arXiv subjects

F. Matteucci

Publications and source records attributed to F. Matteucci.

At least 37 records · Page 2Linked to original sources

arXiv2024

NGC1856: Using machine learning techniques to uncover detailed stellar abundances from MUSE data

We present the first application of the novel approach based on data-driven machine learning methods applied to Multi-Unit Spectroscopic Explorer (MUSE) field data to derive stellar abundances of star clusters. MUSE has been used to target more than 10,000 fields, and it is unique in its ability to study dense stellar fields such as stellar clusters providing spectra for each individual star. We use MUSE data of the extragalactic young stellar cluster NGC 1856, located in the Large Magellanic Cloud (LMC). We present the individual stellar [Fe/H] abundance of 327 cluster members in addition to [Mg/Fe], [Si/Fe], [Ti/Fe], [C/Fe], [Ni/Fe], and [Cr/Fe] abundances of subsample sets. Our results match the LMC abundances obtained in the literature for [Mg/Fe], [Ti/Fe], [Ni/Fe], and [Cr/Fe]. This study is the first to derive [Si/Fe] and [C/Fe] abundances for this cluster. The revolutionary combination of integral-field spectroscopy and data-driven modeling will allow us to understand the chemical enrichment of star clusters and their host galaxies in greater detail expanding our understanding of galaxy evolution.

astro-ph.GA↗

arXiv2024

MINCE II. Neutron capture elements

The MINCE (Measuring at Intermediate metallicity Neutron-Capture Elements) project aims to gather the abundances of neutron-capture elements but also of light elements and iron peak elements in a large sample of giant stars in this metallicity range. T The aim of this work is to study the chemical evolution of galactic sub-components recently identified (i.e. Gaia Sausage Enceladus (GSE), Sequoia). We used high signal-to-noise ratios, high-resolution spectra and standard 1D LTE spectrum synthesis to determine the detailed abundances. We could determine the abundances for up to 10 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu) in 33 stars. The general trends of abundance ratios [n-capture element/Fe] versus [Fe/H] are in agreement with the results found in the literature. When our sample is divided in sub-groups depending on their kinematics, we found that the run of [Sr/Ba] vs [Ba/H] for the stars belonging to the GSE accretion event shows a tight anti-correlation. The results for the Sequoia stars, although based on a very limited sample, shows a [Sr/Ba] systematically higher than the [Sr/Ba] found in the GSE stars at a given [Ba/H] hinting at a different nucleosynthetic history. Stochastic chemical evolution models have been computed to understand the evolution of the GSE chemical composition of Sr and Ba. The first conclusions are that the GSE chemical evolution is similar to the evolution of a dwarf galaxy with galactic winds and inefficient star formation. Detailed abundances of neutron-capture elements have been measured in high-resolution, high signal-to-noise spectra of intermediate metal-poor stars, the metallicity range covered by the MINCE project. These abundances have been compared to detailed stochastic models of galactic chemical evolution.

astro-ph.SR↗

arXiv2023

2D chemical evolution models II. Effects of multiple spiral arm patterns on O, Eu, Fe and Ba abundance gradients

According to observations and numerical simulations, the Milky Way could exhibit several spiral arm modes with multiple pattern speeds, wherein the slower patterns are located at larger Galactocentric distances. Our aim is to quantify the effects of the spiral arms on the azimuthal variations of the chemical abundances for oxygen, iron and for the first time for neutron-capture elements (europium and barium) in the Galactic disc. We assume a model based on multiple spiral arm modes with different pattern speeds. The resulting model represents an updated version of previous 2D chemical evolution models. We apply new analytical prescriptions for the spiral arms in a 2D Galactic disc chemical evolution model, exploring the possibility that the spiral structure is formed by the overlap of chunks with different pattern speeds and spatial extent. The predicted azimuthal variations in abundance gradients are dependent on the considered chemical element. Elements synthesised on short time scales (i.e., oxygen and europium in this study) exhibit larger abundance fluctuations. In fact, for progenitors with short lifetimes, the chemical elements restored into the ISM perfectly trace the star formation perturbed by the passage of the spiral arms. The map of the star formation rate predicted by our chemical evolution model with multiple patterns of spiral arms presents arcs and arms compatible with those revealed by multiple tracers (young upper main sequence stars, Cepheids, and distribution of stars with low radial actions). Finally, our model predictions are in good agreement with the azimuthal variations that emerged from the analysis of Gaia DR3 GSP-Spec [M/H] abundance ratios, if at most recent times the pattern speeds match the Galactic rotational curve at all radii.

