1. Xinjiang Astronomical Observatory, CAS, 150, Science 1-Street, Urumqi, Xinjiang 830011, P. R. China
2. School of Astronomy and Space Science, Nanjing University, 22 Hankou Road, Nanjing, Jiangshu 210093, P. R. China
3. National Space Science Center, CAS, 1-Nanertiao, Zhongguancun, Haidian District, Beijing 100190, P. R. China