1. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
2. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
3. Department of Astronomy, Columbia University, 550 W 120th Str, New York, NY 10027, USA
4. Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, D-72076 Tübingen, Germany