Bulk meteorites and their inclusions exhibit, for many chemical elements, isotopic variability produced by nucleosynthetic events in stars and supernovae before the formation of the Sun. While the exact astrophysical origins of these variations are still a matter of debate, their identification provides insights on the environment of the Sun's birth and the formation of the Solar System. Here we present a new Python tool called SIMPLE (Stellar Interpretation of Meteoritic Data and Plotting for Everyone) designed to compare the isotopic composition of the ejecta from core-collapse supernovae (CCSNe) with meteoritic data. In the present version, the SIMPLE toolkit includes a dataset of 18 CCSN models, from 6 different published sets, with initial masses of 15, 20, and 25 M_⊙ and solar metallicity. SIMPLE is designed to easily extract the isotopic abundances predicted by each CCSN model for any elements and post-process them into the format needed to compare to the meteoritic data, therefore, facilitating their interpretation. As an example of how to use SIMPLE, we analyze the composition of the Ni isotopes in the 18 models and confirm that bulk meteorite Ni anomalies are compatible with material from the innermost Si-rich region of CCSN ejecta. Designed as a collaborative platform, SIMPLE is open-source and welcomes community contributions to enhance its development and dissemination for stellar nucleosynthesis and meteoritic studies. Future enhancements include addition of more model predictions and inclusion of mixing between different layers of supernova ejecta.

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 stellar sources that contributed to the chemical and isotopic composition of the solar system, their mixing in the Galactic environment, and the evolution and longevity of the molecular cloud from which our solar system formed can be probed with isotopic measurements of presolar grains, meteorites, and comets. The isotopic compositions of cometary xenon (Xe) and krypton (Kr) measured in the coma of 67P/Churyumov–Gerasimenko (67P/C-G) are distinct from those of the Sun. As noble gases in cometary ices are thought to be derived from the same molecular cloud from which the Sun formed, such nonsolar isotopic compositions suggest that a compositionally distinct reservoir of noble gases was present in the presolar molecular cloud that is currently absent from 67P/C-G. This likely requires that a noble gas component was cotrapped in volatile ice which was later destroyed by processing within the protoplanetary disk. Here, we show that this hypothetical component was enriched in r -process and potentially p -process isotopes compared to the solar composition, and that these isotopes formed over a hundred million years before coalescence and cosmochemical isolation of the solar system. We discuss a scenario where this component was accreted by the presolar molecular cloud sometime after the formation of water ice containing cotrapped Xe and Kr with the composition of 67P/C-G. Such a component could have been derived from a diffuse, heterogeneous molecular portion of the interstellar medium accreted by the presolar molecular cloud, or incorporated from another molecular cloud via a cloud–cloud collision. These observations support two distinct r -processes contributing to the overall solar r -process abundance pattern.

The light odd- Z elements P, Cl, K, and Sc are underproduced in galactic chemical evolution models compared to spectroscopic observations of stars in the Milky Way. The most promising solution to this puzzle is that some massive stars experience O-C shell mergers, boosting their yields through dynamic, convective-reactive nucleosynthesis. We report how convective macro physics based on 3D 4 π hydrodynamic simulations impacts production in the O shell by postprocessing the M _ZAMS = 15 M _⊙ Z = 0.02 model from the NuGrid dataset. We explore a mixing downturn, boosted velocities, reduced ingestion rate, and convective quenching. Across 24 mixing cases, the pre-explosive yields for [P/Fe], [Cl/Fe], [K/Fe], and [Sc/Fe] are modified by [−0.33, 0.23], [−0.84, 0.64], [−0.78, 1.48], and [−0.36, 1.29] dex, respectively. Cases with a convective downturn with the fastest ingestion rate have the largest enhancement, and production is nonmonotonic with boosted velocities. Which reactions are most important for the convective-reactive element production pathways depends on the mixing case. We parameterize production of ^40 K ( t _1/2 = 1.248 Gyr), an important radiogenic heat source for younger (2–3 Gyr) rocky planets, and find a yield variation exceeding 3 orders of magnitude. This range of initial abundances for ^40 K implies the early geodynamic behavior of silicate mantles in rocky planets can differ greatly from that of Earth. These results underscore the importance of investigating the 3D macro physics of shell merger convection through hydrodynamic simulations to develop a predictive understanding of the origin and variability of the light odd- Z elements and the ^40 K/K ratio in planet host stars.

