On the Origin of Early Superhumps in WZ Sge-Type Stars

Abstract

The origin of early superhumps, which are double-wave periodic modulations seen only during the earliest stage of WZ Sge-type outburst, has not been well understood. Based on recent discovery of two-armed arch-like patterns on Doppler tomograms in conjunction with early superhumps, we propose a new interpretation on the origin of early superhumps, following the new interpretation by Smak (2001) and Ogilvie (2001) of the two-armed pattern seen in IP Peg. If we consider irradiation of the elevated surface of the accretion disk caused by vertical tidal deformation, we can consistently explain the observed features on Doppler tomograms and photometric waves at the same time. We interpret that a combination of low mass-ratios (q) and low mass-transfer rates, necessary to give rise to these tidal effects, discriminate WZ Sge-type stars from other SU UMa-type dwarf novae. Based on recent fluid calculations, such an effect would be observable in higher q systems. We interpret that RZ Leo is an example of such objects.