[1] Univ Oslo, Inst Theoret Astrophys, N-0315 Oslo, Norway
[2] Univ Roma Tor Vergata, Dipartimento Fis, I-00133 Rome, Italy
[3] Max Planck Inst Astrophys, D-85741 Garching, Germany
[4] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
[5] Univ Oslo, Ctr Math Applicat, N-0316 Oslo, Norway
[6] Univ Warsaw Observ, PL-00478 Warsaw, Poland