This research program is supported by a RGC grant of Hong Kong Government and a CRCG grant of The University of Hong Kong, and partially supported by NSF of P.R.C.. We thank J. Gil for his useful suggestions and P.K. Mackeown for a critical reading. The kind help of L. Zhang is highly appreciated.
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998