O–C shell mergers in massive stars are a site for producing the p- nuclei by the γ process, but 1D stellar models rely on mixing length theory (MLT), which does not match the radial velocity profiles of 3D hydrodynamic simulations. We investigate how 3D macrophysics-informed mixing impacts the nucleosynthesis of p- nuclei. We postprocess the O shell of the M _ZAMS = 15 M _⊙ , Z = 0.02 model from the NuGrid stellar data set. Applying a downturn to velocities at the boundary and increasing velocities across the shell as obtained in previous results, we find a nonlinear, nonmonotonic increase in p -nuclei production with a spread of 0.96 dex and find that isotopic ratios can change. Reducing C-shell ingestion rates as found in 3D simulations suppresses production, with spreads of 1.22–1.84 dex across MLT and downturn scenarios. Applying dips to the diffusion profile to mimic quenching events also suppresses production, with a 0.51 dex spread. We analyze the impact of varying all photodisintegration rates of unstable n -deficient isotopes from Se to Po by a factor of 10 up and down. The nuclear physics variations for the MLT and downturn cases have a spread of 0.56–0.78 dex. We also provide which reaction rates are correlated with the p- nuclei and find few correlations shared between mixing scenarios. Our results demonstrate that uncertainties in mixing arising from uncertain 3D macrophysics are as significant as nuclear physics and are crucial for understanding p -nuclei production during O–C shell mergers quantitatively.

The merger between the carbon (C) and oxygen (O) shells hours to days before the collapse of a massive star significantly changes its nucleosynthesis, which is reflected in the elemental ratios observed in supernova remnants (SNRs). We present a nucleosynthesis study of 44Ti production in core-collapse supernovae (CCSNe), highlighting large silicon (Si), sulfur (S), calcium (Ca), and, most importantly, argon (Ar) to neon (Ne) ratios as diagnostics for carbon-oxygen (C-O) shell mergers. We compare yields from eight different sets of CCSNe models to observations of Cassiopeia A (Cas A), and show that C-O shell mergers are consistently the models that best match X-ray and infrared observations. These models produce high Ar/Ne ratios (greater than or similar to 0.1), due to 20Ne depletion and production of 36Ar and 38Ar, while lower ratios are obtained from nonmerger cases. Based on the Ar/Ne diagnostic, we compare the range of expected 44Ti produced by C-O shell mergers, which is up to similar to 20%-30% of the overall 44Ti, but expected to be located outside the reverse shock. Based on the sets of models considered, the photon flux expected from the 44Ti synthesized in the C-O shell merger in Cas A is below the NuSTAR and COSI detection limits, compatible with current limits locating most of the 44Ti interior to the reverse shock, but might be detectable from proposed missions like ASCENT. Finally, for the SNR of 1987A, a dominant C-O merger origin of the observed 44Ti is unlikely based on the observed redshift in its 44Ti line.

Several short-lived radionuclides (SLRs) are know to have existed in the early solar system (ESS). These species, which typically decay with half-lives of the order of a few million years, can be used to probe the timescales of events preceding the birth of the Sun. We investigate the ESS origin of ^53 Mn, produced by core-collapse supernovae (CCSNe) and Type Ia supernovae (SNe Ia), and ^60 Fe, produced exclusively by CCSNe. We model the evolution of the radioactive-to-stable abundance ratios of these SLRs with a Galactic chemical evolution (GCE) framework accounting for different SN yields, SN Ia delay times, and other Galactic features ( K ). A further set of models is calculated assuming that SNe Ia did not contribute any ^53 Mn to the ESS. The predicted ratios are compared to meteoritic ratios to derive a distribution of solar isolation times that includes uncertainties due to stochastic chemical enrichment and measurements of the ESS values. The isolation times are then compared to those of ^107 Pd and ^182 Hf calculated in previous work. A self-consistent solution can be found within the current uncertainties, especially when using the GCE setups with K = 1.6 and 2.3, although the maximum likelihood for the ^60 Fe distribution is typically ∼4–5 Myr shorter than for ^53 Mn. The predicted ^60 Fe/ ^53 Mn ratio, instead, is completely inconsistent with the ESS value; this could be resolved using a larger fraction of faint CCSNe than usually considered in GCE models.

