Observational abundances of CEMP stars with patterns in between those produced by the rapid and slow nucleosynthesis processes (CEMP-rs stars) are currently invoked as evidence of synthesis via the intermediate process in the early AGB evolutionary phase of metal-poor low mass stars. Nevertheless, discriminating between r+s- and i-process hypotheses requires high-precision abundances obtained through advanced spectral modelling techniques. Theoretical models of the i-process have become more robust, incorporating refined stellar modelling and nuclear reaction physics, providing ranges of probable elemental abundances and isotopic ratios predictions to be confronted with observational determinations. We performed a new analysis of a high resolution and high S/N UVES spectrum of TYC 6044-714-1. We derived accurate effective temperature and highly precise atmospheric parameters, element abundances, and isotopic ratios using state-of-the-art 1D non-LTE and 3D non-LTE spectral line modelling. Using the latest AGB nucleosynthesis models, we assessed the possibility of the i-process to act aside the s-process. We find that TYC 6044-714-1 was likely born as a normal in-situ halo star about 13 Gyr ago, pre-enriched by the r-process through a standard Galactic chemical-evolution pathway. The s+r model provides the best overall reproduction of the observed heavy-element abundance pattern and Ba isotopic ratios, yielding excellent agreement across all three s-process peaks. While i+s+r models with increasing overshooting efficiency improve the fit for specific elements, they do not consistently reproduce the full abundance pattern. The i+s+r models require extreme and physically implausible conditions, and predict s-process Ba fractions inconsistent with those inferred from isotopic ratios of the 4934 Å resonance line. We conclude that the pure s+r scenario is the most plausible explanation.

CEMP-rs stars are often interpreted as signatures of intermediate (i-) process nucleosynthesis during early AGB evolution, yet no observed pattern has been shown to favor the i-process over a simple r+s combination. We present a new analysis of TYC 6044-714-1 based on high-resolution UVES spectra and state-of-the-art 1D and 3D non-LTE modelling, deriving precise atmospheric parameters, elemental abundances, and barium isotopic ratios. Modelling of the Ba II 4934 Å line indicates that 86% of barium originates from the s-process, while the derived [Ba/Eu] = 0.25 dex further supports s-process dominance.

The [Ba/Eu] abundance ratio is commonly adopted as a tracer of the relative contributions of the slow (s) and rapid (r) neutron-capture processes. However, at [Fe/H] < -2 dex, Ba can be produced efficiently by both processes, rendering [Ba/Eu] non-deterministic. We propose to use barium isotopic ratio from the fitting of resonance Ba line profiles affected by hyperfine splitting. This approach requires precise atomic and stellar parameters, together with advanced spectral modelling, which, so far, remained insufficiently validated. We aim to provide a robust prescription of line-profile modelling for a reliable determination of the s- and r-processes fractions of barium in ordinary and peculiar stars. We assess the performance of 1D LTE and 1D non-LTE synthesis, and 3D non-LTE abundance corrections to model Ba lines. Alongside barium abundances and its isotopic ratios, we determine Eu and other neutron-capture element abundances to validate the method in the Titans metal-poor benchmark stars. The observational results are compared with the predictions of stochastic Galactic chemical evolution models that account the inhomogeneous mixing in the early times. We find that 1D LTE and 3D non-LTE Ba abundance determinations are equivalent, whereas the 1D non-LTE approach leads to systematic underestimations. These underestimations bias isotopic fractions toward higher r-process contributions. The inferred s- and r-process fractions demonstrate that [Ba/Eu] alone is an ambiguous tracer for ordinary stars within the range -0.8 < [Ba/Eu] < 0 dex. The comparison of our set of models, both for the proto-Milky Way halo and for Gaia-Enceladus galaxy is used to put constraints on the production of Ba at low [Fe/H], especially evaluating the role of rotating massive stars. The method here developed can be applied with confidence to both ordinary stars and peculiar stars enhanced in barium.

