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The first generation of ELT instruments includes an optical-infrared high-resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of $\sim$100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 $\mu$m with the goal of extending it to 0.35-2.4 $\mu$m with the addition of a U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allow ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coud\'e room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases, there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states., Comment: SPIE astronomical telescope and instrumentation 2024, in press

In this work we compute the rates and numbers of different types of stars and phenomena (supernovae, novae, white dwarfs, merging neutron stars, black holes) that contributed to the chemical composition of the Solar System. This process is called "chemical evolution". In particular, we analyse the death rates of stars of all masses, dying either quiescently or explosively. These rates and total star numbers are computed in the context of a revised version of the two-infall model for the chemical evolution of the Milky Way, which reproduces fairly well the observed abundance patterns of several chemical species, the global solar metallicity, and the current gas, stellar, and total surface mass densities. We compute also the total number of stars ever born and still alive as well as the number of stars born up to the formation of the Solar System with a mass and metallicity like the Sun. This latter number will account for all the possible existing Solar Systems which can host life in the solar vicinity. We conclude that, among all the stars (from 0.8 to 100 M$_{\odot}$) born and died from the beginning up to the Solar System formation epoch, which contributed to its chemical composition, 93.00\% are represented by stars dying as single white dwarfs (without interacting significantly with a companion star) and originating in the mass range 0.8-8 M$_{\odot}$, while 5.24$\%$ are neutron stars and 0.73$\%$ are black holes, both originating from supernovae core-collapse (M > 8 M$_{\odot}$); 0.64$\%$ are Type Ia supernovae and 0.40$\%$ are nova systems, both originating from the same mass range as the white dwarfs. The number of stars similar to the Sun born from the beginning up to the Solar System formation, with metallicity in the range 12+log(Fe/H)= 7.50 $\pm$ 0.04 dex is $ \sim 31 \cdot$ 10$^{7}$, and in particular our Sun is the $\sim 2.61 \cdot$ 10$^7$-th star of this kind., Comment: Accepted for publication in Astronomy and Astrophysics (A&A), 11 pages, 9 figures

Relics of ancient accretion events experienced by the Milky Way are predominantly located within the stellar halo of our Galaxy. However, debris from different objects display overlapping distributions in dynamical spaces, making it extremely challenging to properly disentangle their contribution to the build-up of the Galaxy. To shed light on this chaotic context, we started a program aimed at the homogeneous chemical tagging of the local halo of the Milky Way, focusing on the component in retrograde motion, since this is expected to host a large fraction of stars accreted from past mergers. The A Walk on the Retrograde Side (WRS) project targets retrograde halo stars in the Solar Neighborhood having accurate $6$-D phase space information available, measuring the precise chemical abundance of several chemical elements from high-resolution spectroscopy. In this first paper, we present the project and the analysis of high-resolution spectra obtained with UVES at VLT and PEPSI at LBT for $186$ stars. Accurate radial velocity and chemical abundance of several elements have been obtained for all the target stars. In particular we focus on the chemical composition of a specific subset of substructures identified dynamically in the literature. Our study reveals that two among the more recently discovered structures in the retrograde halo, namely Antaeus / L-RL$64$ and ED-$3$, have identical chemical patterns and similar integrals of motion, suggesting a common origin. In turn, the abundance patterns of this unified system differ from that of Gaia-Enceladus, confirming that it is an independent structure. Finally, Sequoia exhibits a different chemistry with respect to that of Gaia-Enceladus at $\mathrm{[Fe/H]} < -1.5$ dex, showcasing an excess of stars with lower Mg and Ca in the common metallicity range., Comment: 21 pages, 17 figures, accepted for publication in A&A

To investigate the relative amount of ejecta from high-mass versus intermediate-mass stars and to trace the chemical evolution of the Galaxy, we have performed with the IRAM 30m and the SMT 10m telescopes a systematic study of Galactic interstellar 18O/17O ratios toward a sample of 421 molecular clouds, covering a galactocentric distance range of 1-22 kpc. The results presented in this paper are based on the J=2-1 transition and encompass 364 sources showing both C18O and C17O detections. The previously suggested 18O/17O gradient is confirmed. For the 41 sources detected with both facilities, good agreement is obtained. A correlation of 18O/17O ratios with heliocentric distance is not found, indicating that beam dilution and linear beam sizes are not relevant. For the subsample of IRAM 30 m high-mass star-forming regions with accurate parallax distances, an unweighted fit gives 18O/17O = (0.12+-0.02)R_GC+(2.38+-0.13) with a correlation coefficient of R = 0.67. While the slope is consistent with our J=1-0 measurement, ratios are systematically lower. This should be caused by larger optical depths of C18O 2-1 lines, w.r.t the corresponding 1-0 transitions, which is supported by RADEX calculations and the fact that C18O/C17O is positively correlated with 13CO/C18O. After considering optical depth effects with C18O J=2-1 reaching typically an optical depth of 0.5, corrected 18O/17O ratios from the J=1-0 and J=2-1 lines become consistent. A good numerical fit to the data is provided by the MWG-12 model, including both rotating stars and novae., Comment: 17 pages, 11 figures, published in ApJS

