1. Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, D-72076 Tübingen, Germany
2. Department of Astronomy and Astrophysics, Eberly College of Science, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
3. Physique Atomique et Astrophysique, Université de Mons – UMONS, B-7000 Mons, Belgium
4. IPNAS, Université de Liège, Sart Tilman, B-4000 Liège, Belgium
5. NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA