[1] NASA, Goddard Space Flight Ctr, SSAI, Greenbelt, MD 20771 USA
[2] NASA, Goddard Space Flight Ctr, Observat Cosmol Lab, Greenbelt, MD 20771 USA
[3] Univ Salamanca, E-37008 Salamanca, Spain
[4] Univ Calif Davis, Dept Phys, Davis, CA 95616 USA
[5] Univ Hawaii, Inst Astron, Honolulu, HI 96822 USA