1. China University of Petroleum–Beijing at Karamay, Anding Road 355, 834000 Karamay, P.R. China
2. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650011, P.R. China
3. Yunnan Observatories, Chinese Academy of Sciences, P.O. Box 110, 650216 Kunming, P.R. China