The Cause of the Superoutburst in SU UMa Stars is Finally Revealed by Kepler Light Curve of V1504 Cygni

We have studied the SC (short cadence) Kepler light curve of an SU UMa star, V1504 Cyg, which extends for a period of about 630 d. All superoutbursts in V1504 Cyg have turned out to be of the precursor-main type and the superhump first appears near the maximum of the precursor. The superhumps grow smoothly from the precursor to the main superoutburst showing that the superoutburst is initiated by the tidal instability (as evidenced by growing superhump) as envisioned in the thermal-tidal instability (TTI) model proposed by Osaki (1989). We have performed power spectral analysis of the light curve of V1504 Cyg. One of outstanding features is an appearance of a negative superhump extending for around 300 d, well over a supercycle. We have found that an appearance of the negative superhump tends to reduce the frequency of occurrence of normal outbursts. Two types of supercycles are recognized in V1504 Cyg, which are similar to those of the Type L and S supercycles in the light curve of VW Hyi, a prototype SU UMa star, introduced by Smak (1985). It is found that the Type L supercycle is the one accompanied with the negative superhump and the Type S is that without the negative superhump. If we adopt a tilted disk as an origin of the negative superhump, two types of the supercycles are understood to be due to a difference in outburst intervals, which is in turn caused by a difference in mass supply from the secondary to different parts of the disk. The frequency of the negative superhump varies systematically during a supercycle in V1504 Cyg. This variation can be used as an indicator of the disk radius variation and we have found that observed disk radius variation in V1504 Cyg fits very well with a prediction of the TTI model.