We examine AM CVn stars, ultra-short-period variables thought to have helium disks, from the stand-point of the disk instability model. By calculating the vertical structure of the helium accretion disks, we have confirmed that the helium accretion disks are thermally unstable, since their thermal-equilibrium curves exhibit the characteristic S-shaped form. We apply these results to AM CVn stars. We have found that the observed large-amplitude photometric variations in AM CVn stars can be explained by the thermal-tidal instability model of helium accretion disks.
A dwarf nova of the WZ Sge-type, EG Cnc, which had a superoutburst in 1996 December after 19 years of dormancy, exhibited, after the end of the main outburst, six repetitive short outbursts with a mean interval of about 7 days. We propose that this rebrightening phenomenon of EG Cnc may have been produced by a thermal disk instability, which could in turn have been caused by a temporal increase in the viscosity at a cold state. We suggest that an increase in viscosity in quiescence could be produced by a highly turbulent state in the disk due to an after-effect of the main outburst. Our numerical simulations based on this model can reproduce very well the observed light curve of EG Cnc. We discuss a possible cause why the WZ Sge-type dwarf novae show post-outburst rebrightening.
The Natal pulses, solitary cyclonic meanders in the Agulhas Current, are reproduced in an ocean general circulation model. The model covers the region around the Agulhas Current with a grid fine enough to reproduce major eddies. The features of the reproduced Natal pulses are consistent with observational evidences in the following respects: they are generated at the Natal Bight when anticyclonic eddies come, move downstream along the Agulhas Current at speeds about 20 km day−1, and grow in its horizontal size as they move. The present simulation shows that the generation and growth of the Natal pulse occurs because of the interaction between the mean flow of the Agulhas Current and an anticyclonic eddy. A supplemental simulation, where the topography of the Natal Bight is modified, indicates that the topography of the Natal Bight does not cause the generation of the Natal pulses, contrary to a previous suggestion.
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