Marco Palla

The formation pathway of the α -poor Milky Way (MW) thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high- α disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that α -poor stars have coexisted with α -rich stars since ∼12.5 Gyr ago. Stellar ages of 0.5 Gyr precision were re-derived for nearly 30 000 subgiant stars using accurate effective temperatures ( T _eff ) from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30% have relative low [ α /Fe] and solar-like metallicities (−0.5 ≤ [Fe/H] ≤ +0.5 dex). These stars exhibit kinematics consistent with those of near solar-age α -poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose T _eff and gravity modified with time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, α -poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.

Context. Understanding the interstellar medium (ISM) requires high-resolution, multi-component mapping to capture its complex physical structure. Nearby spiral galaxies, with their abundant and diverse ISM, provide an ideal laboratory for such a comprehensive analysis at sub-galactic scales. Aims. We investigate dust-to-gas and dust-to-metal ratios (DGR and DMR) as functions of the gas-phase metallicity ( Z ) on spatial scales ranging from 0.6 to 2.3 kpc, in a sample of ten nearby spiral galaxies spanning more than an order of magnitude in stellar mass (9.7 ≤ log( M _* / M _⊙ )≤11.0), star formation rate (SFR, ∼0.3–3 M _⊙ yr ⁻¹ ), and metallicity ranging from 8.3 ≲ 12 + log(O/H) ≲ 8.8. We explore how the DGR– Z and DMR– Z relations are shaped by the assumptions behind the CO-to-H ₂ conversion factor ( α _CO ). Methods. We homogeneously combined maps of dust, atomic gas, molecular gas, and metallicity. Motivated by the diversity in L _{CO(1 − 0)} /SFR ratios and metallicity across our sample, we introduced a hybrid α _CO prescription to distinguish between CO-bright and CO-dark regimes. The derived DGR– Z and DMR– Z relations were compared with other global and resolved observational results, and with the predictions of dust and chemical evolution models. Results. Both DGR– Z and DMR– Z relations are dependent on the adopted α _CO prescription, and no single α _CO can reproduce the properties of the entire sample, motivating the use of a hybrid approach. The DGR increases with metallicity, spanning ∼1 dex across the sampled range, while the DMR remains approximately constant at log(DMR) = − 0.53 ± 0.13, implying that ∼30% of metals are locked into dust grains. This flat behaviour indicates an evolved dust phase where efficient ISM grain growth drives a saturation regime balancing dust formation and destruction.

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.

The evolution of massive stars in binary systems that undergo mass transfer during their lifetime is different from that of single stars. This might affect their chemical yields. While massive stars produce most of the metals in the Universe, only a few studies have investigated the effects of massive binary stars on the chemical evolution of the Milky Way. Following the most recent studies on massive binary-stripped star yields as functions of metallicity, we wish to improve previous results based on single-metallicity model grids. By adopting a detailed model of chemical evolution for our Galaxy, we computed the evolution of 22 chemical species including C, N, O, α-elements, and Fe-peak elements, adopting novel prescriptions for single and binary massive star yields. Our main results can be summarised as follows: (i) consistently with previous predictions, we observe very small differences in the predicted solar abundances and X/Fe versus Fe/H relations even when we included massive binary yields depending on metallicity; (ii) when adopting the new set of stellar yields for massive single stars, as computed by , we were able to reproduce the K solar abundance and the farmer2023 K/Fe versus Fe/H relation, without invoking ad hoc assumptions on nucleosynthesis prescriptions; (iii) our model adopting Farmer's yields for single and binary massive stars can reproduce the X/Fe versus Fe/H relation better for Mg and Ca than standard nucleosynthetic yields adopted in chemical evolution models; and finally, iv) no model reproduced the C/Fe and Ti/Fe versus Fe/H relation well when we adopted the new yields as functions of metallicity.

