The Staggergrid: A grid of 3D stellar atmosphere models. II. Horizontal and temporal averaging and spectral line formation
Abstract
Aims: We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmospheres computed with the Staggercode. The temporally and spatially averaged (hereafter denoted as ⟨3D⟩) models are explored in the light of local thermodynamic equilibrium (LTE) spectral line formation by comparing spectrum calculations using full 3D atmosphere structures with those from ⟨3D⟩ averages.
Methods: We explored methods for computing mean ⟨3D⟩ stratifications from the Staggergrid timedependent 3D radiative hydrodynamical atmosphere models by considering four different reference depth scales (geometrical depth, columnmass density, and two optical depth scales). Furthermore, we investigated the influence of alternative averages (logarithmic, enforced hydrostatic equilibrium, fluxweighted temperatures). For the line formation we computed curves of growth for Fe i and Fe ii lines in LTE.
Results: The resulting ⟨3D⟩ stratifications for the four reference depth scales can be very different. We typically find that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are highest, the differences become considerable and increase for higher T_{eff}, lower log g, and lower [Fe / H]. The differential comparison of spectral line formation shows distinctive differences depending on which ⟨3D⟩ model is applied. The averages over layers of constant columnmass density yield the best mean ⟨3D⟩ representation of the full 3D models for LTE line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to increasing interference by reversed granulation towards higher effective temperature, in particular at low metallicity. Appendix A is available in electronic form at http://www.aanda.orgMean ⟨3D⟩ models are available at the CDS via anonymous ftp to http://cdsarc.ustrasbg.fr (ftp://130.79.128.5) or via http://cdsarc.ustrasbg.fr/vizbin/qcat?J/A+A/560/A8 as well as at http://www.staggerstars.net
 Publication:

Astronomy and Astrophysics
 Pub Date:
 December 2013
 DOI:
 10.1051/00046361/201322252
 arXiv:
 arXiv:1307.3273
 Bibcode:
 2013A&A...560A...8M
 Keywords:

 convection;
 hydrodynamics;
 radiative transfer;
 line: formation;
 stars: abundances;
 stars: atmospheres;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 Accepted for publication in A&