1. Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
2. INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4. Yale Center for Astronomy and Astrophysics, 260 Whitney Avenue, New Haven, CT 06520, USA
5. Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
6. Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
7. Department of Physics and Astronomy, University of Hawaii at Hilo, 200 W. Kawili Street, Hilo, HI 96720, USA
8. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA