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Context

Neutral sodium is an important tracer of the Galactic chemical evolution, a powerful diagnostic of different stellar populations, and the subject of detailed studies of exoplanet atmospheres via transmission spectroscopy.

Aims

This work aims to study and quantify the errors in stellar analyses of Na i lines caused by the use of one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE).

Methods

We studied the line formation of nine Na i lines in FGK dwarfs and giants via, for the first time, 3D non-LTE (NLTE) radiative transfer post-processing with the code Balder on 3D radiation hydrodynamic stellar atmospheres from the Stagger grid spanning Teff = 4000-6500 K, log g = 1.5-5.0, and [Fe/H]= -4 to +0.5.

Results

We find that the 3D NLTE abundance corrections relative to 1D LTE tend to be negative and more positive than the corresponding 1D NLTE corrections. This reflects more efficient overionisation in the steeper temperature gradient of the 3D models. The corrections are typically less severe than -0.1 dex for weak lines, but they become more extreme for saturated lines in low-gravity giants (log g <= 2.0), even reaching -0.7 dex. However, for the D resonance lines, the 3D NLTE corrections relative to 1D LTE become slightly positive at the lowest metallicities in our grid, typically around +0.05 dex at [Fe/H] = -4.

Conclusions

We made our 3D NLTE grid, together with interpolation routines based on radial basis functions and fully connected feedforward neural networks, publicly available. This will enable more accurate determination of sodium abundances in present and forthcoming stellar spectroscopic surveys, particularly for metal-poor stars, as well as a better characterisation of the Na i D lines in exoplanet atmospheres.

Context. Silver is a key tracer of the weak r-process in late-type stars. However, when the assumption of local thermodynamic equilibrium is relaxed, accurate abundance determinations become even more sensitive to complete sets of reliable transition data.

Aims. The aim of this work is to provide accurate and extensive results of excitation energies, radiative transitions, and hyperfine data for Ag I.

Methods. The multiconfiguration Dirac-Hartree-Fock (MCDHF) and relativistic coupled-cluster (RCC) methods were used in the present work. The quantitative and qualitative evaluation approach was applied to the MCDHF transition rates to estimate uncertainties using the terminology of the National Institute of Science and Technology Atomic Spectroscopic Database (NIST ASD).

Results. Excitation energies, transition data, and hyperfine structure constants were calculated for 18 states up to 4d¹⁰8s. Fifty-seven electric dipole (E1) transition rates and weighted oscillator strengths were computed, and their uncertainties were estimated to belong to the following NIST ASD classes: four in AA , 12 in A+, five in A, 13 in B+, six in B, and four in C+ with AA ≤ 1%, A+ ≤ 2%, A ≤ 3%, B+ ≤ 7%, B ≤ 10%, C+ ≤ 18%. The remaining transitions, mainly weak transitions involving the 4d⁹5s² states, were estimated to be in the E class >50%. The computed lifetimes from both the MCDHF and RCC methods are in good mutual agreement and mostly fall within the error bars of available experimental values from laser-induced fluorescence measurements. The 4d⁹5s^{2 2}D_5/2 metastable state, important for establishing the ionization balance, decay through an E2 transition to the ground state. The calculated lifetime is 163 ms. The computed hyperfine interaction constants from the MCDHF and RCC methods are in good agreement and compare well with the scattered experimental constants.

