The nature of the rapidly oscillating Ap stars' pulsations

¹ Centro de Astrofísica e Faculdade de Ciências, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
² UJF-Grenoble 1/CNRS-INSU, Institut de Plantologie et d'Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France

^a e-mail: mcunha@astro.up.pt

Abstract

Chemically peculiar stars are stage to a wide variety of physical phenomena, including diffusion, convection, magnetism and pulsation. Progress in the understanding of these objects, through the study of their oscillations, can help us to characterize these physical phenomena and better understand the way they are coupled in stars. A number of chemically peculiar A-type stars, known as rapidly oscillating Ap (roAp) stars, have been known to exhibit high frequency oscillations since the early 80s. Despite this, the mechanism responsible for driving these oscillations is not fully understood. Currently, the most widely accepted theory states that oscillations in this class of pulsators are excited by the opacity mechanism acting on the hydrogen ionization region, in an envelope where convection has been suppressed by a strong magnetic field. Nevertheless, this theory fails to correctly predict some of the observations for this class of pulsators. In this paper we briefly review the current status of understanding of the driving of pulsations in roAp stars. In particular, we shall emphasize the comparison between predictions of nonadiabatic models of roAp stars with observations of a subset of pulsators of this class for which stringent data on global parameters are available.