astro-ph.GA↗

arXiv2023

Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk

Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Our aim is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs ever analyzed. The current sample covers a wide range in Galactocentric distances (RG), pulsation modes and periods. High-resolution and high S/N spectra collected with different spectrographs were adopted to estimate the atmospheric parameters. Individual distances are based on Gaia trigonometric parallaxes or on near-infrared Period-Luminosity relations. We found that Fe and alpha-element radial gradients based on CCs display a well-defined change in the slope for RG larger than 12 kpc. Radial gradients based on open clusters, covering a wide range in age, display similar trends, meaning that the flattening in the outer disk is an intrinsic feature of the radial gradients since it is independent of age. Empirical evidence indicates that the radial gradient for S is steeper than for Fe. The difference in the slope is a factor of two in the linear fit. We also found that S is, on average, under-abundant compared with O. We performed a detailed comparison with Galactic chemical evolution models and we found that a constant Star Formation Efficiency for RG larger than 12 kpc takes account for the flattening in both Fe and alpha-elements. To further constrain the impact that predicted S yields for massive stars have on radial gradients, we adopted a "toy model" and we found that the flattening in the outermost regions requires a decrease of a factor of four in the current S predictions. Sulfur photospheric abundances, compared with other alpha-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular S abundances in external galaxies.

astro-ph.GA↗

arXiv2023

Analytic solution of Chemical Evolution Models with Type Ia SNe

Context: In the last years, a significant number of works have focused on finding analytic solutions for the chemical enrichment models of galactic systems, including the Milky Way. Some of these solutions, however, cannot account for the enrichment produced by Type Ia SNe due to the presence of the delay time distributions (DTDs) in the models. Aims: We present a new analytic solution for the chemical evolution model of the Galaxy. This solution can be used with different prescriptions of the DTD, including the single and double degenerate scenarios, and allows the inclusion of an arbitrary number of pristine gas infalls. Methods: We integrate the chemical evolution model by extending the instantaneous recycling approximation with the contribution of Type Ia SNe. For those DTDs that lead to non-analytic integrals, we describe them as a superposition of Gaussian, exponential and 1/t functions using a restricted least-squares fitting method. Results: We obtain the exact solution for a chemical model with Type Ia SNe widely used in previous works. This solution can reproduce the expected chemical evolution of the alpha and iron-peak elements in less computing time than numerical integration methods. We compare the pattern in the [Si/Fe] vs. [Fe/H] plane observed by APOGEE DR17 with that predicted by the model. We find the low alpha sequence can be explained by a delayed gas infall. We exploit the applicability of our solution by modelling the chemical evolution of a simulated Milky Way-like galaxy from its star formation history. The implementation of our solution is released as a python package. Conclusions: Our solution constitutes a promising tool for the Galactic Archaeology and is able to model the observed trends in alpha element abundances versus [Fe/H] in the solar neighbourhood. We infer the chemical information of a simulated galaxy modelled without Chemistry.

astro-ph.GA↗

arXiv2023

Time evolution of Ce as traced by APOGEE using giant stars observed with the Kepler, TESS and K2 missions

Abundances of s-capture process elements in stars with exquisite asteroseismic, spectroscopic, and astrometric constraints offer a novel opportunity to study stellar evolution, nucleosynthesis, and Galactic chemical evolution. We aim to investigate one of the least studied s-process elements in the literature, Ce, using stars with asteroseismic constraints from the Kepler, K2 and TESS missions. We combine the global asteroseismic parameters derived from precise light curves obtained by the Kepler, K2 and TESS missions with chemical abundances from the APOGEE DR17 survey and astrometric data from the Gaia mission. Finally, we compute stellar ages using the code PARAM. We investigate the different trends of [Ce/Fe] as a function of [Fe/H], [alpha/Fe] and age considering the dependence on the radial position, specially in the case of K2 targets which cover a large Galactocentric range. We, finally, explore the [Ce/alpha] ratios as a function of age in different Galactocentric intervals. The studied trends display a strong dependence of the Ce abundances on [Fe/H] and star formation history. Indeed, the [Ce/Fe] ratio shows a non-monotonic dependence on [Fe/H] with a peak around -0.2 dex. Moreover, younger stars have higher [Ce/Fe] and [Ce/alpha] ratios than older stars, confirming the latest contribution of low- and intermediate-mass asymptotic giant branch stars to the Galactic chemical enrichment. In addition, the trends of [Ce/Fe] and [Ce/alpha] with age become steeper moving towards the outer regions of the Galactic disc, demonstrating a more intense star formation in the inner regions than in the outer regions. Ce is thus a potentially interesting element to help constraining stellar yields and the inside-out formation of the Milky Way disc. However, the large scatter in all the relations studied here, suggests that spectroscopic uncertainties for this element are still too large.