Silver is an important light neutron-capture element whose stellar abundances can help constrain the origin of the weak r-process. The Sun is an important reference point for such studies; moreover, being a moderately volatile element in CI chondrites, the solar silver abundance is interesting as a diagnostic for the debated Sun-CI abundance versus condensation temperature trend. These studies require accurate silver abundances that go beyond the commonly used assumptions of one-dimensional (1D) atmospheres and local thermodynamic equilibrium (LTE); however, no consistent 3D non-LTE analysis of silver has been available to date. We present a new Ag I model atom built from carefully curated radiative and collisional data, including newly computed oscillator strengths using an ab initio multi-configurational Hartree-Fock method and inelastic hydrogen collision rates based on a combined asymptotic and free-electron-model approach. We assessed modelling uncertainties via targeted sensitivity tests, finding the results to be most sensitive to the hydrogen-collision data. Applying the model to the solar Ag I 328 and 338 nm resonance lines, we find severe positive-abundance corrections from coupled 3D and non-LTE effects. Using revised equivalent-width measurements, we derive a recommended solar 3D non-LTE silver abundance of log ε_Ag = 1.15 ± 0.08. This is an increase of 0.19dex relative to the current reference value. Our ab initio model significantly reduces the discrepancy with the meteoritic value from 0.25 to 0.06 dex; moreover, this residual offset is consistent with what was recently reported for other moderately volatile elements. The Sun provides the benchmark test for the first silver non-LTE model atom presented here. In subsequent work, this model will be applied to determine 3D non-LTE silver abundances in metal-poor dwarfs and giants, enabling improved constraints on Galactic chemical evolution and weak r-process nucleosynthesis.

The multiconfiguration Dirac-Hartree-Fock (MCDHF) and relativistic configuration interaction (RCI) methods are used to provide excitation energies, radiative transition data, lifetimes, Landeg-factors, hyperfine interaction constants and isotope shift parameters for the 99 lowest levels of configurations 1s (2)2snl (n <= 7) + 1s (2)2p (2) in beryllium. Compared with available experimental excitation energies, the average difference with the standard deviation is 7.08 +/- 1.14 cm(-1) (0.011% +/- 0.003%), which demonstrates the excellent theory-observation agreement. The uncertainties of the transition rates are estimated based on two independent methods. The present MCDHF/RCI oscillator strengths and those obtained from the explicitly correlated Gaussian method all agree within 2%, except for four transitions affected by strong cancellation effects. For lifetimes, hyperfine splittings and isotope shifts, the present MCDHF/RCI results show good agreement with the few available experimental values, supporting the reliability of our predictions for many states lacking experimental measurements. These comprehensive results can be used in line identification and diagnostics of astrophysical plasmas.

Context: The stellar origin of the elements molybdenum (Mo, Z = 42) and ruthenium (Ru, Z = 44) is still a matter of debate. Studying their abundances provides valuable insights into nucleosynthesis processes and the broader evolution of neutron-capture elements.

Aims: The aim of this work is to present new observations of Mo and Ru, together with nearby neutron-capture elements strontium (Sr) and zirconium (Zr) for a new sample of 154 giant stars, located in the Galactic disk with metallicities -1 < [Fe/H] < +0.3.

Methods: The abundances were determined under the assumption of the local thermodynamic equilibrium by fitting synthetic spectra. The abundances of Mo were derived from the Mo I lines at 5506 and 5533 Å, while the abundances of Ru were determined from Ru I lines at 4584, and 4757 Å. The abundances of Sr and Zr were measured using lines of Sr II at 4077 and 4215 Å and lines of Zr I at 5385, 6127, and 6134 Å.

Results: We derived Sr, Zr, and Mo abundances for all stars in our sample, and Ru for 110 stars. For most of the giant stars observed in this work, Mo and Ru abundances were determined for the first time. We compare our observations with the signatures from different nucleosynthesis processes.

Conclusions: Both the [Mo/Fe] and [Ru/Fe] in our stars show a decreasing trend with respect to increasing [Fe/H]. This pattern is similar to that of [Zr/Fe], whereas [Sr/Fe] exhibits a relatively flat trend with metallicity. These results are compatible with our previous measurements obtained for dwarf stars over the same metallicity range. Compared to the s-process ratios, all stars show a lower [Zr/Mo] and a higher [Ru/Mo], as expected from classical nucleosynthesis. Still, it is unclear if additional contributions from neutrino-wind components or the i-process is needed to explain the observed scatter of [Zr/Mo] and [Ru/Mo] in the Milky Way disk. Indeed, such a dispersion is consistent with the variations also seen in r-II stars at low metallicity and could therefore result from the combined contributions of r-process and s-process to galactic chemical evolution. The observed [Zr/Mo] and [Ru/Mo] scatter in r-II stars should be constrained by future investigations to define if any contributions of additional nucleosynthesis components are needed.

