[1] CALTECH, Dept Astron, Pasadena, CA 91125 USA
[2] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[3] Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England
[4] Princeton Univ, Dept Astrophys Sci, Princeton, NJ 08544 USA
[5] McKinsey & Co Inc, Seattle, WA USA