Three-dimensional hydrodynamical CO⁵BOLD model atmospheres of red giant stars

II. Spectral line formation in the atmosphere of a giant located near the RGB tip^⋆

¹ Vilnius University Institute of Theoretical Physics and Astronomy, A. Goštauto 12, 01108 Vilnius, Lithuania
e-mail: arunas.kucinskas@tfai.vu.lt; augustinas.ivanauskas@tfai.vu.lt; jonas.klevas@tfai.vu.lt; dainius.prakapavicius@tfai.vu.lt
² Vilnius University Astronomical Observatory, M. K. Čiurlionio 29, 03100 Vilnius, Lithuania
e-mail: Vidas.Dobrovolskas@ff.vu.lt
³ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: msteffen@aip.de
⁴ ZAH Landessternwarte Königstuhl, 69117 Heidelberg, Germany
e-mail: hludwig@lsw.uni-heidelberg.de; elcaffau@lsw.uni-heidelberg.de
⁵ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
e-mail: piercarlo.bonifacio@obspm.fr

Received: 16 August 2012
Accepted: 23 October 2012

Abstract

Aims. We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms (Li i, N i, O i, Na i, Mg i, Al i, Si i, Si ii, S i, K i, Ca i, Ca ii, Ti i, Ti ii, Cr i, Cr ii, Mn i, Fe i, Fe ii, Co i, Ni i, Zn i, Sr ii, Ba ii, and Eu ii), and molecules (CH, CO, C₂, NH, CN, and OH).

Methods. We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (T_eff = 3660 K, log g = 1.0, [M/H] = 0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO⁵BOLD and LHD stellar atmosphere codes, respectively. Both codes share the same atmospheric parameters, chemical composition, equation of state, and opacities, which allowed us to make a strictly differential comparison between the line formation properties predicted in 3D and 1D. The influence of convection on the spectral line formation was assessed with the aid of 3D–1D abundance corrections, which measure the difference between the abundances of chemical species derived with the 3D hydrodynamical and 1D classical model atmospheres.

Results. We find that convection plays a significant role in the spectral line formation in this particular red giant. The derived 3D–1D abundance corrections rarely exceed ± 0.1 dex when lines of neutral atoms and molecules are considered, which is in line with the previous findings for solar-metallicity red giants located on the lower RGB. The situation is different with lines that belong to ionized atoms, or to neutral atoms with high ionization potential. In both cases, the corrections for high-excitation lines (χ > 8 eV) may amount to Δ_3D−1D ~ −0.4 dex. The 3D–1D abundance corrections generally show a significant wavelength dependence; in most cases they are smaller in the near-infrared, at 1600–2500 nm.