The Sun is one of the fundamental benchmarks in astronomy, and there is ever growing interest in precise and accurate determinations of its chemical composition. We present the current state-of-the-art in spectroscopic determinations of the solar elemental abundances, that now routinely employ three-dimensional (3D) radiative-hydrodynamic simulations of the solar photosphere and post-processing in non-local thermodynamic equilibrium (non-LTE). We critically review the recent literature and present recommended present-day and protosolar abundances of the 83 long-lived elements, and discuss their implications vis-& agrave;-vis primitive meteorites, helioseismology, neutrino fluxes, and the Solar Modelling Problem.

Context. Elemental abundance ratios in halo stars provide information on nucleosynthesis, chemical evolution, and accretion of dwarf galaxies at early times in the Milky Way because these ratios depend on the initial mass function (IMF) of core-collapse supernovae (CC SNe) and on the role of Type Ia SNe in making the elements.

Aims. By determining very precise stellar parameters and abundances for a sample of intermediate-metallicity halo stars, we aim to estimate the intrinsic dispersion of various abundance ratios.

Methods. We determined differential abundances of C, O, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, and Zr from high signal-to-noise VLT/UVES spectra for 25 turnoff stars with -2.4 < [Fe/H] < -1.3. Effective temperatures were obtained from profiles of the H beta line and surface gravities from Gaia parallaxes. The spectral analysis was based on 1D model atmospheres assuming local thermodynamic equilibrium (LTE), but we applied 3D non-LTE corrections for several elements.

Results. The dispersion in linear fits to the [X/Fe]-[Fe/H] relations is around a factor of two smaller than that found in previous studies. After correction for measurement errors, the 1 sigma intrinsic dispersion of [X/Fe] at a given metallicity is 0.09 dex for Y and Zr, 0.05-0.07 dex for C, O, and Al, 0.03-0.05 dex for Mg, Ca, Sc, Ti, V, Mn, and Zn, and <0.03 dex for Cr, Co, and Ni. We find strong correlations between the residuals in the [X/Fe]-[Fe/H] fits for the alpha-capture elements (Mg, Al, Ca, Sc, and Ti) and between the residuals for Y and Zr.

Conclusions. Correlations between the residuals in the [X/Fe]-[Fe/H] fits and the effective temperature can be explained by differential atomic diffusion between elements, but its contribution to the scatter of [X/Fe] is of minor importance. Both stochastic effects in sampling the IMF of CC SNe and differences in the Type Ia to CC SNe enrichment ratio between star-forming regions are likely required to explain the intrinsic dispersion of [X/Fe].

Accurate and comprehensive atomic data are essential for the modelling of stellar spectra. Uncertainties in the oscillator strengths of specific lines used for abundance analyses directly translate into uncertainties in the derived elemental abundances; incomplete or biased atomic data sets can impart significant errors in non-local thermodynamic equilibrium (non-LTE) modelling. Theoretical calculations of atomic data are therefore crucial to supplement the limited experimental results. In this work, we present extensive atomic data, including oscillator strengths, transition rates, and lifetimes for 1730 electric-dipole (E1) transitions among 107 levels in neutral sulphur (S I) using the multi-configuration Dirac-Hartree-Fock (MCDHF) and relativistic-configuration-interaction (RCI) methods. These levels belong to the configurations 3p³np (n = 3 - 7), 3p³nf (n = 4, 5), 3s3p⁵, 3p³ns (n = 4 - 7), and 3p³nd (n = 3 - 6). The accuracy of the computed transition rates is assessed by combining the comparison of the differences in transition rates between the Babushkin and Coulomb gauges with a cancellation-factor (CF) analysis. Approximately 16% of the ab initio results achieved an accuracy classification of A-B, corresponding to uncertainties within 10%, as defined by the Atomic Spectra Database of the National Institute of Standards and Technology (NIST ASD). Applying a fine-tuning technique was found to significantly improve the accuracy of the results in the Coulomb gauge, thereby improving the consistency between the Babushkin and Coulomb gauges; about 24% of the fine-tuned transition data are assigned to the accuracy classes A-B.

