Nonradial oscillations of tidally distorted stars and asteroseismology

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1. Pulsational properties of θ Tucanae, one of the δ Scuti type stars in a close binary system, were investigated. Pulsation of this star is described by 13 frequencies, and one of them corresponds to the binary orbital period. However, the mass ratio is only 0.0896. It seems unlikely that oscillation of the primary star is excited by such a light companion. We proposed a new scenario to explain this enigmatic feature ; if the orbital period happens to be close to a quadrupole g-mode of the primary star, the g-mode resonates with the orbital motion. The g-mode eigenspectrum becomes so dense with the decrease of frequency that resonance becomes more likely to occur. As a consequence of resonance, the star is elongated toward its companion, and the radial p-mode eigenfunction is deformed to have an axially symmetric quadrupole component, whose symmetry axis coincides with the direction of the companion. The contribution of the quadrupole component of the p-mode eigenfunction to the apparent luminosity variation changes with time, and a triplet fine structure with an equal spacing of the double of orbital frequency is produced in the power spectrum.2. The color behavior of θ Tucanae with orbital phase was investigated. It was shown that the color variation is well described by two periods. The shorter period is connected to the ellipsoidal shape of the secondary. A reflection effect occurring once each orbital period causes alternative deep and shallow minima. Ten pulsational modes were tested for resonance with the orbital motion. It was found that the dominant pulsation mode shows a definite resonance with the ellipsoidal effect.3. We made spectroscopic observation of Procyon as a team attending the international observational campaign of this star, in order to detect its solar-like oscillations. We are now going on the data analysis.