The Small Magellanic Cloud (SMC) is the host of a rich system of globular clusters (GCs) that span a wide age range. The chemical composition of the SMC clusters is still poorly understood, despite their significance to chemical evolution studies. Here, we provide the first detailed chemical study of evolved giants in three distinct clusters, NGC 121 (10.5 Gyr), NGC 339 (6 Gyr), and NGC 419 (1.4 Gyr). Results are based on high-resolution spectra obtained with FLAMES at the Very Large Telescope. The chemical fingerprints of these clusters closely resemble those of SMC field stars, supporting the SMC's specific history of chemical enrichment relative to the Milky Way. The approximately solar-scaled [alpha/Fe] observed in all three clusters, independent of their [Fe/H], demonstrate the SMC's low star formation efficiency. Compared to their Milky Way counterparts, elements primarily produced by massive stars are severely underrepresented. Particularly, the young cluster NGC 419's extremely low [Zn/Fe] shows that hypernovae have contributed relatively little during the past two Gyr. The three GCs have high [Eu/Fe] values regardless of their age. This suggests that the production of the r-process elements in the SMC was extremely efficient up to 1.5 Gyr ago, with an enrichment timescale comparable to that from Type Ia supernovae. When the properties of the oldest SMC object NGC 121 are compared to those of in-situ Milky Way clusters and accreted clusters linked to the Gaia-Enceladus merger event, it is shown that the SMC had already attained the same metallicity as Gaia-Enceladus but with lower [Fe/H] ratios at the age of NGC 121. This suggests that the chemical enrichment histories of the early SMC and Gaia-Enceladus differed, and that the SMC probably had a lower mass in its early ages than Gaia-Enceladus., Comment: 17 pages, 7 figures, 9 tables. Accepted for publication in Astronomy and Astrophysics

We present the chemical composition of 206 red giant branch stars members of the Small Magellanic Cloud (SMC) using optical, high-resolution spectra collected with the multi-object spectrograph FLAMES-GIRAFFE at the ESO Very Large Telescope. This sample includes stars in three fields located in different positions within the parent galaxy. We analysed the main groups of elements, namely light- (Na), alpha- (O, Mg, Si, Ca, Ti), iron-peak (Sc, V, Fe, Ni, Cu) and s-process elements (Zr, Ba, La). The metallicity distribution of the sample displays a main peak around [Fe/H] ~ -1 dex and a weak metal-poor tail. However, the three fields display [Fe/H] distributions different with each other, in particular a difference of 0.2 dex is found between the mean metallicities of the two most internal fields.The fraction of metal-poor stars increases significantly (from ~1 to ~20%) from the innermost fields to the most external one, likely reflecting an age gradient in the SMC. Also, we found a hint of possible chemically/kinematic distinct substructures. The SMC stars have abundance ratios clearly distinct with respect to the Milky Way stars, in particular for the elements produced by massive stars (like Na, $\alpha$ and most iron-peak elements) that have abundance ratios systematically lower than those measured in our Galaxy. This points out that the massive stars contributed less to the chemical enrichment of the SMC with respect to the Milky Way, according to the low star formation rate expected for this galaxy. Finally, we identified small systematic differences in the abundances of some elements (Na, Ti, V and Zr) in the two innermost fields, suggesting that the chemical enrichment history in the SMC has been not uniform., Comment: 16 pages, 11 figures, 5 tables. Accepted for publication in Astronomy and Astrophysics

We test a scenario in which radial migration could affect the Li abundance pattern of dwarf stars in the solar neighbourhood. This may confirm that the Li abundance in these stars can not serve as a probe for the Li abundance in the interstellar medium. We use the high-quality data (including Li abundances) from the 6th internal Data Release of the Gaia-ESO survey. In this sample, we group stars by similarity in chemical abundances via hierarchical clustering. Our analysis treats both measured Li abundances and upper limits. The Li envelope of the previously identified radially migrated stars is well below the benchmark meteoritic value (<3.26 dex); the star with the highest detected abundance has A(Li) = 2.76 dex. This confirms the previous trends observed for old dwarf stars (median ages $\sim$ 8 Gyr), where Li decreases for [Fe/H]$\gtrsim$0. This result acts as supporting evidence that the abundance of Li measured in the upper envelope of old dwarf stars should not be considered a proxy for the interstellar medium Li. Our scenario also indicates that the stellar yields for [M/H]>0 should not be decreased, as recently proposed in the literature. Our study backs the recent studies that claimed that old dwarfs on the hot side of the dip are efficient probes of the ISM abundance of Li, provided atomic diffusion does not lower significantly the initial Li abundance in the atmospheres of metal-rich objects., Comment: 8 pages, 3 figures. Abridged abstract to fit ArXiv's requirements. Letter published in A&A Letters. Version after the language proofs

The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangle the scenarios of formation and evolution of the Galaxy. We used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance to iron ratio gradients, and their time evolution. We selected member stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw different behaviours for elements belonging to different channels. We found that the youngest clusters in the inner disc have lower metallicity than their older counterpart and they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggested that it might be a bias introduced by the standard spectroscopic analysis producing lower metallicities in low gravity stars. To delineate the shape of the `true' gradient, we should limit our analysis to stars with low surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slow and stationary formation of the thin disc in the latest Gyr. We found a secondary role of clusters' migration in shaping the gradient, with a more prominent role of migration for the oldest clusters., Comment: 25 pages, 14 figures and 4 tables in the main text, 3 figures and 7 tables in the Appendix. Accepted for publication in A&A

