We are grateful to M.A. Malkov for very helpful discussions. This work was supported by the University of Minnesota Supercomputing Institute, by NSF grant AST-9619438 and by NASA grant NAG5-5055. HK's work was supported by Korea Research Foundation Grant (KRF99-015-DI0114).
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001