We are indebted to G. Ferland who provided us with his photoionization code CLOUDY. We thank the referee Chris Impey for his constructive remarks. This work has been supported by the Verbundforschung of the Bundesministerium für Bildung und Forschung under No. 50 OR 96 016.
© European Southern Observatory (ESO) 1999
Online publication: February 22, 1999