The work at the CfA was made possible by support from the visiting scientist program of the Smithsonian Institution. XZ would like to thank L.J. Greenhill and A.S. Trotter for their help in the VLBA data processing. We thank P.T.P. Ho for suggestions to the manuscript. We appreciate the referee, M.-M. Mac Low, for his helpful comments.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000