Nitrogen isotopic ratios are a key tool for tracing Galactic stellar nucleosynthesis. We present the first study of the $^{14}$N/$^{15}$N abundance ratio in the outer regions of the Milky Way (namely, for galactocentric distances, $R_{\rm GC}$, from 12 kpc up to 19 kpc), with the aim to study the stellar nucleosynthesis effects in the global Galactic trend. We analysed IRAM 30m observations towards a sample of 35 sources in the context of the CHEMical complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) project. We derived the $^{14}$N/$^{15}$N ratios from HCN and HNC for 14 and 3 sources, respectively, using the $J$ = 1-0 rotational transition of HN$^{13}$C, H$^{15}$NC, H$^{13}$CN, and HC$^{15}$N. The results found in the outer Galaxy have been combined with previous measurements obtained in the inner Galaxy. We find an overall linear decreasing H$^{13}$CN/HC$^{15}$N ratio with increasing $R_{\rm GC}$. This translates to a parabolic $^{14}$N/$^{15}$N ratio with a peak at 11 kpc. Updated Galactic chemical evolution models have been taken into account and compared with the observations. The parabolic trend of the $^{14}$N/$^{15}$N ratio with $R_{\rm GC}$ can be naturally explained (i) by a model that assumes novae as the main $^{15}$N producers on long timescales ($\ge$1 Gyr) and (ii) by updated stellar yields for low- and intermediate-mass stars., Comment: 19 pages, 8 figures, 6 tables, 4 appendix - Accepted for publication in A&A

With its origin coming from several sources (Big Bang, stars, cosmic rays) and given its strong depletion during its stellar lifetime, the lithium element is of great interest as its chemical evolution in the Milky Way is not well understood at present. To help constrain stellar and galactic chemical evolution models, numerous and precise lithium abundances are necessary for a large range of evolutionary stages, metallicities, and Galactic volume. In the age of stellar parametrization on industrial scales, spectroscopic surveys such as APOGEE, GALAH, RAVE, and LAMOST have used data-driven methods to rapidly and precisely infer stellar labels (atmospheric parameters and abundances). To prepare the ground for future spectroscopic surveys such as 4MOST and WEAVE, we aim to apply machine learning techniques to lithium measurements and analyses. We trained a convolution neural network (CNN), coupling Gaia-ESO Survey iDR6 stellar labels (Teff, log(g), [Fe/H], and A(Li)) and GIRAFFE HR15N spectra, to infer the atm parameters and lithium abundances for ~40,000 stars. We show that the CNN properly learns the physics of the stellar labels, from relevant spectral features through a broad range of evolutionary stages and stellar parameters. The Li feature at 6707.8 A is successfully singled out by our CNN, among the thousands of lines. Rare objects such as Li-rich giants are found in our sample. This level of performance is achieved thanks to a meticulously built, high-quality, and homogeneous training sample. The CNN approach is very well adapted for the next generations of spectroscopic surveys aimed at studying (among other elements) lithium, such as the 4MIDABLE-LR/HR (4MOST Milky Way disk and bulge low- and high-resolution) surveys. In this context, the caveats of ML applications should be appropriately investigated, along with the realistic label uncertainties and upper limits for abundances., Comment: A&A, accepted 05 December 2022, 23 pages, 23 figures. Codes and trained models available at https://github.com/SamirNepal/Li\_CNN\_2022

The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100,000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper (arXiv:2206.02901) introduces the survey results. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. The Gaia-ESO Survey obtained 202,000 spectra of 115,000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022., Comment: 38 pages. A&A in press

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- $\alpha$ 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-$\alpha$ 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-$\alpha$ 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, $\alpha$) of Gaia DR3 stars in different age bins for the considered $\alpha$ 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., Comment: Accepted for publication in Astronomy and Astrophysics (A&A), 17 pages, 16 figures

In the last 15 years different ground-based spectroscopic surveys have been started (and completed) with the general aim of delivering stellar parameters and elemental abundances for large samples of Galactic stars, complementing Gaia astrometry. Among those surveys, the Gaia-ESO Public Spectroscopic Survey (GES), the only one performed on a 8m class telescope, was designed to target 100,000 stars using FLAMES on the ESO VLT (both Giraffe and UVES spectrographs), covering all the Milky Way populations, with a special focus on open star clusters. This article provides an overview of the survey implementation (observations, data quality, analysis and its success, data products, and releases), of the open cluster survey, of the science results and potential, and of the survey legacy. A companion article (Gilmore et al.) reviews the overall survey motivation, strategy, Giraffe pipeline data reduction, organisation, and workflow. The GES has determined homogeneous good-quality radial velocities and stellar parameters for a large fraction of its more than 110,000 unique target stars. Elemental abundances were derived for up to 31 elements for targets observed with UVES. Lithium abundances are delivered for about 1/3 of the sample. The analysis and homogenisation strategies have proven to be successful; several science topics have been addressed by the Gaia-ESO consortium and the community, with many highlight results achieved. The final catalogue has been released through the ESO archive at the end of May 2022, including the complete set of advanced data products. In addition to these results, the Gaia-ESO Survey will leave a very important legacy, for several aspects and for many years to come., Comment: Accepted for publication in Astronomy and Astrophysics. 30 pages, 30 figures, 4 tables

