1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
2. School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
3. Zhejiang University-Purple Mountain Observatory Joint Research Center for Astronomy, Zhejiang University, Hangzhou 310027, China
4. Department of Physics, Purdue University, 525 Northwestern Ave, West Lafayette, IN 47907, USA