1. Yunnan Observatories, Chinese Academy of Sciences, 650216 Kunming, PR China
2. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650216 Kunming, PR China
3. Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
4. Department of Astronomy, Oxford University, Oxford OX1 3RH, UK