The outer Galaxy is an environment with metallicity lower than the Solar one and, because of this, the formation and survival of molecules in star-forming regions located in the inner and outer Galaxy is expected to be different. To gain understanding on how chemistry changes throughout the Milky Way, it is crucial to observe outer Galaxy star-forming regions to constrain models adapted for lower metallicity environments. The project "chemical complexity in star-forming regions of the outer Galaxy" (CHEMOUT) aims to address this problem observing a sample of 35 high-mass star-forming cores at Galactocentric distances up to ~23 kpc with the IRAM 30m telescope in various 3mm and 2mm bands. In this work we analyse observations of methanol (CH3OH), one of the simplest complex organic molecules crucial for organic chemistry in star-forming regions, and of two chemically related species, HCO and formaldehyde (H2CO), towards 15 out of the 35 targets of the CHEMOUT sample. In fact, only targets previously detected in both HCO and H2CO, both precursors of methanol, were considered. We detected CH3OH in all 15 targets. Using a Local Thermodynamic Equilibrium approach, we derive CH3OH excitation temperatures in the range 7 - 16 K and line widths smaller than 4 km/s, consistent with emission from a cold and quiescent envelope. The CH3OH fractional abundances w.r.t. H2 range between ~0.6 x 10^{-9} and ~7.4 x 10^{-9}. These values are comparable to those found in star-forming regions in the inner and local Galaxy. Our results have important implications in the organic, and possibly pre-biotic, chemistry occurring in the outermost star-forming regions of the Galaxy, and can help setting the frontiers of the Galactic habitable zone., Comment: 13 pages, 5 figures, 2 appendices, accepted for publication in A&A

The outer Galaxy is an environment with metallicity lower than the Solar one. Because of this, the formation and survival of molecules in star-forming regions located in the inner and outer Galaxy is expected to be different. To gain understanding on how chemistry changes throughout the Milky Way, it is crucial to observe outer Galaxy star-forming regions to constrain models adapted for lower metallicity environments. In this paper we present a new observational project: chemical complexity in star-forming regions of the outer Galaxy (CHEMOUT). The goal is to unveil the chemical composition in 35 dense molecular clouds associated with star-forming regions of the outer Galaxy through observations obtained with the IRAM 30m telescope. In this first paper, we present the sample, and report the detection at 3~mm of simple organic species HCO+, H13CO+, HCN, c-C3H2, HCO, C4H, and HCS+, of the complex hydrocarbon CH3CCH, and of SiO, CCS and SO. From c-C3H2, we estimate new kinematic heliocentric and Galactocentric distances based on an updated rotation curve of the Galaxy. The detection of the molecular tracers does not seem to have a clear dependence on the Galactocentric distance. We also analyse the HCO+ line profiles. We find high velocity wings in ~71% of the targets, and their occurrence does not depend on the Galactocentric distance. Our results, confirmed by a statistical analysis, show that the presence of organic molecules and tracers of protostellar activity is ubiquitous in the low-metallicity environment of the outer Galaxy. Based on this, and on the additional evidence that small, terrestrial planets are omnipresent in the Galaxy, we support previous claims that the definition of Galactic Habitable Zone should be rediscussed in view of the ubiquitous capacity of the interstellar medium to form organic molecules., Comment: 11 pages, 5 figures, 3 appendices, accepted for publication in A&A

Using hydrodynamical $N$-body simulations, we show that the observed structure and kinematics of the extremely metal-poor, dwarf irregular galaxy DDO 68 is compatible with a merger event with at least two smaller satellite galaxies. We were able to obtain a self-consistent model that simultaneously reproduces several of its observed features, including: the very asymmetric and disturbed shape of the stellar component, the overall HI distribution and its velocity field, the arc-like stellar structure to the west, the low-surface brightness stellar stream to the north. The model implies the interaction of the main progenitor of DDO 68 with two systems with dynamical masses $7\times10^8\,M_{\odot}$ and almost $10^8\,M_{\odot}$ -- 1/20 and 1/150 times the dynamical mass of DDO 68, respectively. We show that the merger between DDO 68 and the most massive of its satellites offers a route to explain the large offset of DDO 68 from the mass-metallicity relation. Assuming that the interacting galaxies have metallicities prior to the merger compatible with those of galaxies with similar stellar masses, we provide quantitative evidence that gas mixing alone does not suffice at diluting the gas of the two components; according to our simulations, the HII regions observed along the Cometary Tail trace the low metallicity of the accreted satellite rather than that of DDO 68's main body. In this case, the mass corresponding to the low metallicity is that of the secondary body and DDO 68 becomes consistent with the mass-metallicity relation., Comment: Accepted for publication in MNRAS. 17 pages, 10 figures

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., Comment: 32 pages, 7 figures, 3 table. Published in Nature Astronomy

We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas and dust yields. We calculate two sets of evolutionary sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from stellar evolution modelling are used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind, to determine the dust produced by these objects. The evolution of AGB stars in the extremely metal-poor domain explored here proves tremendously sensitive to the initial mass of the star. M<2Msun stars experience several third dredge-up events, which favour the gradual surface enrichment of C12 and the formation of significant quantities of carbonaceous dust, of the order of 0.01Msun. The C13 and nitrogen yiel are found to be significantly smaller than in previous explorations of low-mass, metal-poor AGB stars, owing to the weaker proton ingestion episodes experienced during the initial AGB phases. M>5Msun stars experience hot bottom burning and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on C12 nuclei of primary origin dredged-up from the internal regions., Comment: 27 papers, 13 figures. Accepted for publication on Astronomy & Astrophysics

