Optical Flares and Dark-Spot Activity of Red-Dwarf Close Binary Stars

Zhu Liying, zhuly@ynao.ac.cn, Yunnan Astronomical Observatory, China
Qian Shengbang, qsb@ynao.ac.cn, Yunnan Astronomical Observatory
Liu Liang, Yunnan Astronomical Observatory
He Jiajia, Yunnan Astronomical Observatory
Zhao Ergang, Yunnan Astronomical Observatory
Wang Jingjing, Yunnan Astronomical Observatory
Jiang Linqiao, Yunnan Astronomical Observatory

Abstract
At least 80% of all stars in the Galaxy are Red dwarfs (M-type stars). However, the physical properties especially magnetic activities of these most common stars are poorly understood. The serious problem in this field is the significant discrepancy between the theoretical and observational mass-radius relations, i.e., the observed radius is about 10% larger than that computed from theoretical models. Some authors have realized that this discrepancy could attribute to the strong magnetic activities of the components in short-period red-dwarf eclipsing binaries, but observational evidence is lack. We have monitored a set of close binaries which contain at least one M-type star for several years by using several telescopes in China. In this talk, I will summarize some progresses of our research group at YNAO. They are as following: (1) a group of flare events were discovered. The most special case comes from the observations of the M-type eclipsing binary CU Cnc. We found the largest-amplitude flare in R band from this system with the amplitude about 0.52 magnitudes and the duration time about 73 minutes. Meantime, it is the first time to detect quasi-periodic oscillation during and after the flaring energy release of a close eclipsing binary star. This quasi-periodic oscillation was also observed in subsequent solar fare. (2) By analyzing the asymmetry light curves, the coverage of the dark spot on red-dwarf stars were investigated. It is shown that the coverage of the dark spot on red dwarfs in close binary is much higher than that of the Sun. (3) By long-term photometric monitoring, magnetic activity cycles of those red dwarfs were studied. These results will shed light on the evolution and magnetic activity of the most popular stars in the Universe.