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Context. The third Gaia Data Release, which includes BP/RP spectra for 219 million sources, has opened a new window in the exploration of the chemical history and evolution of the Milky Way. The wealth of information encapsulated in these data is far greater than their low resolving power (R=50) at first glance would suggest, as shown in many studies. We zero in on the use of this data for the purpose of the detection of ''new'' metal-poor stars, which are hard to find yet essential for understanding - among other - several aspects of the origin of the Galaxy, star formation and the creation of the elements. Aims. We strive to refine a metal-poor candidate selection method which was developed with simulated Gaia BP/RP spectra, with an ultimate objective of providing the community with both a recipe to select stars for medium/high resolution observations and a catalogue of stellar metallicities. Methods. We used a datased comprised of GALAH DR3 and SAGA database stars in order to verify and adjust to real world data our selection method. For that purpose, we used dereddening as a mean to tackle the issue of extinction, and then we applied our fine-tuned method to select metal-poor candidates, which we thereafter observed and analysed. Results. We were able to infer metallicities for GALAH DR3 and SAGA stars - with color excesses up to E(B-V)<1.5 - with an uncertainty of 0.36 dex, which is good enough for the purpose of identifying new metal-poor stars. Further, we selected 26 metal-poor candidates - via our method - for observations. As spectral analysis showed, 100% of them had [Fe/H]<-2.0, 57% had [Fe/H]<-2.5 and 8% had [Fe/H]<-3.0. We inferred metallicities for these stars with an uncertainty of 0.31 dex, as was proven when comparing to the spectroscopic [Fe/H]. Finally, we assembled a catalogue of metallicities for 10 861 062 stars., Comment: 9 pages, 11 figures, accepted for publication in A&A

We present detailed abundance results based on UVES high dispersion spectra for 7 very and extremely metal-poor stars in the Large Magellanic Cloud. We confirm that all 7 stars, two of which have [Fe/H] $\leq$ --3.0, are the most metal-poor stars discovered so far in the Magellanic Clouds. The element abundance ratios are generally consistent with Milky Way halo stars of similar [Fe/H] values. We find that 2 of the more metal-rich stars in our sample are enhanced in r-process elements. This result contrasts with the literature, where all nine metal-poor LMC stars with higher [Fe/H] values than our sample were found to be rich in r-process elements. The absence of r-process enrichment in stars with lower [Fe/H] values is consistent with a minimum delay timescale of $\sim$100 Myr for the neutron star binary merger process to generate substantial r-process enhancements in the LMC. We find that the occurrence rate of r-process enhancement (r-I or r-II) in our sample of very and extremely metal-poor stars is statistically indistinguishable from that found in the Milky Way's halo, although including stars from the literature sample hints at a larger r-II frequency the LMC. Overall, our results shed light on the earliest epochs of star formation in the LMC that may be applicable to other galaxies of LMC-like mass.

Variations in chemical abundances with evolutionary phase have been identified among stars in globular and open clusters with a wide range of metallicities. In the metal-poor clusters, these variations compare well with predictions from stellar structure and evolution models considering the internal diffusive motions of atoms and ions, collectively known as atomic diffusion, when moderated by an additional mixing process with a fine-tuned efficiency. We present here an investigation of these effects in the Galactic globular cluster NGC 6121 (M4) ([Fe/H] = -1.13) through a detailed chemical abundance analysis of 86 stars using high-resolution ESO/VLT FLAMES spectroscopy. The stars range from the main-sequence turnoff point (TOP) to the red giant branch (RGB) just above the bump. We identify C-N-O and Mg-Al-Si abundance anti-correlations, and confirm the presence of a bimodal population differing by 1 dex in nitrogen abundance. The composition of the second-generation stars imply pollution from both massive (20-40 Msol) and asymptotic giant branch stars. We find evolutionary variations in chemical abundances between the TOP and RGB, which are robust to uncertainties in stellar parameters and modelling assumptions. The variations are weak, but match predictions well when employing efficient additional mixing. Without correcting for Galactic production of lithium, we derive an initial lithium abundance 2.63+-0.10, which is marginally lower than the predicted primordial BBN value., Comment: 21 pages, 12 figures, submitted to MNRAS