Young open clusters (t<200 Myr) have been observed to exhibit several peculiarities in their chemical compositions, from a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to +0.7 dex. Regarding the behaviour of the other $s$-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature. In this work we expand upon our previous analysis of a sample of five young open clusters (IC2391, IC2602, IC4665, NGC2516, and NGC2547) and one star-forming region (NGC2264), with the aim of determining abundances of different neutron-capture elements, mainly CuI, SrI, SrII, YII, ZrII, BaII, LaII, and CeII. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the \textit{Gaia}-ESO survey. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm the super-solar [Ba/Fe] values (from +0.22 to +0.64 dex). Our analysis also points to mildly enhanced [Y/Fe] values (from 0 and +0.3 dex). For the other $s$-process elements we find that [X/Fe] ratios are solar at all ages. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. Thus, we explore different possible scenarios related to the behaviour of spectral lines, from the sensitivity to the presence of magnetic fields to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to higher levels of activity in such young stars. We are still unable to explain these enhancements, but we suggest that other elements (i.e. La) might be more reliable tracer of the $s$-process at young ages and encourage further observations., Comment: 26 pages, 14 figures, accepted for publication in A&A

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., Comment: 17 pages, 10 figures, 3 tables (full tables 1 and 3 only available at the CDS), accepted for publication in Astronomy and Astrophysics. Version 2 includes corrections from language editor

Similarities in the chemical composition of two of the closest Milky Way satellites, namely the Large Magellanic Cloud (LMC) and the Sagittarius (Sgr) dwarf galaxy, have been proposed in the literature, suggesting similar chemical enrichment histories between the two galaxies. This proposition, however, rests on different abundance analyses, which likely introduce various systematics that hamper a fair comparison among the different data sets. In order to bypass this issue (and highlight real similarities and differences between their abundance patterns), we present a homogeneous chemical analysis of 30 giant stars in LMC, 14 giant stars in Sgr and 14 giants in the Milky Way, based on high-resolution spectra taken with the spectrograph UVES-FLAMES. The LMC and Sgr stars, in the considered metallicity range ([Fe/H]>-1.1 dex), show very similar abundance ratios for almost all the elements, with differences only in the heavy s-process elements Ba, La and Nd, suggesting a different contribution by asymptotic giant branch stars. On the other hand, the two galaxies have chemical patterns clearly different from those measured in the Galactic stars, especially for the elements produced by massive stars. This finding suggests the massive stars contributed less to the chemical enrichment of these galaxies with respect to the Milky Way. The derived abundances support similar chemical enrichment histories for the LMC and Sgr., Comment: 21 pages, 8 figures, Accepted for publication by ApJ

Context. High-resolution spectroscopy in the near-infrared (NIR) is a powerful tool for characterising the physical and chemical properties of cool-star atmospheres. The current generation of NIR echelle spectrographs enables the sampling of many spectral features over the full 0.9-2.4 {\mu}m range for a detailed chemical tagging. Aims. Within the Stellar Population Astrophysics Large Program at the TNG, we used a high-resolution (R=50000) NIR spectrum of Arcturus acquired with the GIANO-B echelle spectrograph as a laboratory to define and calibrate an optimal line list and new diagnostic tools to derive accurate stellar parameters and chemical abundances. Methods. We inspected several hundred NIR atomic and molecular lines to derive abundances of 26 different chemical species, including CNO, iron-group, alpha, Z-odd, and neutron-capture elements. We then performed a similar analysis in the optical using Arcturus VLT-UVES spectra. Results. Through the combined NIR and optical analysis we defined a new thermometer and a new gravitometer for giant stars, based on the comparison of carbon (for the thermometer) and oxygen (for the gravitometer) abundances, as derived from atomic and molecular lines. We then derived self-consistent stellar parameters and chemical abundances of Arcturus over the full 4800 - 24500 {\AA} spectral range and compared them with previous studies in the literature. We finally discuss a number of problematic lines that may be affected by deviations from thermal equilibrium and/or chromospheric activity, as traced by the observed variability of He I at 10830 {\AA}., Comment: 16 pages, 14 figures, accepted by A&A

Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. We trace the the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] about 0.3) actually show the highest Li abundances, with A(Li) > 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it., Comment: 5 pages, 3 figures, accepted for publication in Astronomy & Astrophysics Letters

