1. INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
2. Dipartimento di Fisica e Astronomia-Universitá di Bologna, via Gobetti 93/2, I-40129 Bologna, Italy
3. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
4. Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St, Pasadena, California, CA 91101, USA
5. Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
6. California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125, USA
7. Department of Astronomy and Astrophysics, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA
8. Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, USA
9. Department of Astronomy and Astrophysics, The Pennsylvania State University, 403 Davey Lab, University Park, PA 16802, USA
10. Department of Physics, Florida Atlantic University, 777 Glades Rd, Boca Raton, FL 33431, USA
11. IPAC, Mail Code 314-6, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
12. Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
13. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
14. Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA