[1] Max-Planck-Institut für Extraterrestrische Physik, PO Box 1312, Garching bei München 85741, Germany
[2] Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, BP 53, 38041 Grenoble Cedex 9, France
[3] CEA Saclay, DSM/DAPNIA/SAp, 91191 Gif-sur-Yvette Cedex, France
[4] Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, United States
[5] Department of Physics and Astronomy, University of Leicester, Leicester LEI 7RH, United Kingdom