We would like to thank the referee, Prof. P. Haensel, for many helpful comments and suggestions. We would also thank C.M. Chu for his discussion on hydrodynamical instabilities in protoneutron stars. This work was supported in part by the National Natural Science Foundation of China and in part by a RGC grant of Hong Kong.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998