[1] Space Telescope Sci Inst, Baltimore, MD 21218 USA
[2] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
[3] European Space Agcy, Div Astrophys, Dept Space Sci, F-75738 Paris 15, France
[4] Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA
[5] Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA
[6] Arizona State Univ, Dept Phys & Astron, Tempe, AZ 85287 USA
[7] New Mexico State Univ, Dept Astron, Las Cruces, NM 88003 USA
[8] Australian Natl Univ, Mt Stromlo & Siding Spring Observ, Weston, ACT 2611, Australia
[9] Univ Nacl Autonoma Mexico, Inst Astron, Morelia 58090, Michoacan, Mexico
[10] CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA