1. Department of Physics and Astronomy, McMaster University, 1280 Main St. W., Hamilton, ON L8S 4L8, Canada
2. Tsinghua Center for Astrophysics and Physics Department, Tsinghua University, Beijing 100084, China
3. CNRS, CNES, LAM, Aix Marseille University, UMR 7326, F-13388 Marseille 13, France
4. Shanghai Astronomical Observatory, Nandan Road 80, Shanghai 200030, China
5. Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
6. Physics Department, Zhejiang University, Hangzhou 310058, China