Federico Rizzuti

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 low metallicity ( Fe/H łesssim -2 dex), barium can be produced efficiently by both processes, rendering Ba/Eu non-deterministic. We propose using the barium isotopic ratio, from the fitting of resonance line profiles affected by hyperfine splitting. This approach requires precise atomic and stellar parameters, together with advanced spectral modelling, which, so far, had remained insufficiently validated. Ba ii We aim to provide a robust prescription of line-profile modelling for a reliable determination of the s- and r-process fractions of barium in ordinary and peculiar stars. These observational determinations can be used to place constraints on Galactic chemical evolution models, testing yields from different astrophysical sources. We assessed the performance of one-dimensional under local thermodynamic equilibrium (1D LTE) synthesis, 1D non-LTE synthesis, and abundance corrections based on three-dimensional model atmospheres under non-LTE (3D non-LTE) to model Ba lines. Alongside barium abundances and its isotopic ratios, we determined Eu and other neutron-capture element abundances to validate the method in the metal-poor benchmark stars. The observational results are compared with the predictions of stochastic Galactic chemical evolution models that account for 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 towards 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 -0.8 łesssim Ba/Eu łesssim 0 dex range. However, we identify an observational pattern in the Ba/Eu - Ba/Fe plane that can distinguish, at a given Ba/Fe , the dominant neutron capture process. The comparison of our set of models, both for the proto-Milky Way halo and for the Gaia-Enceladus galaxy is used to put constraints on the production of Ba at low metallicity, especially evaluating the role of rotating massive stars. The method developed here can be applied with confidence to both ordinary stars and peculiar stars enhanced in barium.

Only recently, observational studies have started providing measurements for the barium isotopic ratio in metal-poor stars with unprecedented detail. This new approach can be extremely useful in tracing back the origin of neutron-capture elements, since the r- and s-process produce different amounts of barium isotopes, and their astrophysical sites of production are still largely unconstrained. We employ here a stochastic chemical evolution model of the Galactic halo to compare observations to theoretical predictions. We find that in the earliest phases of evolution, both r- and s-process sites are required, with the model and observations agreeing well for Sr, Ba and Eu, possibly requiring a slightly larger s-process production for Sr. The model can actually explain the mixture of r- and s-process material often observed in halo stars. This work shows how is it possible now to use isotopic ratios in addition to elemental ratios to obtain additional constraints useful for the Galactic Archaeology investigation.

While convection has been known to play a key role in stars for many decades, its implementation in one-dimensional stellar evolution codes still represents a major uncertainty today. The purpose of this work is to investigate the impact of initial mass dependent convective boundary mixing (CBM), often referred to as overshooting, on the frequency and type of nuclear burning shell interactions that occur in low metallicity massive stars and the subsequent effect on their fates. Two grids of models were calculated using the Modules for Experiments in Stellar Astrophysics (mesa) code and a 22-isotope nuclear network, each with a different strength of CBM applied. One grid uses the typical CBM value for diffusive overshooting used in literature whereas the other grid uses CBM values guided by the results of 3D convection simulations. Interactions between the carbon, neon and oxygen shells (C-Ne-O) are common throughout both grids. The higher CBM grid also exhibits more frequent H–He and He–C interactions at lower initial masses than in the lower CBM grid. Several models also undergo multiple interaction events during evolution. While future work will be needed to fully assess the impact of the new CBM and the interactions it leads to, one expects interesting effects like unusual nucleosynthesis including more common or enhanced i- and $\gamma$-process nucleosynthesis. Furthermore, SN precursors and a significant change to the pre-SN structure are also expected, with many models not having the commonly expected onion-ring like structure and having a different explosion probability.

Context. The chemical evolution of fluorine is still a matter of debate in Galactic archaeology, especially at low metallicities, where it is particularly challenging to obtain the corresponding chemical abundances from observations. Aims. We present here the first detailed theoretical study of the chemical evolution of fluorine at low metallicities using a stochastic chemical evolution model for the Galactic halo, in light of the most recent data for fluorine, which include observations at lower metallicities down to [Fe/H]∼ −4 dex, more than a factor of 10 lower than previous detections. Methods. We employed a state-of-the-art stochastic chemical evolution model to follow the evolution in the Galactic halo, which has been shown to reproduce the main observables in this Galactic component and the abundance patterns of CNO and neutron-capture elements, and we implemented nucleosynthesis prescriptions for fluorine, focusing on the chemical evolution of this element. Results. By comparing recent observations with model predictions, we confirm the importance of rotating massive stars at low metallicities to explain both the [F/Fe] versus [Fe/H] and [F/O] versus [O/H] diagrams. In particular, we show that we can reach a high [F/Fe] of ∼2 dex at an [Fe/H] of approximately −4 dex, in agreement with recent observations at the lowest metallicities. Conclusions. With a stochastic chemical evolution model for the Galactic halo, we confirm the importance of rotating massive stars as fluorine producers, as suggested in previous studies that used chemical evolution models for the Galactic disc. We also expect an important production of F at high redshifts, in agreement with recent detections of super-solar N by JWST. Further data for fluorine at low metallicities, and also at high redshifts, are needed to put further constraints on the chemical evolution of fluorine and for comparison with our theoretical predictions.

The role of novae as producers of galactic lithium-7 (Li) has been suggested since the 1970s, and it has been reconsidered recently with the detection of beryllium-7 in their outbursts. At the same time, stellar models are moving forward to help us understand the discrepancy between the primordial Li abundance predicted by the standard Big Bang nucleosynthesis theory and the measured value of old dwarf stars. For this work, we followed the evolution of Li in the Galactic thin disc starting from a primordial value of A(Li)= 2.69 dex and applying Li depletion corrections of the stellar model with overshoot to our chemical evolution models. We used the upper envelope of the observational data to constrain the models. In addition to the dwarf main-sequence (MS) stars, our analysis included, for the first time, the early red-giant-branch (RGB) stars. In addition to the renowned Spite plateau of the MS stars at low metallicities, we also confirm the existence of a second A(Li) plateau of the early RGB stars, which can be explained by our model with the corrections from stellar models. Our best-fit model was obtained with an effective averaged Li yield ^L Y _Nova = 2.34 × 10 ^-5 M _⊙ during the whole lifetime of a nova. This reinforces the possibility that novae are the main galactic Li source, together with the stellar models’ ability to resolve the ‘cosmological Li problem’ in this context.

Aims. We determine the contributions of the rapid (r) and slow (s) neutron capture processes to the Ba isotope mixture, along with Ba, Eu, and Sr NLTE abundances, in a sample of very metal-poor stars. The selected stars formed before the contribution from the main s-process in low- and intermediate-mass stars became significant. Some of our sample stars are enhanced in Sr, with [Sr/Ba] reaching up to 0.7. These stars gained their high Sr abundance from a poorly understood process, sometimes referred to in the literature as a light element primary process, which may appear to be a weak s-process or a weak r-process. Our aim is to uncover the nature of this additional Sr source. Methods. The abundances derived from the resonance Ba II 4554 and 4934 Å lines are influenced by the adopted Ba isotope mixture. We computed Ba isotope mixtures corresponding to different r- to s-process contributions (pure r-process, 80%/20%, 50%/50% and 12%/88%, i.e. solar ratio) and determined the corresponding abundances from the Ba II resonance lines in each sample star. Additionally, we determined Ba abundances from weak subordinate Ba II lines, which are unaffected by the adopted Ba isotope mixture. We then compared the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines. Results. We find a higher s-process contribution to Ba isotopes in stars with greater [Sr/Eu] and [Sr/Ba] overabundances, suggesting that the additional Sr synthesis was due to the early s-process occurring in massive stars. Using Sr-enhanced stars, we estimate the [Sr/Ba] ratio produced by the early s-process and obtain [Sr/Ba]_earlyS = 1.1 ± 0.2. The derived value should be regarded as an upper limit, as we cannot definitively exclude the possibility of a contribution to Sr from the weak r-process, which produces Sr but not Ba. Regarding the potential synthesis of Sr and Ba in the i-process in massive stars, our results for Ba isotopes and element abundances argue that there was no detectable contribution from this process within the error bars in our sample stars. Conclusions. In the early Galaxy, before significant main s-process enrichment, barium and strontium were produced primarily by the main r-process and the early s-process, which occurred in rapidly rotating massive stars.

Context. Globular clusters (GCs) are crucial to our understanding of the formation and evolution of our Galaxy. While their abundances of light and iron-peak elements have been extensively studied, research on heavier elements and their possible link to both the multiple stellar population phenomenon and the origin of GCs remains relatively limited. Aims. We aim to analyse the chemical abundances of various neutron-capture elements using GCs as tracers of the Galactic halo. Furthermore, we explore the potential connection between these elements and the multiple stellar population phenomenon in GCs to better constrain the nature of the polluters responsible for the intracluster enrichment. Additionally, we seek to determine the origins of GCs based on their neutron-capture element abundances. Methods. We analysed a sample of 14 GCs spanning a wide metallicity range, [Fe/H] from −0.40 to −2.32, observed as a part of the Gaia-ESO Survey and analysed using a homogeneous methodology. Here we present results for Y, Zr, Ba, La, Ce, Nd, Pr, and Eu obtained from FLAMES-UVES spectra. We compared our results with a stochastic Galactic chemical evolution model. Results. Except for Zr, the Galactic chemical evolution model, when available, closely describes the broad trend displayed by neutron-capture elements in GCs. Moreover, in some clusters, a strong correlation between hot H-burning (Na and Al) and s-process elements suggests a shared nucleosynthetic site, for example asymptotic giant branch stars of different masses and/or fast-rotating massive stars that produced the intracluster pollution. Additionally, we identified clear differences in the [Eu/Mg] ratio between in situ (⟨[Eu/Mg]]⟩=0.14 dex) and ex situ (⟨[Eu/Mg]]⟩=0.32 dex GCs, which reveal their distinct chemical enrichment histories. Finally, on average, the Type II GCs NGC 362, NGC 1261, and NGC 1851 show a spread ratio in s-process elements between second- and first-generation stars that is roughly twice as large as that observed in Type I clusters.