astro-ph.GA↗

arXiv2023

Europium enrichment and hierarchical formation of the Galactic halo

Context. The origin of the large star-to-star variation of the [Eu/Fe] ratios observed in the extremely metal-poor (at [Fe/H]$\leq-3$) stars of the Galactic halo is still a matter of debate.\\ Aims. In this paper, we explore this problem by putting our stochastic chemical evolution model in the hierarchical clustering framework, with the aim of explaining the observed spread in the halo.\\ Methods. We compute the chemical enrichment of Eu occurring in the building blocks that have possibly formed the Galactic halo. In this framework, the enrichment from neutron star mergers can be influenced by the dynamics of the binary systems in the gravitational potential of the original host galaxy. In the least massive systems, the neutron stars can merge outside the host galaxy and so only a small fraction of newly produced Eu can be retained by the parent galaxy itself.\\ Results. In the framework of this new scenario, the accreted merging neutron stars are able to explain the presence of stars with sub-solar [Eu/Fe] ratios at [Fe/H]$\leq-3$, but only if we assume a delay time distribution for merging of the neutron stars $\propto t^{-1.5}$. We confirm the correlation between the dispersion of [Eu/Fe] at a given metallicity and the fraction of massive stars which give origin to neutron star mergers. The mixed scenario, where both neutron star mergers and magneto-rotational supernovae do produce Eu, can explain the observed spread in the Eu abundance also for a delay time distribution for mergers going either as $\propto t^{-1}$ or $\propto t^{-1.5}$.

astro-ph.GA↗

arXiv2023

Detailed $α$ abundance trends in the inner Galactic bulge

In this paper, we aim to derive high-precision alpha-element abundances using CRIRES high-resolution IR spectra of 72 cool M giants of the inner Galactic bulge. Silicon, magnesium, and calcium abundances were determined by fitting a synthetic spectrum for each star. We also incorporated recent theoretical data into our spectroscopic analysis (i.e. updated K-band line list, better broadening parameter estimation, non-local thermodynamic equilibrium (NLTE) corrections). We compare these inner bulge alpha abundance trends with those of solar neighbourhood stars observed with IGRINS using the same line list and analysis technique; we also compare our sample to APOGEE DR17 abundances for inner bulge stars. We investigate bulge membership using spectro-photometric distances and orbital simulations. We construct a chemical-evolution model that fits our metallicity distribution function (MDF) and our alpha-element trends. Among our 72 stars, we find four that are not bulge members. [Si/Fe] and [Mg/Fe] versus [Fe/H] trends show a typical thick disc alpha-element behaviour, except that we do not see any plateau at supersolar metallicities as seen in other works. The NLTE analysis lowers [Mg/Fe] typically by $\sim$0.1 dex, resulting in a noticeably lower trend of [Mg/Fe] versus [Fe/H]. The derived [Ca/Fe] versus [Fe/H] trend has a larger scatter than those for Si and Mg, but is in excellent agreement with local thin and thick disc trends. With our updated analysis, we constructed one of the most detailed studies of the alpha abundance trends of cool M giants in the inner Galactic bulge. We modelled these abundances by adopting a two-infall chemical-evolution model with two distinct gas-infall episodes with timescales of 0.4 Gyr and 2 Gyr, respectively. Based on a very meticulous spectral analysis, we have constructed detailed and precise chemical abundances of Mg, Si, and Ca for cool M giants.

