1. Physics Department, University of Otago, Dunedin 9010, New Zealand
2. TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125, USA
3. Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720, USA
4. Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA