1. Department of Physics, Liaoning University , Shenyang 110036, China
2. National Astronomical Observatories, Chinese Academy of Sciences , Beijing 100101, China
3. University of Chinese Academy of Sciences , Beijing 100049, China
4. Department of Physics, University of Helsinki , Gustaf Hällströmin katu 2, FI-00014 Helsinki, Finland
5. Shanghai Astronomical Observatory, Chinese Academy of Sciences , Shanghai 200030, China
6. South-Western Institute for Astronomy Research, Yunnan University , Kunming 650500, China
7. Shanghai Key Lab for Astrophysics, Shanghai Normal University , Shanghai 200234, China