odd modes ($l=|m|+1$). We estimate the amount of angular momentum transferred by the oscillations for $|m|=1$ and 2, which are typical $|m|$ values for Be stars, and find that the amount is large enough for a decretion disc to form around the star. Therefore, transport of angular momentum by waves stochastically excited in the core of Be stars might be responsible for the Be phenomenon.