Identifying the nucleosynthesis processes behind heavy-element enrichment in stellar atmospheres is challenging. It typically relies on comparing observed abundance-to-iron ratios with theoretical predictions relative to the Sun, but this method is prone to uncertainty due to limitations of classical 1D hydrostatic models. One promising but still underexplored approach is to measure the isotopic composition of stellar atmospheres by focusing on elements that have both slow (s)-process and rapid (r)-process contributions. While the study of total elemental abundances offers a simplified view, isotopic ratios are directly linked to the underlying nucleosynthesis processes. Our aim is to provide a reliable method for quantifying the contributions of the s- and r-processes to barium in stellar atmospheres. This is achieved by determining barium isotopic ratios using 1D atmospheric models in combination with a carefully calibrated microturbulence, based on the comparison between subordinate and resonance Ba lines. In this initial study, we use member stars of the globular cluster NGC 6752 to calibrate the microturbulence (v_mic) value for both subordinate and resonance barium lines across different stellar evolutionary stages. This allows us to provide a reliable estimate of v_mic that can be applied to accurately determine barium abundances and isotopic ratios in stars ranging from the main sequence to the upper red giant branch. The v_mic scale adapted for barium subordinate lines is consistent with that derived from 3D model atmospheres, and thus the T_eff-log g dependent relations of the later can be used safely. The v_mic for the resonance line at λ4934 Angstrom – for the determination of the isotopic ratio – is higher, and depends on the equivalent width (EW). We provide calibrated relations between v_mic and EW for measuring isotopic ratios.

Context. The origin of the Galactic halo is one of the fundamental topics linking the study of galaxy formation and evolution to cosmology. Aims. Our goal is to derive precise and accurate stellar parameters, Mg abundances, and ages for a sample of metal-poor stars with [Fe/H] < - 2 dex from high signal-to-noise and high spectral resolution archival spectra. Methods. We derived effective temperatures from H alpha profiles using three-dimensional non-local thermodynamic equilibrium (3D NLTE) models, and surface gravities and ages from isochrone fitting based on Gaia data. Iron abundances were derived in one-dimensional (1D) NLTE, while Mg abundances were derived in 1D LTE, 1D NLTE, 3D LTE, and 3D NLTE to show the increasing level of accuracy. Results. The sample stars show a tight trend in the [Mg/Fe] versus [Fe/H] plane with a knee located at [Fe/H] approximate to - 2.8 dex, which indicates a low level of stochasticity at the sampled metallicities in this kind of population. Their location in the Lindblad diagram confirms that they belong to the Galactic halo, but does not show a distinct clustering that might be expected for a merger with a single low-mass galaxy. Comparison with chemical evolution models is also not fully definitive on whether the sample stars were born in situ or in accreted low-mass galaxy mergers. Conclusions. Overall, we find two plausible explanations for the chemical sequence traced by the stars in the [Mg/Fe] versus [Fe/H] plane. One is that the sample stars originated in the already formed Milky Way, which at that time (12.5 Gyr ago) was already the main galaxy of its Local Group surroundings. Another explanation is that the sample stars originated in several small galaxies with similar properties, which later merged with the Galaxy. Only accurate spectroscopic analysis such as that done here can reveal trustworthy chemical diagrams required to observe the traces of the Galaxy evolution. Increasing the sample size and the analysis of other elements are required to differentiate between the two hypotheses.

We reanalyze the chemical composition of the metal-poorest tail of the Galactic halo using highly accurate atmospheric parameters Giribaldi et al. (2021, 2023) and cutting-edge 3D NLTE models Amarsi et al (2018). Most [Mg/Fe] versus [Fe/H] diagrams in the literature exhibit significant scatter at [Fe/H] ≲ -2 dex, often interpreted as evidence of inhomogeneous enrichment during the early phases of galaxy evolution Rossi et al. (2021). However, our analysis of observational data reveals that in the range -3.5 < [Fe/H] < -2 dex, the [Mg/Fe] versus [Fe/H] distribution is relatively narrow. This finding suggests a low degree of stochastic enrichment in magnesium during these epochs in the Milky Way halo.

Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H]$\approx -$1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from $-$1 to approximately solar metallicity. This sub-group includes almost ~1/4 of the sample, it is characterised by thin disc kinematics and displays sub-solar $\alpha$-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams.

Separating stars enriched in the s- and r-processes of nucleosynthesis is usually achieved by analyzing the element ratios of s-process elements (like Ba or La) to r-process elements (like Eu). The situation becomes more complex when analyzing CEMP-rs stars, which are carbon-enriched metal-poor objects enriched in a mixture of s- and r-elements. These objects, possibly resulting from the i-process of nucleosynthesis, are notoriously difficult to classify based on elemental ratios. Recent theoretical studies have outlined, however, that the s-, i-, and r-processes produce distinct isotopic mixtures. Here, we propose to analyze a sample of stars known to be enriched in s, r, or r + s elements and to determine the odd-to-even isotopic ratio measured on atomic lines of barium, in order to validate or disprove their assignation.

Representative samples of F-, G-, K-type stars located out of the Solar Neighbourhood has started to be available in spectroscopic surveys. The fraction of metal-poor ([Fe/H]~$\lesssim -0.8$~dex) giants becomes increasingly relevant to far distances. In metal-poor stars, effective temperatures ($T_{\mathrm{eff}}$) based on LTE spectroscopy and on former colour-$T_{\mathrm{eff}}$ relations of still wide use have been reported to be inaccurate. It is necessary to re-calibrate chemical abundances based on these $T_{\mathrm{eff}}$ scales in the multiple available surveys to bring them to the same standard scale for their simultaneous use. For that, a complete sample of standards is required, which so far, is restricted to a few stars with quasi-direct $T_{\mathrm{eff}}$ measurements. We aim at providing a legacy sample of metal-poor standards with proven accurate atmospheric parameters. We add 47 giants to the sample of metal-poor dwarfs of Giribaldi et al. 2021, thereby constituting the Titans metal-poor reference stars. $T_{\mathrm{eff}}$ was derived by 3D non-LTE H$\alpha$ modelling, whose accuracy was tested against interferometry and InfraRed Flux Method (IRFM). Surface gravity (log $g$) was derived by fitting Mg~I~b triplet lines, whose accuracy was tested against asteroseismology. Metallicity was derived using Fe II lines, which was verified to be identical to the [Fe/H] derived from non-LTE spectral synthesis. $T_{\mathrm{eff}}$ from 3D non-LTE H$\alpha$ is equivalent to interferometric and IRFM temperatures within a $\pm$46~K uncertainty. We achieved precision of $\sim$50~K for 34 stars with spectra with the highest S/N. For log $g$, we achieved a total uncertainty of $\pm$0.15~dex. For [Fe/H], we obtained a total uncertainty of $\pm$0.09~dex. We find that the ionization equilibrium of Fe lines under LTE is not valid in metal-poor giants.

The High-Resolution Multi-Object Spectrograph (HRMOS) is a facility instrument that we plan to propose for the Very Large Telescope (VLT) of the European Southern Observatory (ESO), following the initial presentation at the VLT 2030 workshop held at ESO in June 2019. HRMOS provides a combination of capabilities that are essential to carry out breakthrough science across a broad range of active research areas from stellar astrophysics and exoplanet studies to Galactic and Local Group archaeology. HRMOS fills a gap in capabilities amongst the landscape of future instrumentation planned for the next decade. The key characteristics of HRMOS will be high spectral resolution (R = 60000 - 80000) combined with multi-object (20-100) capabilities and long term stability that will provide excellent radial velocity precision and accuracy (10m/s). Initial designs predict that a SNR 100 will be achievable in about one hour for a star with mag(AB) = 15, while with the same exposure time a SNR 30 will be reached for a star with mag(AB) = 17. The combination of high resolution and multiplexing with wavelength coverage extending to relatively blue wavelengths (down to 380 nm), makes HRMOS a spectrograph that will push the boundaries of our knowledge and that is envisioned as a workhorse instrument in the future. The science cases presented in this White Paper include topics and ideas developed by the Core Science Team with the contributions from the astronomical community, also through the wide participation in the first HRMOS Workshop (https://indico.ict.inaf.it/event/1547/) that took place in Firenze (Italy) in October 2021.