Context. The Ca II near-ultraviolet resonance doublet (H&K) and the near-infrared triplet (CaT) are among the strongest features in stellar spectra of FGK-type stars. These spectral lines remain prominent down to extremely low metallicities and are thus useful for providing stellar parameters via ionisation balance, for Galactic chemical evolution, and as radial velocity diagnostics. However, the majority of studies that model these lines in late-type stars still rely on simplified one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE).

Aims. We present 3D non-LTE radiative transfer calculations of the CaT and H&K lines in an extended grid of 3D model atmospheres of a metal-poor FGK type. We investigated the impact of 3D non-LTE effects on abundances, line bisectors, and radial velocities.

Methods. We used a subset of 3D model atmospheres from the recently published STAGGER-grid to synthesise spectra in 3D (non-)LTE with Balder for nine different calcium-to-iron ratios. For comparison, similar calculations were performed in 1D (non-)LTE using models from the MARCS grid.

Results. Abundance corrections for the CaT lines relative to 1D LTE range from +0.1 greater than or similar to Delta(3N)(1L)greater than or similar to -1.0 dex, with more severe corrections for strong lines in giants. With fixed line strength, the abundance corrections become more negative with increasing effective temperature and decreasing surface gravity. Radial velocity corrections relative to 1D LTE based on cross-correlation of the whole line profile range from -0.2 kms(-1) to +1.5 kms(-1), with more severe corrections where the CaT lines are strongest. The corrections are even more severe if the line core alone is used to infer the radial velocity.

Conclusions. The line strengths and shapes, and consequently the abundance and radial velocity corrections, are strongly affected by the chosen radiative transfer assumption, 1/3D (non)-LTE. We release grids of theoretical spectra that can be used to improve the accuracy of stellar spectroscopic analyses based on the Ca II triplet lines.

Post-asymptotic giant branch (Post-AGB) and post-red giant branch (post-RGB) binaries with stable circumbinary discs provide key insights into late stellar and disc evolution, revealing how binary interactions shape disc structure and stellar surface composition. A defining trait of such systems is the observed underabundance of refractory elements in the stellar photosphere relative to volatile elements - photospheric chemical depletion - resulting from the star accreting volatile-rich circumstellar gas. In this study, we investigated the link between photospheric depletion and disc evolution by focusing on post-AGB/post-RGB binaries with low infrared excess (hereafter 'faint disc' targets). We analysed high-resolution optical spectra from HERMES/Mercator and UVES/VLT for 6 Galactic and 2 LMC targets. Using E-iSpec, we homogeneously derived atmospheric parameters and chemical abundances of 29 elements from carbon to europium, and included NLTE corrections for 15 elements from carbon to barium that we calculated using pySME and pre-computed grids of departure coefficients. All targets exhibit 'saturated' depletion patterns, which we characterised using two-piece linear fits defined by three parameters: initial metallicity ([M/H]0), turn-off temperature (Tturn-off), and depletion scale (del 100 K). Among several findings, we highlight the bimodal distribution of Tturn-off in faint disc targets, which allows classification into two subgroups analogous to full discs with continuous, optically thick dust (Tturn-off>1100 K), and transition discs with inner clearing (Tturn-off-<1100 K). Our results imply that faint disc targets likely represent the final stages of disc dissipation, highlighting the diversity of depletion profiles, the complexity of disc-binary interactions, and the need to understand the rarity and evolution of faint disc systems.