1. INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Florence, Italy
2. SRON Netherlands Institute for Space Research, Landleven 12, NL-9747 AD Groningen, the Netherlands
3. Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 970 AV, Groningen, the Netherlands
4. Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, I-50125 Florence, Italy
5. Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, D-85748 Garching, Germany