作者机构:[1]Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China, [2]National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
相关基金:Supported by the National Natural Science Foundation of China (Grant Nos. 10333030 and 10273025)The authors wish to thank Prof. Huang G L. for his helpful suggestions and supports.