1. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA
3. Gemini Observatory, 670 North A'ohoku Place, Hilo, HI 96720, USA
4. Institute for Astronomy, University of Hawaii, Woodlawn Drive, Honolulu, HI 96822, USA
5. Institut d'Astrophysique Spatiale, UMR'CNRS 120, Bâtiment 121, Université de Paris XI, 91405 Orsay Cedex, France
6. McDonald Observatory and Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
7. Department of Space Science, Southwest Research Institute, PO Box 28510, San Antonio, TX 78228, USA