Context. Representative samples of F-, G-, K-type stars located outside of the solar neighbourhood have started to become available in spectroscopic surveys. The fraction of metal-poor ([Fe/H] ≲ −0.8 dex) giants becomes increasingly relevant towards greater distances. In metal-poor stars, effective temperatures (Teff) based on local thermodynamic equilibrium (LTE) spectroscopy and on former colour–Teff relations – which are still widely used – have been reported to be inaccurate. It is necessary to recalibrate chemical abundances based on these Teff scales in the multiple available surveys in order to bring them to the same standard scale for their simultaneous use. To this end, a complete sample of standards is required, which until now has been restricted to a few stars with quasi-direct Teff measurements. Aims. We aim to provide a legacy sample of metal-poor standards with proven accurate atmospheric parameters. We add 47 giants to the TITANS metal-poor reference stars. Methods. We derived Teff using 3D non-LTE Hα modelling, the accuracy of which was tested against interferometry and with the Infra Red Flux Method (IRFM). We derived surface gravity (log ɡ) by fitting Mg I b triplet lines, and tested their accuracy against asteroseismology. Metallicity was derived using Fe II lines, and we find our results to be identical to the [Fe/H] derived from non-LTE spectral synthesis. Results. The Teff that we find using 3D non-LTE Hα is equivalent to interferometric and IRFM temperatures within a ±46 K uncertainty. We achieve precision of ~50 K for 34 stars with spectra with the highest signal-to-noise ratio (S/N). For log ɡ, we achieve a total uncertainty of ±0.15 dex. For [Fe/H], we obtain a total uncertainty of ±0.09 dex. We find that the ionisation equilibrium of Fe lines under LTE is not valid in metal-poor giants. LTE leads to a small but significant metallicity underestimation of ~0.1 dex when derived from weak Fe I lines, and only provided accurate Teff and log ɡ. This bias totally disappears under non-LTE.

The Milky Way accreted several smaller satellite galaxies in its history. These mergers added stars and gas to the Galaxy and affected the properties of the pre-existing stellar populations. Stellar chemical abundances and ages are needed to establish the chronological order of events that occur before, during, and after such mergers. We report precise ages ($\sim$6.5%) and chemical abundances for the Titans, a sample of old metal-poor dwarfs and subgiants with accurate atmospheric parameters. We also obtain ages with an average precision of 10% for a selected sample of dwarf stars from the GALAH survey. We used these stars, located within $\sim$1 kiloparsec of the Sun, to analyse the chronology of the chemical evolution of in-situ and accreted metal-poor stellar populations. We determined ages by isochrone fitting. For the Titans, we determined abundances of Mg, Si, Ca, Ti, Ni, Ba, and Eu using spectrum synthesis. The [Mg/Fe] abundances of the GALAH stars were re-scaled to be consistent with the abundances of the Titans. We separated stellar populations by primarily employing chemical abundances and orbits. We find that star formation in the so-called Gaia-Enceladus or Gaia-Sausage galaxy, the last major system to merge with the Milky Way, lasted at least 3 billion years and got truncated 9.6 $\pm$ 0.2 billion years ago. This marks with very high precision the last stage of its merging process. We also identified stars of a heated metal-poor in-situ population with virtually null net rotation, probably disturbed by several of the early Milky Way mergers. We show that this population is more metal rich than Gaia-Enceladus at any time. The sequence of events uncovered in our analysis supports the hypothesis that Gaia-Enceladus truncated the formation of the high-$\alpha$ disc and caused the gas infall that forms the low-$\alpha$ disc, in agreement with theoretical predictions.