This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

Presolar graphite grains carry the isotopic signatures of their parent stars. A significant fraction of presolar graphites show isotopic abundance anomalies relative to solar for elements such as O, Si, Mg, and Ca, which are compatible with nucleosynthesis in core-collapse supernovae (CCSNe). Therefore, they must have condensed from CCSN ejecta before the formation of the Sun. Their most puzzling abundance signature is the ^22 Ne-enriched component Ne-E(L), interpreted as the effect of the radioactive decay of ^22 Na ( T _1/2 = 2.6 yr). Previous works have shown that if H is ingested into the He shell and not fully destroyed before the explosion, the CCSN shock in the He-shell material produces large amounts of ^22 Na. Here we focus on such CCSN models, showing a radioactive ^26 Al production compatible with grain measurements, and analyze the conditions of ^22 Na nucleosynthesis. In these models, ^22 Na is mostly made in the He shell, with a total ejected mass varying between 2.6 × 10 ^−3 M _⊙ and 1.9 × 10 ^−6 M _⊙ . We show that such ^22 Na may already impact the CCSN light curve 500 days after the explosion, and at later stages it can be the main source powering the CCSN light curve for up to a few years before ^44 Ti decay becomes dominant. Based on the CCSN yields above, the 1274.53 keV γ -ray flux due to ^22 Na decay could be observable for years after the first CCSN light is detected, depending on the distance. This makes CCSNe possible sites to detect a ^22 Na γ -ray signature consistently with the Ne-E(L) component found in presolar graphites. Finally, we discuss the potential contribution from ^22 Na decay to the Galactic positron annihilation rate.

Phosphorus-enhanced (P-rich; [P/Fe] ≳ +0.8) giants have been found among mildly metal-poor field stars, but in only one star in a globular cluster (GC), M4 (NGC 6121). Also, in a sample of bulge spheroid stars, some of them showed a moderate P enhancement in the range +0.5 < [P/Fe] < +1.0. In this paper we derive the P abundance of moderately metal-poor ([Fe/H] ≳ −1) GC stars, aiming to check if the phenomenon could be related to the unusual multiple stellar populations found in most GCs. Here we present the detection of moderately P-enhanced stars among two out of seven bulge GCs (Tonantzintla 1 and NGC 6316), with metallicities similar to those of the bulge-field P-rich stars. Using H -band high-resolution ( R ∼ 22,500) spectra from the APOGEE-2 survey, we present the first high-resolution abundance analysis of [P/Fe] from the P I 16482.932 Å line in a sample of selected bulge GCs. We find that all P-rich stars tend to also be N-rich, which hints at the origin of P-rich stars as second-generation stars in GCs. However no other correlations of P and other elements are found, which are usually indicators of second-generation stars. Further studies with larger samples and comparisons with field stars will be needed before any firm conclusions are drawn.

We provide an overview of the latest advances in the study of phosphorus-rich stars, covering their detailed chemical abundance analyses and innovative mining approaches. Following the discovery of 16 low-mass and low-metallicity stars rich in P, we expanded this sample by demonstrating that a recently identified group of Si-rich giants is also P-rich. A detailed abundance analysis was conducted on the nearinfrared spectra from APOGEE-2 DR17, encompassing 13 elements. Subsequently, a similar analysis was performed on the optical UVES spectra of four P-rich stars, resulting in the abundance determination of 48 light and heavy elements. This comprehensive analysis further refined the chemical fingerprint of these peculiar stars, which was employed to evaluate the plausibility of various nucleosynthetic formation scenarios. In order to obtain a statistically more reliable chemical fingerprint in the future, we explored the use of unsupervised machine learning algorithms to identify additional P-rich stars in extensive spectroscopic surveys, such as APOGEE-2. The primary objective of this research is to identify the progenitor of these stars and determine whether current nucleosynthetic models require revision or if a completely new source of P in the Galaxy is responsible for the existence of the P-rich stars.

This study reports detection of rare p -process isotopes within presolar grains. Presolar grains are relic dust grains from dying stars. These microscopic dust particles are found in primitive solar system materials. Their distinct isotopic compositions record the nucleosynthetic processes in their parent stars and the Galactic chemical environment in which these stars formed. We studied presolar graphite grains of high-density type from the Murchison meteorite and found five grains with subgrains that show enrichments in ^84 Sr compared to the solar abundance. ^84 Sr is the neutron-deficient isotope of strontium that can be produced in the deep oxygen-rich interior of high-mass stars that end their lives as core-collapse supernovae. The observed ^84 Sr excesses cannot be produced in low-mass asymptotic giant branch stars, the source of most high-density presolar graphites found in meteorites. High-density graphites with embedded ^84 Sr excesses are, instead, compatible with a core-collapse supernovae origin. The graphite subgrains condensed from carbon-rich materials in the outer layers of core-collapse supernovae, where ^84 Sr was destroyed by neutron captures during hydrostatic evolution of the stars and their final explosion. Based on current theoretical stellar models, a few percent of contribution from the inner regions of core-collapse supernovae, which are enriched in p -process nuclides, to the outer carbon-rich regions is the most likely explanation for the observed enrichment of ^84 Sr in the subgrains of the high-density graphites. In this study, we present the first observational evidence that core-collapse supernovae produce and eject isotopes made by the p -process.

