1. Department of Astronomy, Beijing Normal University, Beijing 100875, P. R. China
2. Department of Astronomy, University of Geneva, Ch. des Maillettes 51, CH-1290 Versoix, Switzerland
3. South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, P. R. China
4. Institute of Theoretical Physics, Shanxi University, Taiyuan 030006, P. R. China