In spite of the advent of extremely large telescopes in the UV/optical/NIR range, the current generation of 8-10m facilities is likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (>40 300-420 nm goal) at a spectral resolving power of R>20,000, although a lower-resolution, sky-limited mode of R 7,000 is also planned. CUBES will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the Phase B dedicated to detailed design and construction. First science operations are planned for 2028. In this paper, we briefly describe the CUBES project development and goals, the main science cases, the instrument design and the project organization and management.

Beryllium is a light element with one single stable isotope, ^9 Be, which is a pure product of cosmic-ray spallation in the interstellar medium. Beryllium abundances in late-type stars can be used in studies about evolutionary mixing, Galactic chemical evolution, planet engulfment, and the formation of globular clusters. Some of these uses of Be abundances figure among the science cases of the Cassegrain U-Band Efficient Spectrograph (CUBES), a new near-UV low- and medium-resolution spectrograph under development for the Very Large Telescope. Here, we report on a study about beryllium abundances in extremely metal-poor stars in the context of the phase A of CUBES. Our motivation is to understand the limits for the detection of weak lines in extremely metal-poor stars of low Be abundances. We analyze simulated CUBES observations, performed in medium-resolution mode, based on synthetic spectra for four mock stars with [Fe/H] ≤ −3.0. We find that detecting the Be lines is possible in certain cases, but is very challenging and requires high signal-to-noise ratio. Depending on the atmospheric parameters of the target stars, and if signal-to-noise per pixel of about 400 can be achieved, it should be possible to detect Be abundances between log (Be/H) −13.1 and −13.6, with a typical uncertainty of ± 0.15 dex. Using CUBES, the required data for such studies can be obtained for stars that are fainter by two magnitudes with respect to what is possible with current instrumentation.

In the era of Extremely Large Telescopes, the current generation of 8-10m facilities are likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (> 40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R >20, 000 (with a lower-resolution, sky-limited mode of R ~7, 000). With the design focusing on maximizing the instrument throughput (ensuring a Signal to Noise Ratio (SNR) ~20 per high-resolution element at 313 nm for U ~18.5 mag objects in 1h of observations), it will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the detailed design and construction phase. First science operations are planned for 2028.

We introduce the scientific motivations for the development of the Cassegrain U-Band Efficient Spectrograph (CUBES) that is now in construction for the Very Large Telescope. The assembled cases span a broad range of contemporary topics across Solar System, Galactic and extragalactic astronomy, where observations are limited by the performance of current ground-based spectrographs shortwards of 400 nm. A brief background to each case is presented and specific technical requirements on the instrument design that flow-down from each case are identified. These were used as inputs to the CUBES design, that will provide a factor of ten gain in efficiency for astronomical spectroscopy over 300-405 nm, at resolving powers of R ∼ 24,000 and ∼ 7,000. We include performance estimates that demonstrate the ability of CUBES to observe sources that are up to three magnitudes fainter than currently possible at ground-ultraviolet wavelengths, and we place its predicted performance in the context of existing facillities.

Globular clusters host multiple stellar populations that display star-to-star variation of light elements that are affected by hot hydrogen burning (e.g., He, C, N, O). Several scenarios have been suggested to explain these variations. Most involve multiple star formation episodes, where later generations are born from material contaminated by the nucleosynthetic products of the previous stellar generation(s). One difficulty in the modelling of such scenarios is knowing the extent to which processed and pristine material are mixed. In this context, beryllium abundances measured in turn-off stars of different generations can provide new information. Beryllium originates from cosmic-ray spallation and can only be destroyed inside stars. Beryllium abundances can thus directly measure the degree of pollution of the material that formed stars in globular clusters. Turn-off stars in globular clusters are however faint and such studies are beyond the capabilities of current instrumentation. In this work, we show the progress that the CUBES spectrograph will bring to this area. Our simulations indicate that CUBES will enable the detection of variations of about 0.6 dex in the Be abundances between stars from different generations, in several nearby globular clusters with turn-off magnitude down to $V$ = 18 mag.

