A. Kučinskas

Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tucanae

Context. 47 Tuc is among the most metal-rich Galactic globular clusters and its metallicity is similar to that of metal-poor disc s tars and open clusters. Like other globular clusters, it display s variations in the abundances of elements lighter than Si, which is generally interpreted as evidence for the presence of multiple stella r 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 persp ective 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 8.2m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employCO 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-cor relation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. At the same time, we find no convincing ev idence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars whereLi lines were detected reliably,h A(Li)3D NLTEi= 1.78± 0.18 dex, appears to be significantly lower than what is observed in oth er globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abu ndance 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 t hat chemical enrichment history of 47 Tuc was similar to that of other globular clusters, despite the h igher 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 m ain sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with whi ch the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity.

The photospheric solar oxygen project - IV. 3D-NLTE investigation of the 777 nm triplet lines

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. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the OI IR triplet lines at 777 nm in different sets of spectra with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTETD. The idea is to simultaneously derive the oxygen abundance,A(O), and the scaling factor SH that describes the cross-sections for inelastic collisions with neutral hydrogen relative the classical Drawin formula. The best fit of the center-to-limb variation of the triplet lines achieved with the CO5BOLD 3D solar model is clearly of superior quality compared to the line profile fits obtained with standard 1D model atmospheres. Our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 +/- 0.02, with the scaling factor SH in the range between 1.2 and 1.8. 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

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - I. Atmospheric structure of a giant located near the RGB tip

mu

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

-independent microturbulence.

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

We investigate the character and role of convection in the atmosphere of a prototypical red giant located close to the red giant branch (RGB) tip with atmospheric parameters, Teff=3660K, log(g)=1.0, [M/H]=0.0. Differential analysis of the atmospheric structures is performed using the 3D hydrodynamical and 1D classical atmosphere models calculated with the CO5BOLD and LHD codes, respectively. All models share identical atmospheric parameters, elemental composition, opacities and equation-of-state. We find that the atmosphere of this particular red giant consists of two rather distinct regions: the lower atmosphere dominated by convective motions and the upper atmosphere dominated by wave activity. Convective motions form a prominent granulation pattern with an intensity contrast (~18%) which is larger than in the solar models (~15%). The upper atmosphere is frequently traversed by fast shock waves, with vertical and horizontal velocities of up to Mach ~2.5 and ~6.0, respectively. The typical diameter of the granules amounts to ~5Gm which translates into ~400 granules covering the whole stellar surface. The turbulent pressure in the giant model contributes up to ~35% to the total (i.e., gas plus turbulent) pressure which shows that it cannot be neglected in stellar atmosphere and evolutionary modeling. However, there exists no combination of the mixing-length parameter and turbulent pressure that would allow to satisfactorily reproduce the 3D temperature-pressure profile with 1D atmosphere models based on a standard formulation of mixing-length theory.

Galactic globular cluster 47 Tucanae: new ties between the chemical and dynamical evolution of globular clusters?

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.

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

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.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - IV. Oxygen diagnostics in extremely metal-poor red giants with infrared OH lines

Context. It is generally accepted today that Galactic globular clusters (GGCs) consist of at least two generations of stars that are different in their chemical composition and perhaps age. However, knowledge about the kinematical properties of these stellar generations, which may provide important information for constraining evolutionary scenarios of the GGCs, is still limited. Aims. We study the connections between chemical and kinematical properties of different stellar generations in the Galactic globular cluster 47 Tuc. Methods. To achieve this goal, we used abundances of Li, O, and Na determined in 101 main sequence turn-off (TO) stars with the aid of 3D hydrodynamical model atmospheres and NLTE abundance analysis methodology. We divided our sample TO stars into three groups according to their position in the [Li/Na] ‐ [Na/O] plane to study their spatial distribution and kinematical properties. Results. We find that there are statistically significant radial dependencies of lithium and oxygen abundances, A(Li) and A(O), as well as that of [Li/Na] abundance ratio. Our results show that first-generation stars are less centrally concentrated and dynamically hotter than stars belonging to subsequent generations. We also find a significant correlation between the velocity dispersion and O and Na abundance, and between the velocity dispersion and the [Na/O] abundance ratio.

Barium abundance in red giants of NGC 6752 - Non-local thermodynamic equilibrium and three-dimensional 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. nWe 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. nOur 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.

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

Context. Although oxygen is an important tracer of Galactic chemical evolution, measurements of its abundance in the atmospheres of the oldest Galactic stars are still scarce and rather imprecise. This is mainly because only a few spectral lines are available for the abundance diagnostics. At the lowest end of the metallicity scale, oxygen can only be measured in giant stars and in most of cases such measurements rely on a single forbidden [O i] 630 nm line that is very weak and frequently blended with telluric lines. Although molecular OH lines located in the ultraviolet and infrared could also be used for the diagnostics, oxygen abundances obtained from the OH lines and the [O i] 630 nm line are usually discrepant to a level of ∼0.3−0. 4d ex. Aims. We study the influence of convection on the formation of the infrared (IR) OH lines and the forbidden [O i] 630 nm line in the atmospheres of extremely metal-poor (EMP) red giant stars. Our ultimate goal is to clarify whether a realistic treatment of convection with state-of-the-art 3D hydrodynamical model atmospheres may help to bring the oxygen abundances obtained using the two indicators into closer agreement. Methods. We used high-resolution (R = 50000) and high signal-to-noise ratio (S /N ≈ 200−600) spectra of four EMP red giant stars obtained with the VLT CRIRES spectrograph. For each EMP star, 4‐14 IR OH vibrational-rotational lines located in the spectral range of 1514−1548 and 1595−1632 nm were used to determine oxygen abundances by employing standard 1D local thermodynamic equilibrium (LTE) abundance analysis methodology. We then corrected the1D LTE abundances obtained fromeach individual OHline for the 3D hydrodynamical effects, which was done by applying 3D‐1D LTE abundance corrections that were determined using 3D hydrodynamical CO 5 BOLD and 1D hydrostatic LHD model atmospheres. Results. We find that the influence of convection on the formation of [O i] 630 nm line in the atmospheres of EMP giants studied here is minor, which leads to very small 3D‐1D abundance corrections (Δ3D−1D ≤− 0.01 dex). On the contrary, IR OH lines are strongly affected by convection and thus the abundance corrections for these lines are significant, Δ3D−1D ≈− 0.2 ···− 0.3 dex. These abundance corrections do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained using IR OH lines into better agreement with those determined from the [O i] 630 nm line. Since in the EMP red giants IR OH lines are typically at least a factor of two stronger than the [O i] line, OH lines may be useful indicators of oxygen abundances in the EMP stars, provided that the analysis is based on 3D hydrodynamical model atmospheres.