The authors would like to thank the referee of this paper, Dr Xander Tielens, for a frank and far ranging exchange of views. CMW would like to thank the Institut d'Astrophysique de Paris for its hospitality during the initiation of this study. He also wishes to acknowledge travel support from ASI Grant ARS 66-96 and CNR Grant 97.00018.CT02.
© European Southern Observatory (ESO) 1999
Online publication: February 22, 1999