Geometry of giant star model atmospheres: a consistency test

Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden e-mail: [Ulrike.Heiter;Kjell.Eriksson]@astro.uu.se

Received: 27 January 2006
Accepted: 9 March 2006

Abstract

Aims.We investigate the effect of a geometric inconsistency in the calculation of synthetic spectra of giant stars.

Methods.Spectra computed with model atmospheres calculated in spherical geometry while using the plane-parallel approximation for line formation calculations (), as well as the fully plane-parallel case (), are compared to the consistently spherical case ().

Results.We present abundance differences for solar metallicity models with T_eff ranging from 4000 to 6500 K and from 0.5 to 3.0 [cgs]. The effects are smaller for calculations (-0.1 dex in the worst case) than for the case (up to +0.35 dex for minority species and at most -0.04 dex for majority species), both with respect to the case. In the case the differences increase slightly with temperature, while in the case they show a more complex behaviour. In both cases the effects decrease with increasing and increase with equivalent width.

Conclusions.Within the parameter range of F, G and K giants, consistency seems to be less important than using a spherical model atmosphere. The abundance differences due to sphericity effects presented here can be used for error estimation in abundance studies relying on plane-parallel modelling.