Nikolai Piskunov is thanked for help with the calculations of hydrogen profiles. The gf -values provided by Earle Luck are gratefully acknowledged. This work has benefited from financial support from the Swedish Natural Research Council, Robert A. Welch Foundation of Houston, Texas, and National Science Foundation (grants AST9315124 and AST9618414).
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998