In order to investigate the importance of NLTE effects on the
models presented in this paper, we have calculated small number of
representative NLTE models with parameters roughly appropriate for the
Sun and Vega (
Lyr). We include a large number of levels and
lines in NLTE in order to make the model calculations as realistic as
possible. In addition, species that we do not include explicitly
in NLTE are included in the LTE approximation in order to
describe their effects on the model structure and the spectrum,
see [Hauschildt et al.(1997b)Hauschildt, Allard, Alexander, and
Baron] and [Aufdenberg et al.(1997a)Aufdenberg, Hauschildt, Shore, and
Baron] and references therein for
details of the method. This allows us to determine which models
require NLTE and for which models LTE provides an
adequate description of line formation. In the outer, optically thin,
regions of the atmosphere, NLTE effects will always become important
(even in the absence of a chromospheric temperature rise), but for models
of stellar photospheres, it is crucial to know if NLTE effects are
important in the line forming regions. We have computed detailed NLTE
models for two cases: for relatively cool, G-type stars we chose model
parameters approximately appropriate for the Sun; and for hotter stars,
where we chose model parameters appropriate for Vega. The models serve
to demonstrate the effects of NLTE on the structure and spectra, we did
not make any attempt to produce detailed fits to either the Sun or Vega.
The models include a large number of NLTE species and use
the same version of PHOENIX and the same input physics
as the NextGen models.