The interstellar oxygen isotopic ratio of 18O/17O can reflect the relative amount of the secular enrichment by ejecta from high-mass versus intermediate-mass stars. Previous observations found a Galactic gradient of 18O/17O, i.e., low ratios in the Galactic center and large values in the Galactic disk, which supports the insideout formation scenario of our Galaxy. However, the observed objects are not many and, in particular, not so many at large galactocentric distances. Thus we started a systematic study on Galactic interstellar 18O/17O, through observations of C18O and C17O multi-transition lines toward a large sample of 286 sources (at least one order of magnitude larger than previous ones), from the Galactic center region to the far outer Galaxy (~22 kpc). Here we present our observations of J=1-0 lines of C18O and C17O, with the ARO12m and the IRAM 30m telescope. We detected successfully both C18O and C17O 1-0 lines for 34 sources among our IRAM30m sample of 50 targets and for 166 sources among our ARO12m sample of 260 targets. The C18O optical depth effect on our ratio results, evaluated by fitting results of C17O spectra with hyperfine components and our RADEX non-LTE model calculation for the strongest source, was found to be insignificant. Beam dilution does not seem to be a problem either, which was supported by the fact of no systematic variation between the isotopic ratio and the heliocentric distance, and consistent measured ratios from two telescopes for most of those detected sources. Our results, though there are still very few detections made for sources in the outer Galaxy, confirm the apparent 18O/17O gradient of 18O/17O = (0.10+-0.03)R_GC+(2.95+-0.30), with a Pearson's rank correlation coefficient R = 0.69. This is supported by the newest Galactic chemical evolution model including the impact of massive stellar rotators and novae., Comment: 35 pages, 7 figures, published in ApJS

We present observations of fifty square degrees of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes $l = 300$-$350^\circ$ and latitudes $|b| \le 0.5^\circ$. These data have been taken at 0.6 arcminute spatial resolution and 0.1 km/s spectral resolution, providing an unprecedented view of the molecular clouds and gas of the Southern Galactic Plane in the 109-115 GHz $J = 1$-0 transitions of $^{12}$CO, $^{13}$CO, C$^{18}$O and C$^{17}$O. We present a series of velocity-integrated maps, spectra and position-velocity plots that illustrate Galactic arm structures and trace masses on the order of $\sim$10$^{6}$ M$_{\odot}$ per square degree; and include a preliminary catalogue of C$^{18}$O clumps located between $l=330$-$340^\circ$. Together with information about the noise statistics of the survey these data can be retrieved from the Mopra CO website, the PASA data store and the Harvard Dataverse (doi:10.7910/DVN/LH3BDN )., Comment: 53 pages, 40 figures; full data set available via http://newt.phys.unsw.edu.au/mopraco/ ; https://data-portal.hpc.swin.edu.au/dataset/the-mopra-southern-galactic-plane-co-survey-data-release-3 ; or https://doi.org/10.7910/DVN/LH3BDN

We propose here that the lithium decrease at super-solar metallicities observed in high resolution spectroscopic surveys can be explained by the interplay of mixed populations, coming from the inner regions of the Milky Way disc. The lower lithium content of these stars is a consequence of inside-out disc formation, plus radial migration. In this framework, local stars with super-solar metallicities would have migrated to the solar vicinity and depleted their original lithium during their travel time. To arrive to such a result, we took advantage of the AMBRE catalog of lithium abundances combined with chemical evolution models which take into account the contribution to the lithium enrichment by different nucleosynthetic sources. A large proportion of migrated stars can explain the observed lower lithium abundance at super-solar metallicities. We stress that nowadays, there is no stellar model able to predict Li-depletion for such super-solar metallicity stars, and the Solar Li-depletion has to be assumed. In addition, it currently exists no solid quantitative estimate of the proportion of migrated stars in the Solar neighborhood and their travel time. Our results illustrate how important it is to properly include radial migration when comparing chemical evolution models to observations, and that in this case, the lithium decrease at larger metallicities does not necessarily imply that stellar yields have to be modified, contrary to previous claims in literature., Comment: A&A 623, A99 (2019). 5 pages, 2 figures

We present chemical abundances and radial velocities of six HII regions in the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived from deep spectra in the wavelength range 3500 - 10,000 {\AA}, acquired with the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]$\lambda$4363 {\AA} line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the "direct" method. We also derived the oxygen abundances of all the six regions adopting indirect method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity - metallicity relation defined by star-forming galaxies. With the direct-method we find indeed an oxygen abundance of 12+log(O/H)=7.14$\pm$0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.96$\pm$0.09 in the "tail". This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06$\pm$0.03 dex/kpc (or -0.30 dex $R_{25}^{-1}$) with the direct method, and a steeper gradient of -0.12$\pm$0.03 dex/kpc (or -0.59 dex $R_{25}^{-1}$) from the indirect method. For the $\alpha$-element to oxygen ratios we obtain values in agreement with those found in other metal-poor star-forming dwarfs. For nitrogen, instead, we infer much higher values, leading to log(N/O)$\sim-1.4$, at variance with the suggested existence of a tight plateau at $-1.6$ in extremely metal poor dwarfs. The derived helium mass fraction ranges from Y=0.240$\pm$0.005 to Y=0.25$\pm$0.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479$-$522 km/s from the tail to the head of the "comet", consistent with the rotation derived in the HI., Comment: Accepted for publication in MNRAS