Context . In the context of galactic archaeology, the study of the Small Magellanic Cloud (SMC) is of crucial importance, as it represents a unique opportunity to study a nearby massive dwarf system. However, theoretical studies of the chemical evolution of this galaxy are significantly lacking. Aims . In this study, we investigate the chemical enrichment of the SMC galaxy. In addition to the α and Fe-peak elements, we devote particular attention to the evolution of neutron-capture elements with a different origin; namely, r-process (Eu), weak s-process (Zr) and main s-process (Ba, La). Methods . We developed chemical evolution models that use as their input the star formation histories obtained from colour-magnitude diagram fitting. We closely followed the chemical feedback provided by a large variety of nucleosynthetic sources. Our model predictions were compared with recent abundance measurements for the SMC. Results . The developed framework aptly reproduces all the observables for elements up to the Fe-peak. The abundance patterns of n-capture elements are simultaneously reproduced only by assuming an enhanced contribution from the delayed r-process at low metallicity and a top-lighter IMF relative to the reference IMF by Kroupa 2001 (MNRAS, 322, 231). In this way, both the observed very high plateau in [Eu/Fe] and the rising trends in [s-process/Fe] ratios can be reproduced by the models. Conclusions . This study provides, for the first time, information on the evolution of several n-capture elements in a massive dwarf irregular galaxy, also providing insight on several ingredients driving galactic evolution. Moreover, this work provides a test-bed for further modelling of the SMC in the context of the numerous surveys that will target the Magellanic Clouds in the next years.

Context . The accretion of Gaia -Enceladus/Sausage (GES) onto the Milky Way (MW) is one of the most prominent features of the Galactic halo revealed by the combination of the Gaia satellite and large spectroscopic surveys. This massive accretion largely contributes to the local stellar halo mass and was significant enough to alter the formation history and the morphology of the MW. Aims . In this work, we aim to analyse the selection of stars previously identified as belonging to GES with different kinematics and chemical properties to test the hypothesis of a two-phase accretion event. Methods . We apply several statistical tests to assess the significance of the separation between the two populations in GES. We then employ galactic chemical evolution models to investigate the origin of the chemical differences encountered in the analysis. Results . We confirm the presence of two distinct populations, with consistently different dynamical and chemical properties. The low energy population seems to show higher overall abundances, whereas the high-energy one may be more metal-poor. We attribute this difference to the presence of at least two separate populations of stars within Gaia -Enceladus, likely associated with the innermost (low-energy) and outermost (high-energy) regions of the progenitor. The adopted models successfully reproduce the patterns in metal-licity and [ α /M] distributions in an inside-out scenario. Conclusions . Our analysis supports the presence of a former metallicity gradient in Gaia -Enceladus, and reinforces the interpretation of its accretion as a multi-passage event through the Milky Way disc.

We present the chemical abundances of Fe, α -, and neutron-capture elements in 12 metal-poor Small Magellanic Cloud (SMC) giant stars, observed with the high-resolution spectrographs UVES/VLT and MIKE/Magellan. These stars are characterised by [Fe/H] between −2.3 and −1.4 dex, with ten of them with [Fe/H] < −1.8 dex. According to theoretical age-metallicity relations for this galaxy, these stars were formed in the first Gyr of life of the SMC and represent the oldest SMC stars known so far. The [ α /Fe] abundance ratios are enhanced but at a lower level than MW metal-poor stars, as expected according to the slow star formation rate (SFR) of the SMC. The sample exhibits a large star-to-star scatter in all the neutron-capture elements. The two r -process elements measured in this work (Eu and Sm) have abundance ratios from solar up to +1 dex, three of them with [Eu/Fe] > +0.7 dex and labelled as r -II stars. This [ r /Fe] distribution indicates that the r -process in the SMC can be extremely efficient but is still largely affected by the stochastic nature of the main sites of production and the inefficient gas mixing in the early SMC evolution. A similar scatter is observable also for the s -process elements (Y, Ba, La, Ce, and Nd), with the stars richest in Eu also being rich in these s -elements. Also, all the stars exhibit sub-solar [ s /Eu] abundance ratios. At the metallicities of these stars, the production of neutron-capture elements is driven by the r -process because the low-mass AGB stars have not yet evolved and left their s -process signature in the interstellar medium (ISM). In this work, we also present a set of stochastic chemical evolution models tailored for the SMC to validate this scenario.