astro-ph.GA↗

arXiv2022

The cerium content of the Milky Way as revealed by Gaia DR3 GSP-Spec abundances

The recent Gaia Third Data Release contains a homogeneous analysis of millions of high-quality Radial Velocity Spectrometer (RVS) stellar spectra by the GSP-Spec module. This led to the estimation of millions of individual chemical abundances and allows us to chemically map the Milky Way. Among the published GSP-Spec abundances, three heavy-elements produced by neutron-captures in stellar interiors can be found: Ce, Zr and Nd. We use a sample of about 30,000 LTE Ce abundances, selected after applying different combinations of GSP-Spec flags. Thanks to the Gaia DR3 astrometric data and radial velocities, we explore the cerium content in the Milky Way and, in particular, in its halo and disc components. The high quality of the Ce GSP-Spec abundances is quantified thanks to literature comparisons. We found a rather flat [Ce/Fe] versus [M/H] trend. We also found a flat radial gradient in the disc derived from field stars and, independently, from about 50 open clusters, in agreement with previous studies. The [Ce/Fe] vertical gradient has also been estimated. We also report an increasing [Ce/Ca] vs [Ca/H] in the disc, illustrating the late contribution of AGB with respect to SN II. Our cerium abundances in the disc, including the young massive population, are well reproduced by a new three-infall chemical evolution model. Among the halo population, the M 4 globular cluster is found to be enriched in cerium. Moreover, eleven stars with cerium abundances belonging to the Thamnos, Helmi Stream and Gaia-Sausage-Enceladus accreted systems were identified from chemo-dynamical diagnostics. We found that the Helmi Stream could be slightly underabundant in cerium, compared to the two other systems. This work illustrates the high quality of the GSP-Spec chemical abundances, that significantly contributes to unveil the heavy elements evolution history of the Milky Way.

astro-ph.GA↗

arXiv2022

Beyond the two-infall model I. Indications for a recent gas infall with Gaia DR3 chemical abundances

The recent Gaia Data Release 3 (DR3) represents an unparalleled revolution in Galactic Archaeology, providing us with numerous radial velocities chemical abundances for millions of stars, with all-sky coverage. We present a new chemical evolution model for the Galactic disc components (high- and low- $α$ sequence stars) designed to reproduce the new abundance ratios provided by the GSP-spec module for the Gaia DR3 and also constrained by the detailed star formation histories for both the thick and thin disc stars inferred from previous Gaia releases. Sophisticated modeling based on previous Gaia releases has found evidence for narrow episodes of enhanced SF inferred in recent time. Additionally, Gaia DR3 highlighted the presence of young (massive) low-$α$ disc stars which show evidence of a recent chemical impoverishment in several elements. Hence, in this study, we compare Gaia DR3 chemical abundances with the predictions of a three-infall chemical evolution model for the high- and low-$α$ components. The proposed three-infall chemical evolution model nicely reproduces the main features of the abundance ratio [X/Fe] versus [M/H] (X=Mg, Si, Ca, Ti, $α$) of Gaia DR3 stars in different age bins for the considered $α$ elements. Moreover, the most recent gas infall - which started $\sim$ 2.7 Gyr ago - allows us to predict well the Gaia DR3 young population which has experienced a recent chemical impoverishment.

astro-ph.GA↗

arXiv2022

Measurement of the $^{14}$N(n,p)$^{14}$C cross section at the CERN n_TOF facility from sub-thermal energy to 800 keV

Background: The $^{14}$N(n,p)$^{14}$C reaction is of interest in neutron capture therapy, where nitrogen-related dose is the main component due to low-energy neutrons, and in astrophysics, where 14N acts as a neutron poison in the s-process. Several discrepancies remain between the existing data obtained in partial energy ranges: thermal energy, keV region and resonance region. Purpose: Measuring the 14N(n,p)14C cross section from thermal to the resonance region in a single measurement for the first time, including characterization of the first resonances, and providing calculations of Maxwellian averaged cross sections (MACS). Method: Time-of-flight technique. Experimental Area 2 (EAR-2) of the neutron time-of-flight (n_TOF) facility at CERN. $^{10}$B(n,$α$)$^7$Li and $^{235}$U(n,f) reactions as references. Two detection systems running simultaneously, one on-beam and another off-beam. Description of the resonances with the R-matrix code sammy. Results: The cross section has been measured from sub-thermal energy to 800 keV resolving the two first resonances (at 492.7 and 644 keV). A thermal cross-section (1.809$\pm$0.045 b) lower than the two most recent measurements by slightly more than one standard deviation, but in line with the ENDF/B-VIII.0 and JEFF-3.3 evaluations has been obtained. A 1/v energy dependence of the cross section has been confirmed up to tens of keV neutron energy. The low energy tail of the first resonance at 492.7 keV is lower than suggested by evaluated values, while the overall resonance strength agrees with evaluations. Conclusions: Our measurement has allowed to determine the $^{14}$N(n,p) cross-section over a wide energy range for the first time. We have obtained cross-sections with high accuracy (2.5 %) from sub-thermal energy to 800 keV and used these data to calculate the MACS for kT = 5 to kT = 100 keV.

