1. Center for Star and Planet Formation, the Niels Bohr Institute and the Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen, Denmark
2. Rosseland Centre for Solar Physics, Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, N-0315 Oslo, Norway
3. Anton Pannekoek Institute, University of Amsterdam, Science Park 904, C4.149, NL-1098 XH Amsterdam, the Netherlands