All measurements of cosmic star formation must assume an initial distribution of stellar masses -- the stellar initial mass function -- in order to extrapolate from the star-formation rate measured for typically rare, massive stars (> 8 Msun) to the total star-formation rate across the full stellar mass spectrum. The shape of the stellar initial mass function in various galaxy populations underpins our understanding of the formation and evolution of galaxies across cosmic time. Classical determinations of the stellar initial mass function in local galaxies are traditionally made at ultraviolet, optical and near-infrared wavelengths, which cannot be probed in dust-obscured galaxies, especially in distant starbursts, whose apparent star-formation rates are hundreds to thousands of times higher than in our Milky Way, selected at submillimetre (rest-frame far-infrared) wavelengths. The 13C/18O abundance ratio in the cold molecular gas -- which can be probed via the rotational transitions of the 13CO and C18O isotopologues -- is a very sensitive index of the stellar initial mass function, with its determination immune to the pernicious effects of dust. Here we report observations of 13CO and C18O emission for a sample of four dust-enshrouded starbursts at redshifts of approximately two to three, and find unambiguous evidence for a top-heavy stellar initial mass function in all of them. A low 13CO/C18O ratio for all our targets -- alongside a well-tested, detailed chemical evolution model benchmarked on the Milky Way -- implies that there are considerably more massive stars in starburst events than in ordinary star-forming spiral galaxies. This can bring these extraordinary starbursts closer to the `main sequence' of star-forming galaxies, though such main-sequence galaxies may not be immune to changes in initial stellar mass function, depending upon their star-formation densities., Comment: 15 pages, 9 figures. Authors' version. Published online by Nature on 04 June 2018. For extra pptx slides prepared for this work, please see http://www.roe.ac.uk/~rji/IMF_Zhang_2018_slides.pptx

Recent observational results have demonstrated an increase in the surface Na abundance that correlates with stellar mass for red giants between 2 and 3 Msun. This trend supports evolutionary mixing processes as the explanation for Na overabundances seen in some red giants. In this same mass range, the surface Al abundance was shown to be constant. Our main aim was to extend the investigation of the Na and Al surface abundances to giants more massive than 3 Msun. We sought to establish accurately whether the Na abundances keep increasing with stellar mass or a plateau is reached. In addition, we investigated whether mixing can affect the surface abundance of Al in giants more massive than 3 Msun. We obtained new high-resolution spectra of 20 giants in the field of 10 open clusters; 17 of these stars were found to be members of 9 clusters. The giants have masses between 2.5 Msun and 5.6 Msun. Abundances of C, N, and O were determined using spectrum synthesis. The abundances of Na and Al were corrected for non-local thermodynamic equilibrium effects (non-LTE). Moreover, to extend the mass range of our sample, we collected from the literature high-quality C, N, O, and Na abundances of 32 Galactic Cepheids with accurate masses in the range between 3 Msun and 14 Msun. The surface abundances of C, N, O, Na, and Al were compared to predictions of stellar evolution models with and without the inclusion of rotation-induced mixing. The surface abundances of most giants and Cepheids of the sample can be explained by models without rotation. For giants above ~ 2.5 Msun, the Na abundances reach a plateau level of about [Na/Fe] ~ 0.20-0.25 dex (in non-LTE). Our results support previous works that found models with rotation to overestimate the mixing effects in intermediate-mass stars. [abridged], Comment: 17 pages, accepted for publication in A&A

We present intermediate-resolution (R$\sim$1000) spectra in the $\sim$3500-10,000 A range of 14 globular clusters in the magellanic irregular galaxy NGC 4449 acquired with the Multi Object Double Spectrograph on the Large Binocular Telescope. We derived Lick indices in the optical and the CaII-triplet index in the near-infrared in order to infer the clusters' stellar population properties. The inferred cluster ages are typically older than $\sim$9 Gyr, although ages are derived with large uncertainties. The clusters exhibit intermediate metallicities, in the range $-1.2\lesssim$[Fe/H]$\lesssim-0.7$, and typically sub-solar [$\alpha/Fe$] ratios, with a peak at $\sim-0.4$. These properties suggest that i) during the first few Gyrs NGC 4449 formed stars slowly and inefficiently, with galactic winds having possibly contributed to the expulsion of the $\alpha$-elements, and ii) globular clusters in NGC 4449 formed relatively "late", from a medium already enriched in the products of type Ia supernovae. The majority of clusters appear also under-abundant in CN compared to Milky Way halo globular clusters, perhaps because of the lack of a conspicuous N-enriched, second-generation of stars like that observed in Galactic globular clusters. Using the cluster velocities, we infer the dynamical mass of NGC 4449 inside 2.88 kpc to be M($<$2.88 kpc)=$3.15^{+3.16}_{-0.75} \times 10^9~M_\odot$. We also report the serendipitous discovery of a planetary nebula within one of the targeted clusters, a rather rare event., Comment: Accepted for publication in MNRAS; corrected typo in author list

In the light of the new observational data related to fluorine abundances in the solar neighborhood stars, we present here chemical evolution models testing different fluorine nucleosynthesis prescriptions with the aim to best fit those new data related to the abundance ratios [F/O] vs. [O/H] and [F/Fe] vs. [Fe/H]. The adopted chemical evolution models are: i) the classical "two-infall" model which follows the chemical evolution of halo-thick disk and thin disk phases, ii) and the "one-infall" model designed only for the thin disk evolution. We tested the effects on the predicted fluorine abundance ratios of different nucleosynthesis yield sources: AGB stars, Wolf-Rayet stars, Type II and Type Ia supernovae, and novae. We find that the fluorine production is dominated by AGB stars but the Wolf-Rayet stars are required to reproduce the trend of the observed data in the solar neighborhood by J\"onsson et al. (2017a) with our chemical evolution models. In particular, the best model both for the "two-infall" and "one-infall" cases requires an increase by a factor of two of the Wolf-Rayet yields given by Meynet & Arnould (2000). We also show that the novae, even if their yields are still uncertain, could help to better reproduce the secondary behavior of F in the [F/O] vs. [O/H] relation. The inclusion of the fluorine production by Wolf-Rayet stars seems to be essential to reproduce the observed ratio [F/O] vs [O/H] in the solar neighborhood by J\"onsson et al. (2017a). Moreover, the inclusion of novae helps substantially to reproduce the observed fluorine secondary behavior., Comment: 10 pages, 10 figure. Accepted for publication in A&A