nucl-ex↗

arXiv2022

MINCE I. Presentation of the project and of the first year sample

In recent years, Galactic archaeology has become a particularly vibrant field of astronomy, with its main focus set on the oldest stars of our Galaxy. In most cases, these stars have been identified as the most metal-poor. However, the struggle to find these ancient fossils has produced an important bias in the observations - in particular, the intermediate metal-poor stars (-2.5<[Fe/H]< -1.5) have been frequently overlooked. The missing information has consequences for the precise study of the chemical enrichment of our Galaxy, in particular for what concerns neutron-capture elements and it will be only partially covered by future multi-object spectroscopic surveys such as WEAVE and 4MOST. Measuring at Intermediate Metallicity Neutron Capture Elements (MINCE) is gathering the first high-quality spectra (high S/N ratio and high resolution) for several hundreds of bright and metal-poor stars, mainly located in our Galactic halo. We compiled our selection mainly on the basis of Gaia data and determined the stellar atmospheres of our sample and the chemical abundances of each star. In this paper, we present the first sample of 59 spectra of 46 stars. We measured the radial velocities and computed the Galactic orbits for all stars. We found that 8 stars belong to the thin disc, 15 to disrupted satellites, and the remaining cannot be associated to the mentioned structures, and we call them halo stars. For 33 of these stars, we provide abundances for the elements up to zinc. We also show the chemical evolution results for eleven chemical elements, based on recent models. Our observational strategy of using multiple telescopes and spectrographs to acquire high S/N and high-resolution spectra has proven to be very efficient since the present sample was acquired over only about one year of observations. Finally, our target selection strategy proved satisfactory for our purposes.

astro-ph.SR↗

arXiv2022

Chemical evolution of $^{26}$Al and $^{60}$Fe in the Milky Way

We present theoretical mass estimates of $^{26}$Al and $^{60}$Fe throughout the Galaxy, performed with a numerical chemical evolution model including detailed nucleosynthesis prescriptions for stable and radioactive nuclides. We compared the results for several sets of stellar yields taken from the literature, for massive, low and intermediate mass stars, nova systems (only for $^{26}$Al) and supernovae Type Ia.We then computed the total masses of $^{26}$Al and $^{60}$Fe in the Galaxy. We studied the bulge and the disc of the Galaxy in a galactocentric radius range 0-22 kpc. We assumed that the bulge region (within 2 kpc) evolved quickly suffering a strong star formation burst, while the disc formed more slowly and inside-out. We compared our results with the $^{26}$Al mass observed by the $γ$-ray surveys COMPTEL and INTEGRAL to select the best model. Concerning $^{60}$Fe, we do not have any observed mass value so we just performed a theoretical prediction for future observations. In conclusion, low, intermediate mass stars and Type Ia supernovae contribute negligibly to the two isotopes, while massive stars are the dominant source. The nova contribution is, however, necessary to reproduce the observations of $^{26}$Al. Our best model predicts $2.12$ M$_{\odot}$ of $^{26}$Al, in agreement with observations, while for $^{60}$Fe our best mass estimate is $\sim 1.05$ M$_{\odot}$. We also predicted the present injection rate of $^{26}$Al and $^{60}$Fe in the Galaxy and compared it with previous results, and we found a larger present time injection rate along the disc.

astro-ph.GA↗

arXiv2022

Detailed Chemical Abundances of Star Clusters in the Large Magellanic Cloud

We derive the first detailed chemical abundances of three star clusters in the Large Magellanic Cloud (LMC), NGC1831 (436+/-22 Myr), NGC1856 (350+/-18 Myr) and [SL63]268 (1230+/-62 Myr) using integrated-light spectroscopic observations obtained with the Magellan Echelle spectrograph on Magellan Baade telescope. We derive [Fe/H], [Mg/Fe], [Ti/Fe], [Ca/Fe], [Ni/Fe], [Mn/Fe], [Cr/Fe] and [Na/Fe] for the three clusters. Overall, our results match the LMC abundances obtained in the literature as well as those predicted by detailed chemical evolution models. For clusters NGC1831 and NGC1856, the [Mg/Fe] ratios appear to be slightly depleted compared to [Ca/Fe] and [Ti/Fe]. This could be hinting at the well-known Mg-Al abundance anti-correlation observed in several Milky Way globular clusters. We note, however, that higher signal-to-noise observations are needed to confirm such a scenario, particularly for NGC 1831. We also find a slightly enhanced integrated-light [Na/Fe] ratio for cluster [SL63]268 compared to those from the LMC field stars, possibly supporting a scenario of intracluster abundance variations. We stress that detailed abundance analysis of individual stars in these LMC clusters is required to confirm the presence or absence of MSPs.