Context. Recent campaigns of observations have provided new measurements of the carbon isotopes in the most metal-poor stars of the Galaxy. These stars are so metal-poor that they could only have been enriched by one or few generations of massive progenitors. However, explaining the primary production of ¹³C and the low ¹²C/¹³C ratio measured in these stars is challenging. Aims. Making use of the most up-to-date models for zero-metal and low-metallicity stars, we investigate possible sources of ¹³C at low metallicity and verify whether massive stars could be solely responsible for the ¹²C/¹³C ratio observed in halo stars. Methods. We employed the stochastic model for Galactic chemical evolution GEMS to reproduce the evolution of CNO elements and ¹²C/¹³C ratio, including the enrichment from rotating massive stars, some of which show the occurrence of H-He shell mergers. Results. We find that stars without H-He shell mergers do not produce enough ¹³C to be compatible with the observations. Instead, primary production via shell mergers and subsequent ejection during the supernova explosion can explain a ratio of 30 <¹²C/¹³C < 100. The observations are best reproduced assuming a large frequency of shell mergers. A ratio of ¹²C/¹³C < 30 can only be reproduced by assuming an outer layer ejection and no explosion, but requiring a higher level of production of ¹²C and ¹³C. Conclusions. Zero-metal and low-metallicity spinstars with H-He shell mergers appear as the most plausible scenario to explain the low ¹²C/¹³C ratio in carbon-enhanced metal-poor (CEMP) stars. The entire range of ¹²C/¹³C values can be explained by assuming that some stars fully explode, while others only eject their outer layers. Shell mergers are also expected to be more frequent and productive, which is allowed by the current uncertainties related to the treatment of convection in stellar modelling.

Context. Recent observations by JWST have revealed supersolar ¹⁴N abundances in galaxies at very high redshift. On the other hand, these galaxies show subsolar metallicity. The observed N/O ratios are difficult to reproduce in the framework of chemical evolution models for the Milky Way. Aims. Our aim is to reproduce these high N/O ratios with chemical evolution models, assuming different histories of star formation triggering galactic winds, coupled with detailed nucleosynthesis prescriptions for ¹⁴N, ¹²C, ¹⁶O, and ⁵⁶Fe. Methods. We computed several models for small galaxies (10⁹−10¹⁰ M_⊙) with a high star formation efficiency and strong galactic winds. These winds are assumed to be differential, mainly carrying out the products of the explosion of core-collapse supernovae. Results. We find that only models with high star formation rates, a normal initial mass function, and differential galactic winds can reproduce the observed chemical abundances. We also find that, with the same assumptions about star formation and galactic winds, but with a very rapid formation resulting from fast gas infall, we can also reproduce the estimated ages of these objects. We find no necessity to invoke peculiar nucleosynthesis from population III stars, very massive stars, and supermassive stars.

Context. The chemical abundances of elements such as barium and the lanthanides are essential to understand the nucleosynthesis of heavy elements in the early Universe as well as the contribution of different neutron capture processes (for example slow versus rapid) at different epochs. Aims. The Chemical Evolution of R-process Elements in Stars (CERES) project aims to provide a homogeneous analysis of a sample of metal-poor stars ([Fe/H]<−1.5) to improve our understanding of the nucleosynthesis of neutron capture elements, in particular the r-process elements, in the early Galaxy. Methods. Our data consist of a sample of high resolution and high signal-to-noise ratio UVES spectra. The chemical abundances were derived through spectrum synthesis, using the same model atmospheres and stellar parameters as derived in the first paper of the CERES series. Results. We measured chemical abundances or upper limits of seven heavy neutron capture elements (Ba, La, Ce, Pr, Nd, Sm, and Eu) for a sample of 52 metal-poor giant stars. We estimated through the mean shift clustering algorithm that at [Ba/H]=−2.4 and [Fe/H]=−2.4 a variation in the trend of [X/Ba], with X=La,Nd,Sm,Eu, versus [Ba/H] occurs. This result suggests that, for [Ba/H]<−2.4, Ba nucleosynthesis in the Milky Way halo is primarily due to the r-process, while for [Ba/H]>−2.4 the effect of the s-process contribution begins to be visible. In our sample, stars with [Ba/Eu] compatible with a Solar System pure r-process value (hereafter, r-pure) do not show any particular trend compared to other stars, suggesting r-pure stars may form in similar environments to stars with less pure r-process enrichments. Conclusions. Homogeneous investigations of high resolution and signal-to-noise ratio spectra are crucial for studying the heavy elements formation, as they provide abundances that can be used to test nucleosynthesis models as well as Galactic chemical evolution models.