Context. The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. Aims. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the O i IR triplet lines at 777 nm in different sets of spectra.Methods. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor S H that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semiempirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models. Results. With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor S H is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (S H = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D. Conclusions. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74 < A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 ± 0.02. All 1D non-LTE models give much lower oxygen abundances, by up to −0.15 dex. This is mainly a consequence of the assumption of a μ-independent microturbulence. An independent determination of the relevant collisional cross-sections is essential to substantially improve the accuracy of the oxygen abundance derived from the O i IR triplet.

We present new synthetic broad-band photometric colors for late-type giants based on synthetic spectra calculated with the PHOENIX model atmosphere code. The grid covers effective temperatures T eff = 3000 . . . 5000 K, gravities log g = −0.5 . . . +3.5, and metallicities [M/H] = +0.5 . . . −4.0. We show that individual broad-band photometric colors are strongly affected by model parameters such as molecular opacities, gravity, microturbulent velocity, and stellar mass. Our exploratory 3D modeling of a prototypical late-type giant shows that convection has a noticeable effect on the photometric colors too, as it alters significantly both the vertical and horizontal thermal structures in the outer atmosphere. The differences between colors calculated with full 3D hydrodynamical and 1D model atmospheres are significant (e.g., ∆(V −K) ∼ 0.2 mag), translating into offsets in effective temperature of up to ∼70 K. For a sample of 74 late-type giants in the Solar neighborhood, with interferometric effective temperatures and broad-band photometry available in the literature, we compare observed colors with a new PHOENIX grid of synthetic photometric colors, as well as with photometric colors calculated with the MARCS and ATLAS model atmosphere codes. We find good agreement of the new synthetic colors with observations and published T eff -color and color-color relations, especially in theDeviations from the observed trends in the T eff -color planes are generally within ±100 K for T eff = 3500 to 4800 K. Synthetic colors calculated with different stellar atmosphere models agree to ±100 K, within a large range of effective temperatures and gravities. The comparison of the observed and synthetic spectra of late-type giants shows that discrepancies result from the differences both in the strengths of various spectral lines/bands (especially those of molecular bands, such as TiO, H 2 O, CO) and the continuum level. Finally, we derive several new T eff -log g-color relations for late-type giants at solar-metallicity (valid for T eff = 3500 to 4800 K), based both on the observed effective temperatures and colors of the nearby giants, and synthetic colors produced with PHOENIX, MARCS and ATLAS model atmospheres.

Context. The cluster 47 Tuc is among the most metal-rich Galactic globular clusters and its metallicity is similar to that of metal-poor disc stars and open clusters. Like other globular clusters, it displays variations in the abundances of elements lighter than Si, which is generally interpreted as evidence of the presence of multiple stellar populations. Aims. We aim to determine abundances of Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order to study the evolution of light elements in this cluster and to put our results in perspective with observations of other globular and open clusters, as well as with field stars. Methods. We use medium resolution spectra obtained with the GIRAFFE spectrograph at the ESO 8.2 m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employ CO 5 BOLD hydrodynamical simulations to assess the impact of stellar granulation on the line formation and inferred abundances. Results. Our results confirm the existence of Na-O abundance anti-correlation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. At the same time, we find no convincing evidence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were detected reliably, A(Li) 3D NLTE = 1.78 ± 0.18 dex, appears to be significantly lower than what is observed in other globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abundance observed in 47 Tuc is about 0.1 dex lower than the lowest Li abundance observed among the un-depleted stars of the metal-poor open cluster NGC 2243. Conclusions. The correlations/anti-correlations among light element abundances confirm that chemical enrichment history of 47 Tuc was similar to that of other globular clusters, despite the higher metallicity of 47 Tuc. The lithium abundances in 47 Tuc, when put into context with observations in other clusters and field stars, suggest that stars that are more metal-rich than [Fe/H] ∼ −1.0 experience significant lithium depletion during their lifetime on the main sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with which the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity.

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 various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB (T eff ≈ 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 assessed 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 the CO 5 BOLD and 1D LHD 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, Ti i, Fe i, and Ni i, the abundance corrections strongly depend both on the 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-intermediate χ they quickly increase with decreasing metallicity, reaching in their extremes −0.6 to −0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Lines of neutral elements with higher ionization potentials (E ion 10 eV) generally behave very similarly to lines of ionized elements characterized by low ionization potentials (E ion 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 differences between the temperature profiles in the 3D and 1D model atmospheres. Abundance corrections of molecular lines are very sensitive to the 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 potential, the abundance corrections show little variation within the wavelength range studied here, 400−1600 nm. We also find that an approximate treatment of scattering in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thin, atmosphere) leads to 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.

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 Ba ii, and Eu ii), and molecules (CH, CO, C 2 , 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 CO 5 BOLD 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.