[1] Space Telescope Sci Inst, 3700 San Martin Ave, Baltimore, MD 21218 USA
[2] Haverford Coll, Dept Phys & Astron, Haverford, PA 19041 USA
[3] Univ Calif San Diego, Dept Phys & Astron, La Jolla, CA 92093 USA
[4] Arizona State Univ, Sch Earth & Space Explorat, Tempe, AZ 85287 USA
[5] Univ Louisville, Dept Phys & Astron, Louisville, KY 40292 USA