The main body of this work has been carried out at Armagh Observatory, supported by PPARC grant GR/K04613. We acknowledge support at Armagh in terms of both software and hardware by the STARLINK Project, funded by the UK PPARC. The work was completed while one of the authors (V. A.) held a National Research Council-NASA/GSFC Research Associateship at Goddard Space Flight Center.
We are indebted with P. Hauschildt and F. Allard for computing the model photosphere and the opacity table used as a base for our calculations. We are also grateful to G. Severino for providing the Na-H collisional rates, as well for a number of valuable and stimulating discussions.
We wish to thank S. D. Jordan for his useful comments on the manuscript.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998