We present phosphorus abundance measurements for a total of 102 giant stars, including 82 stars in 24 open clusters and 20 Cepheids, based on high-resolution near-infrared spectra obtained with GIANO-B. Evolution of phosphorus abundance, despite its astrophysical and biological significance, remains poorly understood due to a scarcity of observational data. By combining precise stellar parameters from the optical, a robust line selection and measurement method, we measure phosphorus abundances using available P i lines. Our analysis confirms a declining trend in [P/Fe] with increasing [Fe/H] around solar metallicity for clusters and Cepheids, consistent with previous studies. We also report a [P/Fe]–age relation among open clusters older than 1 Gyr, indicating a time-dependent enrichment pattern. Such a pattern can be explained by the different stellar formation history of their parental gas, with more efficient stellar formation in the gas of older clusters (thus with higher phosphorus abundances). [P/Fe] shows a flat trend among Cepheids and clusters younger than 1 Gyr (along with three Cepheids inside open clusters), possibly hinting at the phosphorus contribution from the previous-generation low-mass stars. Such a trend suggests that the young clusters share a nearly common chemical history, with a mild increase in phosphorus production by low-mass stars.

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 (Formula presented), 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.

Context. The Milky Way halo has been built-up over cosmic time through the accretion and dissolution of star clusters and dwarf galaxies as well as through their complex interactions with the Galactic disc. Traces of these accreted structures persist to the present day in the chemical and kinematic properties of stars and their orbits and allow for the disentangling of the accretion history of the Galaxy through observations of Milky Way stars.

Aims. We utilised 6D phase-space information in combination with [Fe/H] measurements to facilitate a clustering analysis of stars using their kinematics and chemistry simultaneously, a technique known as chemo-kinematic tagging. We aim to associate large halo substructure groups with Milky Way halo globular clusters, stellar streams, and satellite galaxies in order to investigate the common origins of these groups of structures.

Methods. We implemented t-distributed stochastic neighbour embedding (t-SNE) to perform dimensionality reduction and identify stars from clusters and streams that are co-localised in the kinematic and chemical parameter space. We used the orbital parameters E, J_r, J_z, L_z, r_apo, r_peri, and eccentricity as well as [Fe/H] as input into the algorithm, and we performed a clustering analysis for a sample of 5347 stars from 229 individual Milky Way substructures.

Results. Most notably, we recovered several large-scale structures that have been reported in the literature, including GSE, Thamnos, Sequoia, I’itoi, LMS-1/Wukong, Sagittarius, Kraken/Koala, the splashed disc, and a candidate structure recently found in the literature. We assigned globular cluster populations to each of these accreted structures and find that 44% of Milky Way globular clusters are consistent with having an accreted origin. In addition, we find that the chemo-dynamic properties of Omega Cen are consistent with a common accretion with the Thamnos structure. Finally, we identified many small-scale structures, including several stream-progenitor associations, and a connection between the Orphan-Chenab stream and the Grus II ultra-faint dwarf galaxy, which supports previous findings that these two objects were brought into the Galaxy in the same accretion event.

Modern spectroscopic surveys output large data volumes. Theoretical models provide a means to transform the information encoded in these data to measurements of physical stellar properties. However, in detail, the models are incomplete and simplified, and prohibit interpretation of the fine details in spectra. Instead, the available data provide an opportunity to use data-driven, differential analysis techniques, as a means towards understanding spectral signatures. We deploy such an analysis to examine core helium-fusing red clump (RC) and shell hydrogen-fusing red giant branch (RGB) stars, to uncover signatures of evolutionary state imprinted in optical stellar spectra. We exploit 786 pairs of RC and RGB stars from the GALAH survey, chosen to minimize spectral differences, with evolutionary state classifications from TESS and K2 asteroseismology. We report sub- per cent residual, systematic spectral differences between the two classes of stars, and show that these residuals are significant compared to a reference sample of RC–RC and RGB–RGB pairs selected using the same criteria. First, we report systematic differences in the Swan (⁠⁠C₂) band and CN bands caused by stellar evolution and a difference in mass, where RGB stars at similar stellar parameters have higher masses than RC stars. Secondly, we observe systematic differences in the line-width of the H  and H  lines caused by a difference in microturbulence, as measured by GALAH, where we measure higher microturbulence in RC stars than RGB stars. This work demonstrates the ability of large surveys to uncover the subtle spectroscopic signatures of stellar evolution using model-free, data-driven methods.

The Galactic evolution of copper remains poorly understood, partly due to the strong departures from local thermodynamic equilibrium (LTE) affecting Cu I lines. A key source of uncertainty in non-LTE modelling is the treatment of inelastic Cu + H collisions. We present new rate coefficients based on a combined asymptotic LCAO (linear combination of atomic orbitals) and free electron model approach, which show significant differences from previous calculations. Applying these updated rates to non-LTE stellar modelling, we find reduced line-to-line scatter and improved consistency between metal-poor dwarfs and giants. Our non-LTE analysis reveals a strong upturn in the [Cu/Fe] trend towards lower [Fe/H] < −1.7. We show that this may reflect the interplay between external enrichment of Cu-rich material of the Milky Way halo at low metallicities, and metallicity-dependent Cu yields from rapidly rotating massive stars. This highlights the unique diagnostic potential of accurate Cu abundances for understanding both stellar and Galactic evolution.

The stars of the Milky Way carry the chemical history of our Galaxy in their atmospheres as they journey through its vast expanse. Like barcodes, we can extract the chemical fingerprints of stars from high-resolution spectroscopy. The fourth data release (DR4) of the Galactic Archaeology with HERMES (GALAH) Survey, based on a decade of observations, provides the chemical abundances of up to 32 elements for 917 588 stars that also have exquisite astrometric data from the Gaia satellite. For the first time, these elements include life-essential nitrogen to complement carbon, and oxygen as well as more measurements of rare-earth elements critical to modern-life electronics, offering unparalleled insights into the chemical composition of the Milky Way. For this release, we use neural networks to simultaneously fit stellar parameters and abundances across the whole wavelength range, leveraging synthetic grids computed with Spectroscopy Made Easy. These grids account for atomic line formation in non-local thermodynamic equilibrium for 14 elements. In a two-iteration process, we first fit stellar labels to all 1 085 520 spectra, then co-add repeated observations and refine these labels using astrometric data from Gaia and 2MASS photometry, improving the accuracy and precision of stellar parameters and abundances. Our validation thoroughly assesses the reliability of spectroscopic measurements and highlights key caveats. GALAH DR4 represents yet another milestone in Galactic archaeology, combining detailed chemical compositions from multiple nucleosynthetic channels with kinematic information and age estimates. The resulting dataset, covering nearly a million stars, opens new avenues for understanding not only the chemical and dynamical history of the Milky Way but also the broader questions of the origin of elements and the evolution of planets, stars, and galaxies.

Large spectroscopic surveys aim to consistently compute stellar parameters of very diverse stars, while minimizing systematic errors. We explore the use of stellar clusters as benchmarks to verify the precision of spectroscopic parameters in the fourth data release (DR4) of the GALAH survey. We examine 58 open and globular clusters and associations to validate measurements of temperature, gravity, chemical abundances, and stellar ages. We focus on identifying systematic errors and understanding trends between stellar parameters, particularly temperature and chemical abundances. We identify trends by stacking measurements of chemical abundances against effective temperature and modelling them with splines. We also re-fit spectra in three clusters with the Spectroscopy Made Easy and Korg packages to reproduce the trends in DR4 and to search for their origin by varying temperature and gravity priors, linelists, and the spectral continuum. Trends are consistent between clusters of different ages and metallicities, can reach amplitudes of ~0.5 dex, and differ for dwarfs and giants. We use the derived trends to correct the DR4 abundances of 24 and 31 chemical elements for dwarfs and giants, respectively, and publish a detrended catalogue. While the origin of the trends could not be pinpointed, we found that: (i) photometric priors affect derived abundances, (ii) temperature, metallicity, and continuum levels are degenerate in spectral fitting, and it is hard to break the degeneracy even by using independent measurements, (iii) the completeness of the linelist used in spectral synthesis is essential for cool stars, and (iv) different spectral fitting codes produce significantly different iron abundances for stars of all temperatures. We conclude that clusters can be used to characterise the systematic errors of parameters produced in large surveys, but further research is needed to explain the origin of the trends.

