On the properties of strange modes |
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| Authors: | Hideyuki Saio Norman H Baker & Alfred Gautschy |
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| Institution: | Astronomical Institute, School of Science, Tohoku University, Sendai 980-8578, Japan,;Department of Astronomy, Columbia University, New York, NY 10027, USA,;Astronomisches Institut, Universität Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland |
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| Abstract: | Properties of the so-called strange modes occurring in linear stability calculations of stellar models are discussed. The behaviour of these modes is compared for two different sets of stellar models, for very massive zero-age main-sequence stars and for luminous hydrogen-deficient stars, both with high luminosity-to-mass ratios. We have found that the peculiar behaviour of the frequencies of the strange modes with the change of a control parameter is caused by the pulsation amplitude of a particular eigenmode being strongly confined to the outer part of the envelope, around the density inversion zone. The frequency of a strange mode changes because the depth of the confinement zone changes with the control parameter. Weakly non-adiabatic strange modes tend to be overstable because the amplitude confinement quenches the effect of radiative damping. On the other hand, extremely non-adiabatic strange modes become overstable because the perturbation of radiation force (gradient of radiation pressure) provides a restoring force that can be out of phase with the density perturbation. We discuss this mechanism by using a plane-parallel two-zone model. |
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| Keywords: | stars: chemically peculiar stars: oscillations stars: variables: other |
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