[1] Univ Florida, Dept Astron, Bryant Space Sci Ctr 211, Gainesville, FL 32611 USA
[2] Univ Zurich, Inst Theoret Phys, CH-8057 Zurich, Switzerland
[3] ETH, Dept Phys, CH-8093 Zurich, Switzerland
[4] Indiana Univ, Dept Astron, Bloomington, IN 47405 USA