1. Simons Fellow, Department of Astronomy, Columbia University, NY 10027, USA
2. Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
3. Subdepartment of Astrophysics, University of Oxford, Oxford, OX1 3RH, UK
4. Sagan Fellow, Department of Astronomy, University of Washington, Seattle, WA 98195, USA