J. Klevas

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

Aims. We utilize state-of-the-art three-dimensional (3D) hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of vario us atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch , RGB (Teff ≈ 5000 K, log g = 2.5), and characterized by four different metallicities, [M/H] = 0.0,−1.0,−2.0,−3.0. Methods. The role of convection in the spectral line formation is asse ssed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. The 3D hydrodynamical and classical 1D model atmospheres used in this study were calculated with theCO 5 BOLD and 1DLHD codes, respectively. Identical atmospheric parameters, chemical composition, equation of state, and opacities were used with both codes, therefore allowing a strictly differential analysis of the line formation properties in the 3D and 1D models. Results. We find that for lines of certain neutral atoms, such as Mg i, Tii, Fei, and Nii, the abundance corrections strongly depend both on metallicity of a given model atmosphere and the line excitation potential, χ. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity (≤ ±0.1 dex), for lines of neutral elements with low ionization potential and low-to-intermediateχ they quickly increase with decreasing metallicity, reachi ng in their extremes to−0.6···− 0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamica l models. Lines of neutral elements with higher ionization potential s (Eion& 10 eV) generally behave very similarly to lines of ionized elements characterized with low ionization potentials ( Eion . 6 eV). In the latter case, the abundance corrections are small (generally, ≤ ±0.1 dex) and are caused by approximately equal contributions from the horizontal temperature fluctuations and di fferences between the temperature profiles in the 3D and 1D model atmospheres. A bundance corrections of molecular lines are very sensitive to metallicity of the underlying model atmosphere and may be larger (in absolute value) than∼ −0.5 dex at [M/H] =−3.0 (∼ −1.5 dex in the case of CO). At fixed metallicity and excitation potent ial, the abundance corrections show little variation withi n the wavelength range studied here, 400− 1600 nm. We also find that an approximate treatment of scatter ing in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thi n, atmosphere) leads to the abundance corrections that are altered by less than∼ 0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines.

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

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 (Lii, Ni, Oi, Nai, Mgi, Ali, Sii, Siii, Si, Ki, Cai, Caii, Tii, Tiii, Cri, Crii, Mni, Fei, Feii, Coi, Nii, Zni, Srii, Baii, and Euii), 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 (Teff=3660 K, log g=1.0, [M/H]=0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with theCO 5 BOLD andLHD stellar atmosphere codes, respectively. Both codes share t he 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 for mation 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 spectra l 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. I n both cases, the corrections for high-excitation lines (χ > 8 eV) may amount to �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.

Lithium spectral line formation in stellar atmospheres - The impact of convection and NLTE effects

Different simplified approaches are used to account for the non-local thermodynamic equilibrium (NLTE) effects with 3D hydrodynamical model atmospheres. In certain cases, chemical abundances are derived in 1D NLTE and corrected for the 3D effects by adding 3D-1D LTE abundance corrections (3D+NLTE approach). Alternatively, average model atmospheres are sometimes used to substitute for the full 3D hydrodynamical models. We tested whether the results obtained using these simplified schemes (i.e., 3D+NLTE, 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 [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 line by assessing strengths of synthetic 670.8 nm line profiles, computed using 3D/1D NLTE/LTE approaches. 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. 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 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- 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.

Chemical abundances in metal-poor giants: limitations imposed by the use of classical 1D stellar atmosphere models

In this work we have used 3D hydrodynamical (CO5BOLD) and 1D hydrostatic (LHD) stellar atmosphere models to study the importance of convection and horizontal temperature inhomogeneities in stellar abundance work related to late-type giants. We have found that for a number of key elements, such as Na, Mg, Si, Ca, Ti, Fe, Ni, Zn, Ba, Eu, differences in abundances predicted by 3D and 1D models are typically minor (< 0.1 dex) at solar metallicity. However, at [M/H] = -3 they become larger and reach to -0.5...-0.8 dex. In case of neutral atoms and fixed metallicity, the largest abundance differences were obtained for the spectral lines with lowest excitation potential, while for ionized species the largest 3D-1D abundance differences were found for lines of highest excitation potential. The large abundance differences at low metallicity are caused by large horizontal temperature fluctuations and lower mean temperature in the outer layers of the 3D hydrodynamical model compared with its 1D counterpart.

Abundances of Na, Mg, and K in the atmospheres of red giant branch stars of Galactic globular cluster 47 Tucanae

We study the abundances of Na, Mg, and K in the atmospheres of 32 RGB stars in the Galactic globular cluster (GGC) 47 Tuc, with the goal to investigate the possible existence of Na-K and Mg-K correlations/anti-correlations, similar to those that were recently discovered in two other GGCs, NGC 2419 and 2808. The abundances of K, Na, and Mg were determined using high-resolution 2df spectra obtained with the AAT. The 1D NLTE abundance estimates were obtained using 1D hydrostatic ATLAS9 model atmospheres and spectral line profiles synthesized with the MULTI package. We also used 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance corrections,

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - V. Oxygen abundance in the metal-poor giant HD 122563 from OH UV lines

\Delta_{\rm 3D-1D~LTE}

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - VI. First chromosphere model of a late-type giant

, for the spectral lines of Na, Mg, and K used in our study. These abundance corrections were used to understand the role of convection in the formation of spectral lines, as well as to estimate the differences in the abundances obtained with the 3D hydrodynamical and 1D hydrostatic model atmospheres. The average element-to-iron abundance ratios and their RMS variations due to star-to-star abundance spreads determined in our sample of RGB stars were

Abundances of Mg and K in the atmospheres of turn-off starsin Galactic globular cluster 47 Tucanae

\langle{\rm [Na/Fe]}\rangle^{\rm 1D~NLTE}=0.42\pm0.13

Can we trust elemental abundances derived in late-type giants with the classical 1D stellar atmosphere models?

,

Abundance of zinc in the red giants of Galactic globular cluster 47 Tucanae

\langle{\rm [Mg/Fe]}\rangle^{\rm 1D~NLTE}=0.41\pm0.11