We thank M.P. Haynes for pointing out the paper of F.H. Briggs (1990) and for valuable discussions which stimulated this work. Further, we thank F.H. Briggs, M.M. Romanova, and an anonymous referee for valuable comments on this work. We thank I.L. Tregillis for critical checking of an early version of this work. This work was supported in part by NSF grant AST-9320068.
© European Southern Observatory (ESO) 1998
Online publication: September 17, 1998