In this study we investigate the chemical enrichment of the rapid neutron-capture process in the Small Magellanic Cloud (SMC). We measured the [Eu/Fe] abundance ratio of 209 giant stars that are confirmed members of the SMC, providing the first extensive dataset of Eu abundances in this galaxy across its full metallicity range, spanning more than 1.5 dex. We compared the Eu abundances with those of Mg and Ba to evaluate the efficiency of the r -process relative to α-capture and s -process nucleosynthesis. The SMC shows enhanced [Eu/Fe] values at all metallicities (comparable with the values measured in the Milky Way) and a clear decline as [Fe/H] increases (from approximately −1.75 dex to approximately −0.5 dex), which is consistent with the onset of Type Ia supernovae. In contrast, [Eu/Mg] is enhanced by about +0.5 dex at all [Fe/H] and thus significantly above the values observed in Milky Way stars, where [Eu/Mg] remains close to the solar value, reflecting comparable production of r -process and α-capture elements. Moreover, [Ba/Eu] increases with metallicity beginning at [Fe/H] ≈ −1.5 dex, namely at a lower metallicity with respect to the Milky Way, where [Ba/Eu] starts to increase around [Fe/H] ≈ −1 dex. Our findings suggest the SMC has a higher production of Eu (with respect to the α-elements) than the Milky Way, but it is still in line with what has been observed in other dwarf systems within the Local Group. We confirm that galaxies with star formation efficiencies lower than the Milky Way have a high [Eu/α], probably indicating stronger efficiency of the delayed sources of the r -process at low metallicities.

To fully characterize the formation and evolution of galaxies, we need to observe their stars, gas, and dust on resolved spatial scales. We present the ALPINE-CRISTAL-JWST survey, which combines kiloparsec-resolved imaging and spectroscopy from the Hubble Space Telescope, JWST, and Atacama Large Millimeter/submillimeter Array for 18 representative main-sequence galaxies at z = 4–6 and log ( M * / M ⊙ ) > 9.5 to study their star formation, chemical properties, and extended gas reservoirs. The cospatial measurements resolving the ionized gas, molecular gas, stars, and dust on 1–2 kpc scales make this a unique benchmark sample for the study of galaxy formation and evolution at z ∼ 5, connecting the Epoch of Reionization with the cosmic noon. In this paper, we outline the survey goals and sample selection, and present a summary of the available data for the 18 galaxies. In addition, we measure spatially integrated quantities (such as global gas metallicity), test different star formation rate indicators, and quantify the presence of H α halos. Our targeted galaxies are relatively metal rich (10%–70% solar), complementary to JWST samples at lower stellar mass, and there is broad agreement between different star formation indicators. One galaxy has the signature of an active galactic nuclei (AGN) based on its emission-line ratios. Six show broad H α emission suggesting type 1 AGN candidates. We conclude with an outlook on the exciting science that will be pursued with this unique sample in forthcoming papers.

We present the results of the first systematic and dedicated high-resolution chemical analysis of the Thamnos substructure, a candidate relic of the process of hierarchical merging of the Milky Way. The analysis was performed in comparison with the Gaia -Sausage-Enceladus (GSE) remnant, within the fully self-consistent and homogeneous framework established by the ‘A Walk on the Retrograde Side’ (WRS) project. We analysed high-resolution and high signal-to-noise ratio spectra obtained with UVES at VLT for 212 red giant branch stars classified as candidate members of Thamnos and GSE, based on selections in the space of the integrals of motion. We derived precise abundances for 16 atomic species. Compared to GSE, stars attributed to the Thamnos substructure are, on average, more metal-poor, yet most of them show higher [X/Fe] abundance ratios in several elements, such as Na, Mg, Al, Ca, Cu, and Zn, as well as lower [Eu/Fe]. The majority of candidate Thamnos stars show chemical signatures more consistent with the in situ Milky Way halo rather than a typical low-mass accreted dwarf galaxy. Our findings are further supported by comparisons with tailored galactic chemical evolution models, which fall short in reproducing the observed enhancement in the α-elements, but are able to fit the more metal-poor component present in the Thamnos substructure. These results confirm a high level of contamination in the Thamnos substructure from the in situ population and to a lesser degree from GSE, while still leaving room for a genuine accreted population from a small disrupted dwarf galaxy.