1. Department of Astronomy, University of Michigan , 1085 S. University Avenue, Ann Arbor, MI 48109 , USA
2. Department of Astronomy, Yale University , PO. Box 208101, New Haven, CT 06520 , USA
3. Institute of Astronomy, University of Cambridge , Madingley Road, CB3 0HA , UK
4. Department of Physics, University of Chicago , Chicago, IL 60637 , USA
5. Max-Planck-Insitut für extraterrestrische Physik , Giessenbachstrasse, D-85748 Garching , Germany