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We utilize state-of-the-art 3D hydrodynamical and classical 1D stellar modelatmospheres to study the influence of convection on the formation properties ofvarious atomic and molecular spectral lines in the atmospheres of four redgiant stars, located close to the base of the red giant branch, RGB($T_{\mathrm eff}\approx5000$ K, $\log g=2.5$), and characterized by fourdifferent metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. The role of convectionin the spectral line formation is assessed with the aid of abundancecorrections, i.e., the differences in abundances predicted for a givenequivalent width of a particular spectral line with the 3D and 1D modelatmospheres. We find that for lines of certain neutral atoms the abundancecorrections strongly depend both on metallicity of a given model atmosphere andthe line excitation potential. While abundance corrections for all lines ofboth neutral and ionized elements tend to be small at solar metallicity, forlines of neutral elements with low ionization potential and low-to-intermediate$\chi$ they quickly increase with decreasing metallicity, reaching in theirextremes to -0.6...-0.8 dex. In all such cases the large abundance correctionsare due to horizontal temperature fluctuations in the 3D hydrodynamical models.Abundance corrections of molecular lines are very sensitive to metallicity ofthe underlying model atmosphere and may be larger (in absolute value) than -0.5dex at [M/H] = -3.0 (-1.5 dex in the case of CO). We also find that anapproximate treatment of scattering in the 3D model calculations leads to theabundance corrections that are altered by less than ~0.1 dex, both for atomicand molecular (CO) lines, with respect to the model where scattering is treatedas true absorption throughout the entire atmosphere, with the largestdifferences for the resonance and low-excitation lines.

astronomy and astrophysics

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant  stars. III. Line formation in the atmospheres of giants located close to the  base of RGB