1. National Astronomical Observatories, Chinese Academy of Sciences, 20 Datun Road, Beijing 100101, China
2. Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
3. School of Astronomy and Space sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
4. Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
5. Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA