1. INAF Osservatorio Astrofisico di Arcetri, Largo E. Fermi, I-50122 Firenze, Italy
2. Department of Physics, Sapienza University of Rome, I-00185 Rome, Italy
3. Department of Physics, University of Rome Tor Vergata, I-00133, Rome, Italy
4. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China