Accretion disc instabilities in cataclysmic variable stars

• Main Author: Wynn, Rebecca
• Published: University of Leicester 2000
• Subjects: 523.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.696857

A 1D hydrodynamical code is used to model the viscous evolution of VY Scl stars, which are a subclass of Cataclysmic Variable. Low states arise as a result of occasional drops in the mass transfer rate, which probably result from the passage of starspots across the inner Lagrangian point on the secondary star. The model includes the heating of the accretion disc by irradiation from the white dwarf and shows that outbursts from the low state can be suppressed if the temperature of the white dwarf is sufficiently high (Twd 40 000 K). A magnetic propeller model is used to show that the quiescent value of the viscosity parameter of the accretion disc within WZ Sge is likely to be occo d 0.02, in agreement with estimates of ojcold f r other dwarf novae. Assuming the white dwarf in WZ Sge to be weakly magnetic it is shown that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner accretion disc. This has the effect of stabilizing the inner disc and allowing the outer disc to accumulate mass. Numerical models yield an estimated recurrence time of rec 30 10 yr, in agreement with the observed recurrence time of trec 33 yr. The model is also used to follow WZ Sge through outburst, producing lightcurves that are in good agreement with observation. Finally, high-speed K-band photometry of WZ Sge is presented. Analysis of the data reveals a strong oscillation at 27.88 0.01 s, along with weaker oscillations at slightly longer periods. The principal oscillation is attributed to the presence of a rapidly rotating weakly magnetic white dwarf, and possible explanations for the weaker oscillations are discussed. The long term brightness variation in the K-band lightcurves is analysed, providing tentative evidence of a precessing, elliptical disc. The observational properties can be explained if the white dwarf possesses a weak magnetic field.