1. School of Astronomy and Space Science, Nanjing University, Nanjing 210023, Jiangsu, China
2. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, China
3. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching bei München, Germany