SUPERHUMP TIMING IN SU URSAE MAJORIS SYSTEMS - IMPLICATIONS OF THE DATA FOR THE PRECESSING DISK MODEL

We tabulate the most current data for the orbital period, superhump period, and mass ratio in SU Ursae Majoris type variables to test models of the superhump timing mechanism. Contrary to earlier reports, the correlation between the superhump period excess and the orbital period is not significant. However, the correlation between the superhump period excess and the mass ratio is significant at the 4.7-sigma-level. This correlation was predicted by the Whitehurst model which explains superhumps as periodic enhancements of tidal dissipation in an eccentric, precessing accretion disk. We compare these data with the results of recent hydrodynamical simulations and with our own calculations of coplanar, restricted three-body orbits as a function of mass ratio for mass ratios less than unity. We find that all reliable mass ratios for SU UMa systems are consistent with the upper limit of 0.22 placed by the model. We also computed the degree of instability as a function of mass ratio. We attribute the delays in the onset of superhumps observed in WZ Sge and WX Cet to the relatively small instability expected for their mass ratios. The theoretical superhump period computed by Whitehurst for a mass ratio of 0.15 agrees well with the data, but the values computed by Hirose & Osaki are systematically too high. Stringent further tests of the model are afforded theoretically by trying to reproduce the observed superhump period-mass ratio relationship, and from observation by further exploring the time delay-mass ratio relationship. If the validity of the model can be established in this way, superhump timing observations can serve as a much-needed tool for determining mass ratios in SU UMa systems. Finally, we review the status of proposed models for the superoutburst cycle in SU UMa systems.