Introduction

In a recent paper we have presented a grid of plane parallel model atmospheres for dwarf stars ($\log(g)\geq 3.5$) and in the temperature range $3000\leq \hbox{$\,T_{\rm eff}$}\leq 10000\,{\rm K}$. The NG models are intended for the analysis and modeling of main sequence stars and, where applicable, sub-giants. Spherical extension effects become more important for smaller gravities, which is the reason why the NG-grid stopped at $\log(g)= 3.5$. In this paper, we present an extension of the NG grid to smaller gravities by including the effects of spherical geometry in the model calculations. This includes, for example, the calculation of the hydrostatic structure in spherical geometry and the effects of spherical radiative transfer on the model atmospheres.

In order to retain compatibility with the NG grid, the NG-giant grid presented here uses the same parameterization of the element abundances as the NG grid. Therefore, the models presented here are not applicable to chemically peculiar stars such as, e.g., Carbon stars. However, there are many possible applications for these models. For example, the Period-luminosity (PL) relation of Cepheids is of fundamental importance for the determination of the extragalactic distance scale. have used the atmosphere models presented in this paper to connect evolutionary models to observed colors for Cepheids with very good results.

In the next section we give a brief overview over the model construction and the differences from the NG grid. Then we discuss the results, in particular the effects of spherical symmetry and we end with a summary of the paper.

have used the atmosphere models presented in this paper to connect evolutionary models to observed colors for Cepheids with very good results.

In the next section we give a brief overview over the model construction and the differences from the NG grid. Then we discuss the results, in particular the effects of spherical symmetry and we end with a summary of the paper.