Yoji Osaki

Thermal and Tidal Instabilities in Accretion Disks of Dwarf Novae

Various outbursting phenomena in dwarf novae are now thought to be caused by variable rates of accretion onto white dwarf components through accretion disks in cataclysmic binary systems. In this review, I discuss the thermal instability (the ordinary disk instability) and the tidal instability in accretion disks as causes of outbursts in dwarf novae. In the first part, we discuss the disk instability model for normal outbursts of dwarf novae; in particular we consider following three problems: the narrowness of transition fronts, the delay of UV flux to the optical at the rise of outburst, and the variation of disk radius in an outburst-quiescence cycle.

Ginga observations of the dwarf novae BV Pup and V426 Oph

Ginga X-ray observations of the high excitation dwarf nova BV Pup and the suspected DQ Her dwarf nova V426 Oph are presented. The observation of BV Pup took place during optical quiescence and showed no detectable X-ray flux, implying the X-ray luminosity was below that observed by Exosat during the decline from an outburst. An optical spectrum obtained near the time of the X-ray observations showed no high-excitation He II 4686, so that it is not clear if this was an unusual low-excitation time for BV Pup or if this is a normal phase of the outburst cycle. V426 Oph was observed for four days near maximum outburst brightness with overlapping X-ray coverage on three days. The 2-10 keV flux was a factor of 2 below that observed by Exosat during a quiescent state. There is evidence for a 28 min timescale in the X-ray, and optical data which are not strictly periodic in nature.

Excitation mechanisms of solar oscillations

Excitation mechanisms of solar oscillations are reviewed. Two excitation mechanisms have been proposed for solar p-mode oscillations: the linear overstability due to the so-called κ-mechanism and the stochastic excitation by turbulent convection. Here we discuss the stochastic excitation mechanism in a greater detail because it is more likely to be the possible explanation than the former mechanism. In this mechanism, the amplitudes of oscillations are determined by the balance of the excitation rate by turbulent convection to the damping rate. We examine these two effects separately. In particular, we discuss the acoustic energy generation by turbulent convection and the question of quadrupole versus dipole radiation.

Hydrodynamic Simulation of Accretion Disks in Cataclysmic Variables

The non-axisymmetric configurations of accretion disks in cataclysmic variables are studied by two-dimensional hydrodynamical calculations. The particle-hydrodynamic method first developed by Lin and Pringle (1976) is used, and time evolution of accretion disks under a constant mass supply rate from the secondary is followed until the system settles to a quasi-steady state. It is found that a binary system with comparable masses of component stars settles to a steady state of an elongated disk fixed in the rotating frame of the binary while in the case of low mass secondary it settles to a periodic oscillating state in which a non-axisymmetric (eccentric) structure is formed and it rotates slowly in the inertial frame of reference. The period of oscillation is a few percent longer than the orbital period of binary, and it offers a natural explanation to the “superhump” periodicity of SU UMa stars. Our results thus confirm basically those of Whitehurst (1988) who has discovered the tidal instability of an accretion disk in a low mass secondary.