Pattern formation in rapidly oscillating peculiar A stars

Abstract

Peculiar A stars harbour pairs of antipodal spots, detectable in magnetic-field variation and chemical-abundance anomalies, and which are inclined from the axis of rotation. Many of the stars are observed to oscillate nonradially with frequencies of high-order acoustic modes. The oscillations appear to be dipolar, with axes that are almost always more-or-less aligned with the spots. It is known theoretically that when the spots produce the dominating aspherical influence on the dynamics of the oscillations, there is always an oscillation eigenmode that is constrained to be aligned with the spots, in accord with the observations. But under some circumstances the spots may not have complete dynamical dominance, and Coriolis precession can prevent a pure mode from remaining aligned. Yet, the oscillations appear to be aligned. Here I investigate the proposal that in such circumstances what is being observed is not a single oscillation eigenmode, stationary with respect to the rotating star, but an ensemble of precessing modes whose envelope is almost stationary, and almost aligned with the spots. I present a one-dimensional toy model of a slowly drifting (standing) acoustic mode in a medium with thermal “spots”, and show that under appropriate conditions stationary, non-precessing, mode envelopes are possible.