Context. Although current observations indicate that there are two distinct sequences of disk stars in the [ α /M] versus [M/H] parameter space, further complexity is evident in the chemical makeup of the Milky Way and consequently suggests a complicated evolutionary history. Aims. We developed two-infall galactic chemical evolution (GCE) models consistent with the Galactic chemical map. Methods. We obtained new GCE models simulating the chemical evolution of the Milky Way, as constrained by a golden sample of 394 000 stellar abundances of the Milky Way Mapper survey from data release 19 of SDSS-V. The separation between the chemical thin and thick disks was defined using [Mg/M]. We used the chemical evolution environment OMEGA+ combined with Levenberg-Marquardt (LM) and bootstrapping algorithms for the optimization and error estimation. We simulated the entire Galactic disk and considered six galactocentric regions, allowing for a more detailed analysis of the formation of the inner, middle, and outer Galaxy. We investigated the evolution of α , odd-Z, and iron-peak elements, covering 15 species altogether. Results. The chemical thin and thick disks are separated by Mg observations, which the other α -elements show similar trends with, while odd-Z species demonstrate different patterns as functions of metallicity. In the inner Galactic disk regions, the locus of the low-Mg sequence is gradually shifted toward higher metallicity, while the high-Mg phase is less populated. The best-fit GCE models show a well-defined peak in the rate of the infalling matter as a function of the Galactic age, confirming a merger event about 10 Gyr ago. We show that the timescale of gas accretion, the exact time of the second infall and the ratio between the surface mass densities associated with the second infall event and the formation event vary with the distance from the Galactic center. According to the models, the disk was assembled within a timescale of (0.32±0.02) Gyr during a primary formation phase, followed by an increasing accretion rate over a (0.55±0.06) Gyr-timescale and a relaxation phase that lasted (2.86±0.70) Gyr, with a second peak seen for the infall rate at (4.13±0.19) Gyr. Conclusions. Our best Galaxy evolution models are consistent with an inside-out formation scenario of the Milky Way disk and in agreement with the findings of recent chemodynamical simulations.

In this work, we present a new approach to produce spectroscopic constants and model first-principles synthetic spectra for all molecules of astrophysical interest. We have generalized our previous diatomic molecule simulation framework, employing transition-optimized shifted Hermite (TOSH) theory, thereby enabling the modeling of polyatomic rotational constants for molecules with three or more atoms. These capabilities are now provided by our new code Epimetheus. As a first validation of our approach, we confront our predictions and assess their accuracy against the well-studied triatomic molecule ozone 666 ( ^16 O _3 ), in addition to eight of its potential isotopomers: ozone 668 ( ^16 O ^16 O ^18 O), 686 ( ^16 O ^18 O ^16 O), 667 ( ^16 O ^16 O ^17 O), 676 ( ^16 O ^17 O ^16 O), 688 ( ^16 O ^18 O ^18 O), 868 ( ^18 O ^16 O ^18 O), 888 ( ^18 O _3 ), and 777 ( ^17 O _3 ). We then assess the accuracy of these rotational constants using the Epimetheus data in our code Pandora, and generate synthetic molecular spectra. The ozone spectra presented here are purely infrared and not Raman. Epimetheus builds upon the work from our previous code Prometheus, which used the TOSH theory to account for anharmonicity for the fundamental ν = 0 → ν = 1 band, going further to now account for triatomic molecules. This is combined with thermal profile modeling for the rotational transitions. We have found that this extended method performs well, typically approximating the spectroscopic constants with errors of less than 2%. Some issues do arise depending on the symmetry group of the ozone isotopomer. From these spectroscopic constants and using our own spectral modeling code, we show that we can provide the data to produce appreciable molecular spectra, which are good approximations until high-resolution studies can be done.

