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

TL;DR

This study models the pulsational stability of the rapidly rotating Be star HD 163868 using a 2D oscillation code that fully incorporates the Coriolis force, aligning theoretical predictions with observations.

Contribution

It introduces a new 2D oscillation code accounting for the Coriolis force and applies it to explain observed pulsation modes in HD 163868.

Findings

Both prograde and retrograde overstable modes identified

Low degree odd r-modes destabilized by the kappa-mechanism confirmed

High degree, retrograde m=1 modes explain ultra-low frequency observations

Abstract

We study the pulsational stability of the rapidly rotating Be star HD 163868 using a newly developed 2D oscillation code which takes the Coriolis force fully into account and compare our results with observations (MOST) and recent other stability analyses of this ~ 6 Msun star. 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 kappa-mechanism. The ultra-low frequency modes that could not be explained in a previous analysis are interpreted as high degree, retrograde m=1 modes. A reasonably good fit to the observed oscillation spectrum is possible if we assume that only even modes are observed. This requires a nearly equator-on view of the observed star, consistent with the measured high v sin i value of 250 km/s.