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

Publications of the Astronomical Society of Japan, Jun 2013

We have studied the short-cadence Kepler light curve of an SU UMa star, V1504 Cyg, which covers a period of $$\sim$$ 630 d. All superoutbursts in V1504 Cyg have turned out to be of precursor-main types, 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 was initiated by a tidal instability (as evidenced by the growing superhump) as envisioned in the thermal-tidal instability (TTI) model proposed by Osaki (1989, PASJ, 41, 1005). We performed a power spectral analysis of the light curve of V1504 Cyg. One of the outstanding features is the appearance of a negative superhump extending over around 300 d, well over a supercycle. We found that the appearance of the negative superhump tends to decrease 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 Type S supercycles in the light curve of VW Hyi, a prototype SU UMa star, introduced by Smak (1985, Acta Astron., 35, 357). It is found that the Type L supercycle is the one accompanied by the negative superhump, and 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 the outburst interval, 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 the observed disk-radius variation in V1504 Cyg fits very well with a prediction of the TTI model.

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Osaki, Yoji, Kato, Taichi. The Cause of the Superoutburst in SU UMa Stars is Finally Revealed by Kepler Light Curve of V1504 Cygni, Publications of the Astronomical Society of Japan, 2013, DOI: 10.1093/pasj/65.3.50