In massive stars (initial mass of ≳9 M _⊙ ), the weak s - (slow neutron capture) process produces elements between Fe and Zr, enriching the Galaxy with these elements through core-collapse supernova explosions. The weak s -process nucleosynthesis is driven by neutrons produced in the ^22 Ne( α , n ) ^25 Mg reaction during convective He core and C shell burning. The yields of heavy elements thus depend on the ^22 Ne( α , n ) ^25 Mg and the competitive ^22 Ne( α , γ ) ^26 Mg reaction rates, which are dominated by several narrow-resonance reactions. While the accuracy of these rates has been under debate for decades, recent experimental efforts, including ours, drastically reduced these uncertainties. In this work, we use a set of 280 massive star nucleosynthesis models calculated using different ^22 Ne( α , n ) ^25 Mg and ^22 Ne( α , γ ) ^26 Mg rates and a galactic chemical evolution (GCE) study to probe their impact on the weak s -process elemental abundances in the Galaxy. The GCE was computed with the OMEGA+ code, using the new sets of stellar yields with different ^22 Ne+ α rates. From GCE, we find that these rates are causing up to 0.45 dex of variations in the [Cu/Fe], [Ga/Fe], and [Ge/Fe] ratios predicted at solar metallicity. The greatest impact on the stellar nucleosynthesis and GCE results derives from uncertainties in the ( α , n ) strength ( ωγ _( _α _, _n _) ) of the E _x = 11.32 MeV resonance. We show that variations observed in the GCE calculations for weak s -process elements become negligibly smaller than dispersions found in observations once the ωγ _( _α _, _n _) is accurately determined within the uncertainty of 10%–20% (typically reported experimental errors for the resonance) in future nuclear physics experiments.

Neutron capture reactions are the main contributors to the synthesis of the heavy elements through the s-process. Together with ^13C(α,n)16O, which has recently been measured by the LUNA collaboration in an energy region inside the Gamow peak, 22Ne(α,n)25Mg is the other main neutron source in stars. Its cross section is mostly unknown in the relevant stellar energy (450 keV < Ecm < 750 keV), where only upper limits from direct experiments and highly uncertain estimates from indirect sources exist. The ERC project SHADES (UniNa/INFN) aims to provide for the first time direct cross section data in this region and to reduce the uncertainties of higher energy resonance parameters. High sensitivity measurements will be performed with the new LUNA-MV accelerator at the INFN-LNGS laboratory in Italy: the energy sensitivity of the SHADES hybrid neutron detector, together with the low background environment of the LNGS and the high beam current of the new accelerator promises to improve the sensitivity by over 2 orders of magnitude over the state of the art, allowing to finally probe the unexplored low-energy cross section. Here we present an overview of the project and first results on the setup characterization.

Radioactive nuclei with lifetimes on the order of millions of years can reveal the formation history of the Sun and active nucleosynthesis occurring at the time and place of its birth1,2. Among such nuclei whose decay signatures are found in the oldest meteorites, 205Pb is a powerful example, as it is produced exclusively by slow neutron captures (the s process), with most being synthesized in asymptotic giant branch (AGB) stars3-5. However, making accurate abundance predictions for 205Pb has so far been impossible because the weak decay rates of 205Pb and 205Tl are very uncertain at stellar temperatures6,7. To constrain these decay rates, we measured for the first time the bound-state β- decay of fully ionized 205Tl81+, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate8 and our 10% experimental uncertainty has eliminated the main nuclear-physics limitation. With new, experimentally backed decay rates, we used AGB stellar models to calculate 205Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the 205Pb/204Pb ratio from meteorites9-11, we determined the isolation time of solar material inside its parent molecular cloud. We find positive isolation times that are consistent with the other s-process short-lived radioactive nuclei found in the early Solar System. Our results reaffirm the site of the Sun's birth as a long-lived, giant molecular cloud and support the use of the 205Pb-205Tl decay system as a chronometer in the early Solar System.

Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the r-process contribution by their stable isobars are defined as s-only nuclei. For a long time the abundance of ^{204}Pb, the heaviest s-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, ^{204}Tl (t_{1/2}=3.78  yr), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on ^{204}Tl, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of ^{204}Tl produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the s-process temperatures of kT≈8  keV and kT≈30  keV, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new ^{204}Tl MACS, the uncertainty arising from the ^{204}Tl(n,γ) cross section on the s-process abundance of ^{204}Pb has been reduced from ∼30% down to +8%/-6%, and the s-process calculations are in agreement with the latest solar system abundance of ^{204}Pb reported by K. Lodders in 2021.

Context. The oldest stars in the Milky Way are metal-poor with [Fe/H] < -1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of the r-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of the r-process elements in the early Galaxy. Aims. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-0901, HD 6268, HD 121135, and HD 195636 (-1.5 > [Fe/H] > -3.0). Methods. The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex. Results. Based on their kinematical properties, we show that HE 1523-0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature. Conclusions. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (-1.62 > [Fe/H] > -1.12). The most metal-poor stars in our sample, HE 1523-0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metal-licity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523-0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderline r-process enrichment between limited-r and r-I star. Significant relative differences are observed between the r-process signatures in these stars.