F. Bocchino is grateful for stimulating discussions with J. Raymond at CfA in Cambridge, MA, USA, and with R. Bandiera at the Osservatorio Astrofisico di Arcetri, Italy. The authors wish to thank S. Serio, F. Reale and G. Peres for suggestions and comments. This work was supported by Ministero per l'Univerisita' e per la Ricerca Scientifica.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999