[1] Peking Univ, Kavli Inst Astron & Astrophys, Beijing 100871, Peoples R China
[2] Peking Univ, Dept Astron, Beijing 100871, Peoples R China
[3] Univ Sheffield, Dept Phys & Astron, Sheffield S3 7RH, S Yorkshire, England
[4] Chinese Acad Sci, Key Lab Opt Astron, Natl Astron Observ, Beijing 100012, Peoples R China