R-mode oscillations in accreting white dwarfs in cataclysmic variables

Abstract

Some cataclysmic variables show short-period ($\sim$200 to $\sim$2000 s) light variations, which are attributable to nonradial pulsations of the accreting primary white dwarf. We regard these periodic variations as r-mode (global Rossby-wave) oscillations which are presumably excited mechanically and confined into hydrogen-rich layers in the white dwarf. Often observed modulations in amplitudes and frequencies even during an observing season may be interpreted as beatings among densely distributed r-mode frequencies. Making the r-mode frequency distribution of a model to be consistent (by adjusting the rotation frequency) with the observed pulsation frequencies, we obtain the best-fit rotation rate for each case. The rotation periods thus obtained for 17 accreting white dwarfs lie between 4 and 10 minutes. Pulsation frequencies observed sometime after a dwarf-nova outburst tend to be higher than the pre-outburst values, so that the best-fit rotation rate for the post-outburst frequencies is also higher. This indicates a spin-up of the hydrogen-rich layers to occur by an enhanced accretion at the outburst. We also discuss the relation between the rotation frequencies of pulsating accreting white dwarfs and the orbital periods.