Dr. DAI Zhibin from Yunnan Observatories of the Chinese Academy of Sciences and his collaborators showed the evolution of the quiescent disk surrounding a superoutburst (SO) of the dwarf nova TW Virginis from portions of the Kepler K2 Short Cadence light curve at quiescence.

This study, published in The Astronomical Journal, provided the observational evidences on the thermal-tidal instability (TTI) model, and challenged the “standard” cataclysmic variables (CV) evolution theory.

SO appearing in the accreting white dwarf system is a quasi-periodic transient extensively investigated since the early years, due to the large amplitude inferior to that of supernova. However, the traditional ground-based observations cannot carry out any systematic study on a two-week SO due to the inconsistent outburst interval. Therefore, the uncertainty features of the SO can only be composed from the patches of the observation data.

In 2014, Kepler detects three continuous outbursts on TW Virgins. The complicated and irregular changes in light curve seem to appear randomly as usual.

In this study, Dr. DAI and his collaborators explored the complete, regular but unstable modulations from the quiescent data around a SO using their own data analysis method. The quiescent modulations were classified into three types with the different profile of the second hump. The researchers explained the apparently irregular modulations detected in most of cataclysmic variables at quiescence.

Based on these classified light curves, they constructed a synthetic quiescence-outburst evolution model of the disk around the white dwarf, showing a complete evolution progress lasting more than one month. TW Virginis arises from the quiescence to the SO at first, then decays from the SO to the quiescence, and finally goes back to the normal outburst.

This study confirms the disk luminosity distributions, hotspot sizes and positions predicted by the TTI model. It also challenges the "standard" CV evolution theory. "The theoretical mass transfer rates from the secondary are generally over-estimated for most of the accreting white dwarf systems," said Dr. DAI.