1. National Astronomical Observatories, Chinese Academy of Sciences , Beijing 100101, China
2. University of Chinese Academy of Sciences , Beijing 100049, China
3. Beijing No. 25 High School , Beijing 100006, China
4. Tianjin Astrophysics Center, Tianjin Normal University , Tianjin 300387, China
5. Department of Physics and Astronomy, University of Wyoming , Laramie, WY 82071, USA