1. Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
2. National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, China
3. European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
4. National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042, China
5. Department of Physics, Yunan University, 2 Cuihu North Road, Kunming 650091, China