New light on the driving mechanism in roAp stars

I. Effects of metallicity

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France e-mail: stheado@ast.obs-mip.fr
² Institut d'Astrophysique et de Géophysique de Liège, Allé du 6 Août 17, 4000 Liège, Belgium
³ Observatoire de Paris, LESIA, CNRS UMR 8109, 5 place J. Janssen, 92195 Meudon, France
⁴ Department of Physics, Wichita State University, Wichita, KS 67260-0032, USA

Received: 1 July 2008
Accepted: 3 October 2008

Abstract

Context. Observations suggest that a relationship exists between the driving mechanism of roAp star pulsations and the heavy element distribution in these stars.

Aims. We attempt to study the effects of local and global metallicity variations on the excitation mechanism of high order p-modes in A star models.

Methods. We developed stellar evolutionary models to describe magnetic A stars with different global metallicity or local metal accumulation profiles. These models were computed with CLES (“Code Liègeois d'évolution stellaire”), and the stability of our models was assessed with the non-adiabatic oscillation code MAD.

Results. Our models reproduce the blue edge of the roAp star instability strip, but generate a red edge hotter than the observed one, regardless of metallicity. Surprisingly, we find that an increase in opacity inside the driving region can produce a lower amount of driving, which we refer to as the “inverse κ-mechanism”.