1. Physics Department and Tsinghua Center for Astrophysics, Tsinghua University , Beijing 100084 , China
2. Beijing Planetarium, Beijing Academy of Science and Technology , Beijing 100044 , China
3. Department of Astronomy, University of California , Berkeley, CA 94720-3411 , USA
4. INAF Osservatorio Astronomico di Padova , Vicolo dell’Osservatorio 5, 35122 Padova , Italy
5. Institute of Space Sciences (ICE, CSIC) , Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona , Spain
6. The School of Physics and Astronomy, Tel Aviv University , Tel Aviv 69978 , Israel
7. Xinjiang Astronomical Observatory, Chinese Academy of Sciences , Urumqi, Xinjiang 830011 , China
8. School of Astronomy and Space Science, University of Chinese Academy of Sciences , Beijing 100049 , China