On the nature of strange modes in massive stars

Details On the nature of strange modes in massive stars On the nature of strange modes in massive stars

Feb 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.284..821p&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 284, Issue 4, pp. 821-829.

Astronomy and Astrophysics

Astronomy

19

Stars: Early-Type, Stars: Oscillations.

Scientific paper

We investigate the recently discovered, highly unstable strange modes in models of massive main-sequence stars calculated with new opacities. We calculate these non-adiabatic radial modes by locating resonant features in the calculated responses to forcing potentials with complex frequencies. The modes are found to be confined to the surface layers around and above a convection zone in which a density inversion occurs and radiation pressure dominates. In order to gain an understanding of these modes, we develop a plane approximation and make the approximation of zero thermal time-scale. We then have a third-order, non-self- adjoint eigenvalue problem. The normal modes may be calculated analytically in simple cases, and a more general analytic asymptotic treatment is possible. We show that instability nearly always occurs when radiation pressure dominates, and that this is a result of the coupling of two almost isothermal wave-like modes with a non-oscillatory secular mode which dominates the asymptotic behaviour when radiation pressure is important. The interaction causes neighbouring neutral wave-like modes to coalesce and become unstable. The importance of the secular mode is brought about because the radiation pressure perturbation depends only on the temperature perturbation, and this is a non-local function of the displacement because the zero thermal time-scale allows instantaneous communication of thermal information.

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