We present a spectroscopic and photometric calibration to derive effective temperatures T-eff and metallicities [Fe/H] for M dwarfs, based on a principal component analysis (PCA) of 147 spectral indices measured off moderate resolution (R similar to 11000), high S/N (>100) spectra in the lambda lambda 8390-8834 region, plus the J-H colour. Internal uncertainties, estimated by the residuals, are 81 K and 0.12 dex, respectively, for T-eff and [Fe/H], the calibrations being valid for 3050 K < T 4100 K and -0.45 < [Fe/H] 0.50 dex. The PCA calibration is a competitive model-independent method to derive T-eff and [Fe/H] for large samples of M dwarfs, well suited to the available database of far-red spectra. The median uncertainties are 105 K and 0.23 dex for T-eff and [Fe/H], respectively, estimated by Monte Carlo simulations. We compare our values to other works based on photometric and spectroscopic techniques and find median differences 75 +/- 273 K and 0.02 +/- 0.31 dex for T-eff and [Fe/H], respectively, achieving good accuracy but relatively low precision. We find considerable disagreement in the literature between atmospheric parameters for stars in common. We use the new calibration to derive T-eff and [Fe/H] for 178 K7-M5 dwarfs, many previously unstudied. Our metallicity distribution function for nearby M dwarfs peaks at [Fe/H]similar to-0.10 dex, in good agreement with the RAVE distribution for GK dwarfs. We present radial velocities (internal precision 1.4 kms(-1)) for 99 objects without previous measurements. The kinematics of the sample shows it to be fully dominated by thin/thick disc stars, excepting the well-known high-velocity Kapteyn's star.

Context. Several large stellar spectroscopic surveys are producing enormous amounts of data that can be used to determine stellar atmospheric parameters and chemical abundances. Nonetheless, the accuracy achieved in the derived astrophysical parameters is still insufficient, mainly because of the paucity of adequate calibrators, particularly in the metal-poor regime ([Fe/H] ≤−1.0).Aims. Our aim is to increase the number of metal-poor stellar calibrators that have accurate parameters. Here, we introduce the TITANS metal-poor reference stars: a sample of 41 dwarf and subgiant stars with accurate, but model-dependent, parameters.Methods. Effective temperatures (T eff ) were derived by fitting observed Hα profiles with synthetic lines computed using three-dimensional (3D) hydrodynamic model atmospheres that take local thermodynamic equilibrium departures into account (non-LTE effects). Surface gravities (log g ) were computed using evolutionary tracks and parallaxes from Gaia early-data release 3.Results. The same methods recover the T eff values of the Gaia benchmark stars, which are mostly based on interferometric measurements, with a 1σ dispersion of ±50 K. We assume this to be the accuracy of the Hα profiles computed from 3D non-LTE models for metal-poor dwarfs and subgiants, although this is likely an upper-bound estimate dominated by the uncertainty of the standard T eff values. We achieved an internal precision typically between 30 and 40 K; these errors are dominated by instrumental effects. The final total uncertainty for the T eff values of the TITANS are thus estimated to be of the order of 1%. The typical error for log g is ≤0.04 dex. In addition, we identified a few members of Gaia -Enceladus, of Sequoia, and of the Helmi stream in our sample. These stars can pave the way for the accurate chemical characterisation of these Galactic substructures.Conclusions. Using the TITANS as a reference, large stellar surveys will be able to improve the internal calibration of their astrophysical parameters. Ultimately, this sample will help users of data from Gaia and large surveys to reach their goal of redefining our understanding of stars, stellar systems, and the Milky Way.