1. Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn , Nussallee 14-16, D-53115 Bonn , Germany
2. Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute , V Holešovičkách 2, CZ-180 00 Praha 8 , Czech Republic
3. National Astronomical Observatories, Chinese Academy of Sciences (NAOC) , 20A Datun Road, Chaoyang District Beijing, 100101 , People's Republic of China
4. School of Astronomy and Space Science, Nanjing University , Nanjing 210093 , People’s Republic of China
5. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education , Nanjing 210093 , People’s Republic of China