[1] Columbia Univ, Dept Phys, New York, NY 10027 USA
[2] Columbia Univ, Columbia Astrophys Lab, New York, NY 10027 USA
[3] Ctr Astro Space, Lebedev Phys Inst, Moscow 117810, Russia
[4] Leiden Univ, NL-2300 RA Leiden, Netherlands
[5] Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA
[6] Max Planck Inst Extraterr Phys, D-37075 Garching, Germany