Lithium’s susceptibility to burning in stellar interiors makes it an invaluable tracer for delineating the evolutionary pathways of stars, offering insights into the processes governing their development. Observationally, the complex Li production and depletion mechanisms in stars manifest themselves as Li plateaus, and as Li-enhanced and Li-depleted regions of the HR diagram. The Li-dip represents a narrow range in effective temperature close to the main-sequence turn-off, where stars have slightly super-solar masses and strongly depleted Li. To study the modification of Li through stellar evolution, we measure 3D non-local thermodynamic equilibrium (NLTE) Li abundance for 581 149 stars released in GALAH DR3. We describe a novel method that fits the observed spectra using a combination of 3D NLTE Li line profiles with blending metal-line strength that are optimized on a star-by-star basis. Furthermore, realistic errors are determined by a Monte Carlo nested sampling algorithm which samples the posterior distribution of the fitted spectral parameters. The method is validated by recovering parameters from a synthetic spectrum and comparing to 26 stars in the Hypatia catalogue. We find 228 613 Li detections, and 352 536 Li upper limits. Our abundance measurements are generally lower than GALAH DR3, with a mean difference of 0.23 dex. For the first time, we trace the evolution of Li-dip stars beyond the main sequence turn-off and up the subgiant branch. This is the first 3D NLTE analysis of Li applied to a large spectroscopic survey, and opens up a new era of precision analysis of abundances for large surveys.

Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for.

Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D₁ and K I resonance lines on transmission spectra.

Methods. We modeled the CLV of the Na I D₁ and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between µ = 0.1 and µ = 1.0.

Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%).

Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST.

Context. Traditional one-dimensional hydrostatic model atmospheres introduce systematic modelling errors into spectroscopic analyses of FGK-type stars.

Aims. We present an updated version of the STAGGER-grid of three-dimensional model atmospheres, and explore the accuracy of postprocessing methods in preparation for spectral synthesis.

Methods. New and old models were (re)computed following an updated workflow, including an updated opacity binning technique. Spectroscopic tests were performed in three-dimensional local thermodynamic equilibrium for a grid of 216 fictitious Fe I lines, spanning a wide range of oscillator strengths, excitation potentials, and central wavelengths, and eight model atmospheres that cover the stellar atmospheric parameter range (T_eff, log g, and [Fe/H]) of FGK-type stars. Using this grid, the impact of vertical and horizontal resolutions, and temporal sampling of model atmospheres on spectroscopic diagnostics, was tested.

Results. We find that downsampling the horizontal mesh from its original size of 240 × 240 grid cells to 80 × 80 cells, in other words, sampling every third grid cell, introduces minimal errors on the equivalent width and normalised line flux across the line and stellar parameter space. Regarding temporal sampling, we find that sampling ten statistically independent snapshots is sufficient to accurately model the shape of spectral line profiles. For equivalent widths, a subsample consisting of only two snapshots is sufficient, introducing an abundance error of less than 0.015 dex.

Conclusions. We have computed 32 new model atmospheres and recomputed 116 old ones present in the original grid. The public release of the STAGGER-grid contains 243 models and the processed snapshots can be used to improve the accuracy of spectroscopic analyses.