1. South-Western Institute for Astronomy Research, Yunnan University, Chenggong District, Kunming 650500, P. R. China
2. Department of Astronomy, Beijing Normal University, Beijing 100875, P. R. China
3. Department of Astronomy, Yunnan University, Chenggong District, Kunming 650500, P. R. China
4. Department of Astronomy, China West Normal University, Nanchong 637009, P. R. China
5. Department of Astronomy, Peking University, Beijing 100871, P. R. China