We present results of a search for extremely metal-poor (EMP) stars in the Large Magellanic Cloud, which can provide crucial information about the properties of the first stars as well as on the formation conditions prevalent during the earliest stages of star formation in dwarf galaxies. Our search utilised SkyMapper photometry, together with parallax and proper motion cuts (from Gaia), colour-magnitude cuts (by selecting the red giant branch region) and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of photometric candidates were taken using the ANU 2.3m telescope/WiFeS spectrograph, from which 7 stars with [Fe/H] $\leq$ -2.75 were identified, two of which have [Fe/H] $\leq$ -3. Radial velocities, derived from the CaII triplet lines, closely match the outer rotation curve of the LMC for the majority of the candidates in our sample. Therefore, our targets are robustly members of the LMC based on their 6D phase-space information (coordinates, spectrophotometric distance, proper motions and radial velocities), and they constitute the most metal-poor stars so far discovered in this galaxy., Comment: Accepted for publication in MNRAS

Context: The first stars produced the first heavy elements and set the stage for the formation of the first galaxies. Accurate chemical abundances of ultra metal-poor stars ([Fe/H]<-4) can be used to infer properties of the first stars, and thus the formation mechanism for low-mass second generation stars in the early universe. Spectroscopic studies have shown that most second generation stars are carbon-enhanced with one notable exception SDSS J102915.14+172927.9. Aims: We reanalyse the composition of SDSS J102915.14+172927.9. Methods: We developed a tailored 3D model atmosphere for SDSS J102915.14+172927.9 with the Stagger-code, making use of an improved surface gravity estimate based on the Gaia DR3 parallax. This model was used as input in the radiative transfer code Balder to compute 3D non-LTE synthetic spectra. These spectra were then used to infer abundances for Mg, Si, Ca, Fe and Ni, and upper limits on Li, Na and Al. 3D LTE synthetic spectra were computed with Scate to infer the abundance of Ti and upper limits on C and N. Results: In contrast to earlier works based on 1D non-LTE corrections to 3D LTE results, we are able to achieve ionisation balance for Ca I and Ca II when employing our consistent 3D non-LTE treatment. Moreover, the elemental abundances are systematically higher than those found in earlier works. In particular, [Fe/H] increases by 0.57 dex, and the upper limits of C and N increase by 0.90 dex and 1.82 dex, respectively. Conclusions: We find that Population III progenitors with masses 10-20 M_sun exploding with energy E<=3*10^{51} erg can reproduce our 3D non-LTE abundance pattern. Contrary to previous work, we obtain higher upper limits on the carbon abundance that are ``marginally consistent'' with star formation through atomic line cooling, and as such, prevent strong conclusions about the formation mechanism of this low mass star., Comment: 14 pages, 9 figures, Accepted for publication in Astronomy & Astrophysics

The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangle the scenarios of formation and evolution of the Galaxy. We used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance to iron ratio gradients, and their time evolution. We selected member stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw different behaviours for elements belonging to different channels. We found that the youngest clusters in the inner disc have lower metallicity than their older counterpart and they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggested that it might be a bias introduced by the standard spectroscopic analysis producing lower metallicities in low gravity stars. To delineate the shape of the `true' gradient, we should limit our analysis to stars with low surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slow and stationary formation of the thin disc in the latest Gyr. We found a secondary role of clusters' migration in shaping the gradient, with a more prominent role of migration for the oldest clusters., Comment: 25 pages, 14 figures and 4 tables in the main text, 3 figures and 7 tables in the Appendix. Accepted for publication in A&A

It is well known that cool star atmospheres depart from local thermodynamic equilibrium (LTE). Accurate abundance determination requires taking those effects into account, but the necessary non-LTE calculations are often lacking. Our goal is to provide detailed estimates of NLTE effects for FGK type stars for all spectral lines from the ultraviolet to the infrared that are potentially useful as abundance diagnostics. The first paper in this series focusses on the light elements Na, Mg and Al. The code PySME is used to compute curves-of-growth for 2158 MARCS model atmospheres in a wide parameter range. Nine abundance points are used to construct individual line curves-of-growth by calculating the equivalent widths of 35 Na lines, 134 Mg lines, and 34 Al lines. The lines are selected from the ultra-violet to the near infrared wavelength range. We demonstrate the power of the new grids with LTE and NLTE abundance analysis by means of equivalent width measurements of five benchmark stars; the Sun, Arcturus, HD84937, HD140283 and HD122563. For Na, the NLTE abundances are lower than in LTE and show markedly reduced line-to-line scatter in the metal-poor stars. For Mg, we confirm previous reports of a significant 0.25 dex LTE ionization imbalance in metal-poor stars that is only slightly improved in NLTE (0.18 dex). LTE abundances based on Mg II lines agree better with models of Galactic chemical evolution. For Al, NLTE calculations strongly reduce a 0.6 dex ionization imbalance seen in LTE for the metal-poor stars. The abundance corrections presented in this work are in good agreement with previous studies for the subset of lines that overlap, except for strongly saturated lines., Comment: 12 pages main text, 7 figures, 14 pages appendix. Accepted for publication in A&A

