[1] Stockholm Observ, SE-13336 Saltsjobaden, Sweden
[2] Lawrence Livermore Natl Lab, IGPP, Livermore, CA 94550 USA
[3] Univ Calif Davis, Dept Phys, Davis, CA 95616 USA
[4] Univ Oslo, Inst Theoret Astrophys, N-0315 Oslo, Norway
[5] Univ London Imperial Coll Sci Technol & Med, Blackett Lab, Astrophys Grp, London SW7 2BZ, England