1. Department of Astronomy, Tsinghua University, Beijing 100084, China
2. National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
3. School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
4. Chinese Academy of Sciences South America Center for Astronomy, Beijing 100101, China
5. Tianjin Astrophysics Center, Tianjin Normal University, Tianjin 300387, China
6. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748, Garching, Germany
7. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA