1. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3. Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
4. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
5. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada