[1] Tsinghua Univ, Dept Astron, Beijing 100084, Peoples R China
[2] Univ Oxford, Sub Dept Astrophys, Dept Phys, Denys Wilkinson Bldg,Keble Rd, Oxford OX1 3RH, England
[3] Chinese Acad Sci, Natl Astron Observ, 20A Datun Rd, Beijing 100101, Peoples R China
[4] Beijing Normal Univ, Inst Frontiers Astron & Astrophys, Beijing 102206, Peoples R China
[5] Univ Chinese Acad Sci, Sch Astron & Space Sci, Beijing 100049, Peoples R China