ON THE NATURE OF STRANGE MODES IN MASSIVE STARS

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 f eatures 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 occur s and radiation pressure dominates. In order to gain an understanding of these modes, we develop a plane approximation and make the approxim ation of zero thermal time-scale. We then have a third-order, non-self -adjoint eigenvalue problem, The normal modes may be calculated analyt ically in simple cases, and a more general analytic asymptotic treatme nt is possible. We show that instability nearly always occurs when rad iation pressure dominates, and that this is a result of the coupling o f 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 m odes to coalesce and become unstable. The importance of the secular mo de is brought about because the radiation pressure perturbation depend s only on the temperature perturbation, and this is a non-local functi on of the displacement because the zero thermal timescale allows insta ntaneous communication of thermal information.