Hideyuki Saio

Line profile variations caused by low-frequency nonradial pulsations of rapidly rotating stars

Line profile variations caused by low-frequency nonradial g-mode oscillations of a rotating star are presented. For a nonradial oscillation mode whose oscillation frequency in the corotating frame is comparable to or smaller than the rotation frequency, the latitudinal dependence of the oscillation amplitude deviates significantly from that for a nonrotating star. Rotation usually causes oscillations confined to a narrow equatorial belt as well as causing toroidal velocity fields. The concentration of the amplitude of the oscillation toward the equator leads to a reduction in the strength of the bumps in line profiles if the maximum velocity of nonradial pulsation is fixed. For the same normalization, the line profile variations due to the sectoral prograde mode are most visible, while those caused by retrograde waves are almost invisible due to a cancellation effect. The toroidal component generated by stellar rotation appreciably affects the line profile variation of some modes. 24 refs.

Nitrogen and helium enhancement in the progenitor of supernova 1987 A

Why the progenitor of supernova 1987A was a blue supergiant has been a fundamental question, because the occurrence of a type II supernova from such a progenitor had not been known before. Recent ultraviolet observations have provided crucial evidence that bears on this problem. Emission lines of CNO elements show that the expansion velocity of the emitting gas is <30 km s–1, and that the abundance ratios of N/C and N/O are larger than the solar ratios by factors of ∼30 and 10, respectively1,2, indicating that the ultraviolet emissions come from circumstellar material which had been processed by hydrogen burning in the interior and ejected into space when the progenitor was a red supergiant. The progenitor of SN1987A must therefore have evolved first to a red supergiant and then back to the blue. From calculations of massive star evolution, we show how the star can undergo blue–red–blue evolution. By comparing these theoretical models with the abundance information from the ultraviolet observations, we conclude that the hydrogen-rich envelope of the progenitor was as massive as 7–11 M⊙ and almost the whole envelope was uniformly mixed, probably because of rapid rotation.

Energy of oscillation : the role of the negative energy mode in overstable nonradial oscillations of rotating stars

In the present treatment of adiabatic nonradial oscillation energies in both nonrotating and uniformly rotating stars, the resonance between two positive energy modes yields an avoided crossing, while the resonance between one positive energy mode and a negative one yields an overstable mode. It is found that while the oscillation energies of p- and g-modes in nonrotating stars, and g-modes of rotating stars, are definitely positive, the energy of an oscillatory convective mode of rotating stars is positive or negative, depending on the ratio of the frequencies of rotation to those of oscillation. For a negative energy oscillation, amplitude increases when energy is extracted from, rather than fed into, the oscillation; when a resonance occurs between a negative energy mode and a positive one, energy flows from the negative energy mode to the positive one, thereby forming an overstable oscillation. 16 refs.

Low-frequency oscillations of rotating massive stars

Stellar rotation significantly modifies the property of nonradial oscillations when the frequencies (in the co-rotating frame) are comparable to or less than the frequency of rotation. The angular dependence of the amplitude of such an oscillation cannot be expressed by a single spherical harmonic, Ylm (θ, φ). The amplitude of a g-mode tends to be confined to a narrow equatorial region compared to the non-rotating case. In addition to g-modes, which exist in a radiative equilibrium region, inertial (oscillatory convective) modes exist in a convective region. Half of the inertial modes have negative energy, while all the g-modes have positive energy. When the oscillation frequency of a positive energy mode is close to that of a negative energy mode, resonance coupling between the two modes occurs and energy flows from the negative energy mode to the positive one to increase the amplitude of both modes; i.e., to lead to the overstability of the oscillations. Therefore, overstable g-mode oscillations are possible for all rotating stars with inner convective and outer radiative regions. The resonance excitation of g-modes in the radiative envelope by inertial oscillations in the rotating convective core give natural explanation for rapid variations of early type stars. From observed periods of a variable early type star we can obtain information on the superadiabatic temperature gradient and the angular rotation frequency of the convective core.

Asymptotic analysis of inertial waves in the convective envelope of the sun

Inertial waves are propagative in convective regions of rotating stars. Half of the inertial waves have positive energy of oscillations and the other half negative energy. The inertial waves with negative energy become overstable when they are in resonance with waves having positive energy such as internal gravity waves or when they dissipate energy of oscillations through nonadiabatic effects. We calculate a frequency spectrum of inertial (oscillatory convective) modes with negative energy propagating in the surface convective zone of the sun by using an asymptotic method of nonradial oscillations of rotating stars. It is shown that the inertial modes have large amplitudes only at high latitudes. The inertial modes with negative energy have very low frequencies seen in the corotating frame and hence if they are observed in an inertial frame their frequencies are approximately equal to −mΩ⊙.

A New Progenitor Model of Type Ia Supernovae

A new progenitor model of Type Ia supernovae (SNe Ia) is proposed. The model consists of a carbon-oxygen white dwarf (0.8–1.2 M⊙) and a low-mass red giant star (0.8–1.5 M⊙) with a helium core (0.2–0.4 M⊙). When a red giant fills its inner critical Roche lobe and its mass transfer rate exceeds a critical value, a common envelope state is realized. Then the mass accretion rate onto the white dwarf, i.e., the mass transfer rate is tuned up to be Ṁ= 8.5 x10−7 (MWD/ M⊙−0.52) M⊙ yr−1, where MWD is the mass of the white dwarf. This rate is high enough to suppress the hydrogen shell flashes, but too low for carbon to be ignited off-center. When the carbon-oxygen core mass grows to the Chandrasekhar limit during the common envelope phase, a Type Ia supernova explosion is expected to occur.

Growth rate of white dwarf mass in binaries

The growth rate of a white dwarf which accretes hydrogen-rich or helium matter is studied. If the accretion rate is relatively small, unstable shell flash occurs and during which the envelope mass is lost. We have followed the evolutions of shell flashes by steady state approach with wind mass loss solutions to determined the mass lost from the system for wide range of binary parameters. The time-dependent models are also calculated in some cases. The mass loss due to the Roche lobe overflow are taken into account. This results seriously affects the existing scenarios on the origin of the type I supernova or on the neutron star formation induced by accretion.