A disk origin for superhumps in Ursae Majoris variables

Abstract

As cataclysmic variables, SU Ursae Majoris stars^1,2 are short-period binary systems consisting of a red dwarf star that fills its Roche-lobe and spills material to an accretion disk around a white dwarf. SU UMa systems undergo outbursts (episodes of increased mass transfer in the disk) of two distinct types: normal outbursts that last a few days, and superoutbursts that last up to 2 weeks. Superhumps are periodic ∼30% brightness variations of unknown origin seen only during the superoutbursts. The superhump period is several per cent longer than the binary orbital period. Both the accretion disk^3,4 and the red dwarf⁵ have been proposed as possible sites for the superhump. Although superhumps occur in both eclipsing and non-eclipsing systems, observations of the eclipsing systems Z Cha and OY Car may help to discriminate between these two classes of models. Whitehurst et al.⁵ have shown that the superhumps in Z Cha are not deeply eclipsed, and argue that superhumps therefore cannot originate in the accretion disk. In their starspot model, superhumps arise in a bright active region located in the equatorial belt of the red dwarf. By contrast, the light curve synthesis results reported here show explicitly that disk models produce a shallow partial eclipse of the superhump light, compatible with the eclipse depth measurements of Whitehurst et al., provided the superhump source is not concentrated into a small region of the disk. The starspot model is shown to be inconsistent with the observed shapes of superhumps unless radiation emerges from the starspot in a narrow (≲35°) fan beam with its long axis in the direction of stellar latitude.