astro-ph.GA↗

arXiv2021

A relic from a past merger event in the Large Magellanic Cloud

According to the standard cosmological scenario, the large galaxies that we observe today have reached their current mass via mergers with smaller galaxy satellites (Moore et al.1999). This hierarchical process is expected to take place on smaller scales for the satellites themselves, that should build-up from the accretion of smaller building blocks (D'Onghia & Lake 2008). The best chance we have to test this prediction is by looking at the most massive satellite of the Milky Way (MW): the Large Magellanic Cloud (LMC). Smaller galaxies have been revealed to orbit around the LMC (Erkal & Belokurov 2020, Patel et al. 2020), but so far the only evidence for mutual interactions is related to the orbital interplay with the nearby Small Magellanic Cloud (SMC), which is the most massive LMC satellite. In this work, we report the likely discovery of a past merger event that the LMC experienced with a galaxy with a low star formation efficiency and likely having a stellar mass similar to those of dwarf spheroidal galaxies. This former LMC satellite has now completely dissolved, depositing the old globular cluster (GC) NGC 2005 as part of its debris. This GC is the only surviving witness of this ancient merger event, recognizable through its peculiar chemical composition. This discovery is the observational evidence that the process of hierarchical assembly has worked also in shaping our closest satellites.

astro-ph.GA↗

arXiv2021

The Gaia-ESO Survey: Galactic evolution of lithium from iDR6

We exploit the unique characteristics of a sample of open clusters (OCs) and field stars for which high-precision 7Li abundances and stellar parameters are homogeneously derived by the Gaia-ESO Survey (GES). We derive possibly undepleted 7Li abundances for 26 OCs and star forming regions with ages from young to old spanning a large range of Galactocentric distances, which allows us to reconstruct the local late Galactic evolution of lithium as well as its current abundance gradient along the disc. Field stars are added to look further back in time and to constrain 7Li evolution in other Galactic components. The data are then compared to theoretical tracks from chemical evolution models that implement different 7Li forges. We find that the upper envelope of the 7Li abundances measured in field stars of nearly solar metallicities traces very well the level of lithium enrichment attained by the ISM as inferred from observations of cluster stars. We confirm previous findings that the abundance of 7Li in the solar neighbourhood does not decrease at supersolar metallicity. The comparison of the data with the chemical evolution model predictions favours a scenario in which the majority of the 7Li abundance in meteorites comes from novae. Current data also seem to suggest that the nova rate flattens out at later times. This requirement might have implications for the masses of the white dwarf nova progenitors and deserves further investigation. Neutrino-induced reactions taking place in core-collapse supernovae also produce some fresh lithium. This likely makes a negligible contribution to the meteoritic abundance, but could be responsible for a mild increase of the 7Li abundance in the ISM of low-metallicity systems that would counterbalance the astration processes.

astro-ph.GA↗

arXiv2021

APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods

The analysis of the APOGEE DR16 data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [$α$/Fe] vs. [Fe/H] abundance ratio space: the so-called high-$α$ sequence, classically associated to an old population of stars in the thick disc, and the low-$α$ sequence, which mostly comprises relatively young stars in the thin disc. We perform a Bayesian analysis based on a Markov Chain Monte Carlo method to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample. An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regions are assembled on shorter time-scales compared to the external ones. In the outer disc (with radii $R>6$ kpc), the chemical dilution due to a late accretion event of gas with primordial chemical composition is the main driver of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$α$ sequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-$α$ phase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from $\sim$3.0 to 4.7 Gyr, between the first and second gas infall events for all the analyzed Galactocentric regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$α$ stars are distributed in the abundance space at different Galactocentric distances.

astro-ph.GA↗

arXiv2021

Constraints on stellar rotation from the evolution of Sr and Ba in the Galactic halo

Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.

astro-ph.GA↗