The detection of $^6$Li in Spite plateau stars contradicts the standard Big Bang nucleosynthesis prediction, known as the second cosmological lithium problem. We measure the isotopic ratio $^6$Li/$^7$Li in three Spite plateau stars: HD 84937, HD 140283, and LP 815-43. We use 3D NLTE radiative transfer and for the first time apply this to high resolution, high-S/N data from the ultra-stable VLT/ESPRESSO spectrograph. These are amongst the best spectra ever taken of any metal-poor stars. As the measurement of $^6$Li/$^7$Li is degenerate with other physical stellar parameters, we employ Markov chain Monte Carlo methods to find the probability distributions of measured parameters. As a test of systematics we also use three different fitting methods. We do not detect $^6$Li in any of the three stars, and find consistent results between our different methods. We estimate 2$\sigma$ upper limits to $^6$Li/$^7$Li of 0.7%, 0.6%, and 1.7% respectively for HD 84937, HD 140283, and LP 815-43. Our results indicate that there is no second cosmological lithium problem, as there is no evidence of $^6$Li in Spite Plateau stars., Comment: 17 pages, 9 figures, accepted for publication in MNRAS

Large spectroscopic surveys of the Milky Way need to be calibrated against a sample of benchmark stars to ensure the reliable determination of atmospheric parameters. We present new fundamental stellar parameters of seven giant and subgiant stars that will serve as benchmarks. The aim is to reach a precision of 1% in the effective temperature. This precision is essential for accurate determinations of the full set of fundamental parameters and abundances of stars observed by the surveys. We observed HD121370 (etaBoo), HD161797 (muHer), HD175955, HD182736, HD185351, HD188512 (betaAql), and HD189349 using the high angular resolution optical interferometric instrument PAVO/CHARA. The limb-darkening corrections were determined from 3D model atmospheres based on the STAGGER grid. The Teff were determined directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. We estimated surface gravities from comparisons to Dartmouth stellar evolution model tracks. The spectroscopic observations were collected from the ELODIE and FIES spectrographs. We estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron. For six of the seven stars we measure Teff to better than 1%. For one star, HD189349, the uncertainty in Teff is 2% due to an uncertain bolometric flux. We do not recommend this star as a benchmark until this measurement can be improved. Median uncertainties for all stars in logg and [Fe/H]} are 0.034dex and 0.07dex, respectively. All the fundamental stellar parameters were based on consistently combining interferometric observations, 3D limb-darkening modelling and spectroscopic analysis. This paper follows our previous papers including dwarfs and metal-poor stars., Comment: 14 pages, 8 figures, 9 tables, tables A.1.-A.7. will be submitted electronically, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:2006.05411

Stellar models applied to large stellar surveys of the Milky Way need to be properly tested against a sample of stars with highly reliable fundamental stellar parameters. We have established a program aiming to deliver such a sample. We present new fundamental stellar parameters of nine dwarfs that will be used as benchmarks for large stellar surveys. One of these stars is the solar-twin 18Sco, which is one of the Gaia-ESO benchmarks. The goal is to reach a precision of 1% in Teff. This precision is important for accurate determinations of the full set of fundamental parameters and abundances of stars observed by the surveys. We observed HD131156 (xiBoo), HD146233 (18Sco), HD152391, HD173701, HD185395 (thetaCyg), HD186408 (16CygA), HD186427 (16CygB), HD190360 and HD207978 (15Peg) using the high angular resolution optical interferometric instrument PAVO/CHARA. We derived limb-darkening corrections from 3D model atmospheres and determined Teff directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE spectrograph and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analyses of unblended lines of neutral and singly ionized iron. For eight of the nine stars, we measure the Teff less than 1%, and for one star better than 2%. We determined the median uncertainties in logg and Fe/H as 0.015dex and 0.05dex, respectively. This study presents updated fundamental stellar parameters of nine dwarfs that can be used as a new set of benchmarks. All parameters were based on consistently combining interferometric observations, 3D limb-darkening modelling and spectroscopic analysis. The next paper will extend our sample to metal-rich giants., Comment: 15 pages, 10 figures, 9 tables, tables 10-16 will be submitted electronically, accepted for publication in A&A