Non-radial oscillations of the rapidly rotating Be star HD 163868

Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 28 February 2007
Accepted: 18 April 2007

Abstract

Context.Oscillations in rotating stars with frequency of the same order or smaller than the rotation rate Ω cannot be described by a single spherical harmonic due to the effect of the Coriolis force. This is a serious complication which is usually treated by writing the eigenfunctions as a (truncated) sum of different spherical harmonic degrees for a given m-value, or by neglecting the θ part of the rotation vector (Traditional Approximation).

Aims.We aim for a more adequate treatment of the coupling with higher angular degrees for low frequency oscillations of rotating stars by taking the Coriolis force fully into account, so that the coupling is included in the analysis (up to the grid resolution) and to compare the results with analyses based on the above mentioned approximations.

Methods.To this end a new, more efficient version of a 2D(r, θ) implicit oscillation code was developed in which no a priori assumptions about the θ variation of the eigenfunctions is made, enabling a better treatment of the rotational truncation of g-modes near the stellar poles, and for which ~150 gridpoints in θ are feasible. We test the code by comparing the simulated oscillation spectrum with that obtained by the MOST photometry for the 6  Be star HD 163868.

Results.We find both prograde and retrograde overstable modes (although more prograde than retrograde modes) and confirm the existence of low degree odd r-modes, destabilised by the κ-mechanism. The ultra-low frequency modes that could not be explained in a previous analysis are interpretated as (high degree) retrograde modes with i.e. 0. A reasonably good fit to the observed oscillation spectrum is possible if we assume that only even modes are observed (no unstable r-modes visible). This requires a nearly equator-on view of the observed star, consistent with the measured high value of 250 km s^-1.