1. School of Astronomy and Space Science, Nanjing University , Nanjing 210093 , China
2. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education , Nanjing 210093 , China
3. Collaborative Innovation Center of Modern Astronomy and Space Exploration , Nanjing 210093 , China
4. Department of Astronomy and Astrophysics, University of California , 1156 high street, Santa Cruz, CA 95064 , USA
5. Apache Point Observatory, New Mexico State University , P.O. Box 59, Sunspot, NM 88349-0059 , USA
6. Sternberg Astronomical Institute, Moscow State University , Moscow 119992 , Russia