1. Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands
2. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands
3. Department of Physics and Astronomy, University College London, Gower St., London WC1E 6BT, UK
4. I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
5. Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA
6. Centre for Star and Planet Formation, Niels Bohr Institute & Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K., Denmark
7. Center for Space and Habitability, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
8. Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching, Germany