Kappa-mechanism excitation of retrograde mixed modes in rotating B-type stars

R. H. D. Townsend$^{1,2}$

1 - Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
2 - Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK

I examine the stability of retrograde mixed modes in rotating B-type stars. These modes can be regarded as a hybridization between the Rossby modes that arise from conservation of vorticity, and the Poincar\'{e} modes that are gravity waves modified by the Coriolis force. Using a non-adiabatic pulsation code based around the traditional approximation, I find that the modes are unstable in mid- to late-B type stars, due to the same iron-bump opacity mechanism usually associated with SPB and $\beta$ Cep stars. At one half of the critical rotation rate, the instability for $m=1\ldots4$ modes spans the spectral types B4 to A0. Inertial-frame periods of the unstable modes range from 100 days down to a fraction of a day, while normalized growth rates can reach in excess of $10^{-5}$.

I discuss the relevance of these findings to the mass-loss mechanism of Be stars, and to the pulsation of the putative Maia class of variable star. I also outline some of the questions raised by this discovery of a wholly-new class of pulsational instability in early-type stars.

Reference: MNRAS
Status: Manuscript has been submitted

Weblink: http://www.star.ucl.ac.uk/~rhdt/publications/

Comments: 10 pages, 6 figures

Email: rhdt@bartol.udel.edu