1. Lecturer in astronomy at the Astronomisk Institut, Aarhus Universitet, DK 8000 Aarhus C, Denmark, and a research associate at NORDITA, Copenhagen, Denmark.
2. Lecturer in astronomy and applied mathematics at the Institute of Astronomy and the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 OHA, England.
3. Professor in the Department of Astrophysical, Planetary, and Atmospheric Sciences and a fellow of the Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, Colorado 80309.