1. School of Physics and Technology, Wuhan University, Wuhan 430072, China
2. Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
3. National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing, 100101, China
4. School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, China
5. Department of Astronomy, Beijing Normal University, Beijing, 100875, China
6. Department of Physics and Astronomy, University of Southampton, Highfield SO17 1BJ, UK