1. Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
2. School of Physics and Astronomy, Yunnan University, 2 North Cuihu Road, Kunming, Yunnan 650091, China
3. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-8046 Garching bei Muenchen, Germany
4. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District Beijing 100012, China