Zhu, L. Y, Yunnan Observatories, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, P.R. China Key Laboratory for the Structure and Evolution of Celestial Bodies, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, P. R. China. Physical Sciences Department, Graduate University of the Chinese Academy of Sciences, Yuquan Road 19, Sijingshang Block, 100049 Beijing, China