Delays in dwarf novae II: VWHyi, the tidal instability and enhanced mass transfer models

We discuss the multi–wavelength predictions of the two models proposed for SUUMa stars, i.e the enhanced mass transfer (EMT) and the tidal thermal instability (TTI) models. We focus on the systematic differences of the suggested scenarios before discussing the model predictions together with the observations of the best-studied SUUMa system, VWHyi. We find that assuming the standard form of the viscosity parameter α, both models predict only outbursts being triggered at the inner edge of the accretion disc. In the TTI model the superoutbursts are triggered when the outer radius of the disc reaches a certain value, i.e. the 3:1 resonance radius. In contrast, the EMT scenario predicts superoutbursts when the disc mass exceeds a critical value. This causes the EMT model to be much more sensitive to mass transfer variations than the TTI model. In both models we find the predicted UV and EUV delays in agreement with the observations of VWHyi for αh/αc∼ 4. In addition both models can generate precursor outbursts which are more pronounced at short wavelengths, in agreement with observations. Variations found in the observed light curve of single systems (e.g. VWHyi) as well as the difference between ordinary SUUMa stars and ERUMa systems are a natural outcome of the EMT model while the TTI model fails to explain them.