Lithium spectral line formation in stellar atmospheres

The impact of convection and NLTE effects

¹ Institute of Theoretical Physics and Astronomy, Vilnius University, A. Goštauto 12, 01108 Vilnius, Lithuania
e-mail: jonas.klevas@tfai.vu.lt
² Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁴ ZAH Landessternwarte Königstuhl, 69117 Heidelberg, Germany

Received: 24 April 2015
Accepted: 23 October 2015

Abstract

Aims. Because of the complexities involved in treating spectral line formation in full 3D and non-local thermodynamic equilibrium (NLTE), different simplified approaches are sometimes used to account for the NLTE effects with 3D hydrodynamical model atmospheres. In certain cases, chemical abundances are derived in 1D NLTE and then corrected for the 3D effects by adding 3D–1D LTE (Local Thermodynamic Equilibrium, LTE) abundance corrections (3D+NLTE approach). Alternatively, average ⟨3D⟩ model atmospheres are sometimes used to substitute for the full 3D hydrodynamical models.

Methods. In this work we tested whether the results obtained using these simplified schemes (3D+NLTE, ⟨3D⟩ NLTE) may reproduce those derived using the full 3D NLTE computations. The tests were made using 3D hydrodynamical CO⁵BOLD model atmospheres of the main sequence (MS), main sequence turn-off (TO), subgiant (SGB), and red giant branch (RGB) stars, all at two metallicities, [ M / H ] = 0.0 and −2.0. Our goal was to investigate the role of 3D and NLTE effects on the formation of the 670.8 nm lithium resonance line. This was done by assessing differences in the strengths of synthetic 670.8 nm line profiles, which were computed using 3D/1D NLTE/LTE approaches.

Results. Our results show that Li 670.8 nm line strengths obtained using different methodologies differ only slightly in most of the models at solar metallicity studied here. However, the line strengths predicted with the 3D NLTE and 3D+NLTE approaches become significantly different at subsolar metallicities. At [ M / H ] = −2.0, this may lead to (3D NLTE) – (3D+NLTE) differences in the predicted lithium abundance of ~0.46 and ~0.31 dex in the TO and RGB stars respectively. On the other hand, NLTE line strengths computed with the average ⟨3D⟩ and 1D model atmospheres are similar to those obtained with the full 3D NLTE approach for MS, TO, SGB, and RGB stars, at all metallicities; 3D − ⟨3D⟩ and 3D − 1D differences in the predicted abundances are always less than ~0.04 dex and ~0.08 dex, respectively. However, neither of the simplified approaches can reliably substitute 3D NLTE spectral synthesis when precision is required.