We present a radial velocity analysis of 20 solar neighborhood RR Lyrae and 3 Population II Cepheids variables. We obtained high-resolution, moderate-to-high signal-to-noise ratio spectra for most stars and obtained spectra were covering different pulsation phases for each star. To estimate the gamma (center-of-mass) velocities of the program stars, we use two independent methods. The first, `classic' method is based on RR Lyrae radial velocity curve templates. The second method is based on the analysis of absorption line profile asymmetry to determine both the pulsational and the gamma velocities. This second method is based on the Least Squares Deconvolution (LSD) technique applied to analyze the line asymmetry that occurs in the spectra. We obtain measurements of the pulsation component of the radial velocity with an accuracy of $\pm$ 3.5 km s$^{-1}$. The gamma velocity was determined with an accuracy $\pm$ 10 km s$^{-1}$, even for those stars having a small number of spectra. The main advantage of this method is the possibility to get the estimation of gamma velocity even from one spectroscopic observation with uncertain pulsation phase. A detailed investigation of the LSD profile asymmetry shows that the projection factor $p$ varies as a function of the pulsation phase -- this is a key parameter which converts observed spectral line radial velocity variations into photospheric pulsation velocities. As a byproduct of our study, we present 41 densely-spaced synthetic grids of LSD profile bisectors that are based on atmospheric models of RR Lyr covering all pulsation phases., Comment: 17 pages, 16 figures, accepted for publication in MNRAS; doi:10.1093/mnras/stx2944

Lithium abundance in most of the warm metal-poor main sequence stars shows a constant plateau (A(Li)~2.2 dex) and then the upper envelope of the lithium vs. metallicity distribution increases as we approach solar metallicity. Meteorites, which carry information about the chemical composition of the interstellar medium at the solar system formation time, show a lithium abundance A(Li)~3.26 dex. This pattern reflects the Li enrichment history of the interstellar medium during the Galaxy lifetime. After the initial Li production in Big Bang Nucleosynthesis, the sources of the enrichment include AGB stars, low-mass red giants, novae, type II supernovae, and Galactic cosmic rays. The total amount of enriched Li is sensitive to the relative contribution of these sources. Thus different Li enrichment histories are expected in the Galactic thick and thin disc. We investigate the main sequence stars observed with UVES in Gaia-ESO Survey iDR4 catalog and find a Li-[alpha/Fe] anticorrelation independent of [Fe/H], Teff, and log(g). Since in stellar evolution different {\alpha} enhancements at the same metallicity do not lead to a measurable Li abundance change, the anticorrelation indicates that more Li is produced during the Galactic thin disc phase than during the Galactic thick disc phase. We also find a correlation between the abundance of Li and s-process elements Ba and Y, and they both decrease above the solar metallicity, which can be explained in the framework of the adopted Galactic chemical evolution models., Comment: 15 pages, 15 figures, published in A&A, data online at CDS

We test the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the our Galaxy have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] vs. [Fe/H] patterns with the ones observed in Galactic halo stars. The [O/Fe] ratio deviates substantially from the observed abundance ratios in the Galactic halo stars for [Fe/H] > -2 dex, while they overlap for lower metallicities. On the other hand, for the neutron capture elements, the discrepancy is extended at all the metallicities, suggesting that the majority of stars in the halo are likely to have been formed in situ. We present the results for a model considering the effects of an enriched gas stripped from dwarf satellites on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the adopted infall time-scale, and the presence of a threshold in the gas for star formation., Comment: 4 pages, 2 figures, to appear in Proceedings of IAU Symposium 334 "Rediscovering our Galaxy" in Potsdam (Germany), July 10-14, 2017

We present deep 3500$-$10000 $\AA$ spectra of H II regions and planetary nebulae (PNe) in the starburst irregular galaxy NGC 4449, acquired with the Multi Object Double Spectrograph at the Large Binocular Telescope. Using the "direct" method, we derived the abundance of He, N, O, Ne, Ar, and S in six H II regions and in four PNe in NGC 4449. This is the first case of PNe studied in a starburst irregular outside the Local Group. Our H II region and PN sample extends over a galacto-centric distance range of $\approx$2 kpc and spans $\approx$0.2 dex in oxygen abundance, with average values of $12+\log(O/H)=8.37 \pm 0.05$ and $8.3 \pm 0.1$ for H II regions and PNe, respectively. PNe and H II regions exhibit similar oxygen abundances in the galacto-centric distance range of overlap, while PNe appear more than $\sim$1 dex enhanced in nitrogen with respect to H II regions. The latter result is the natural consequence of N being mostly synthesized in intermediate-mass stars and brought to the stellar surface during dredge-up episodes. On the other hand, the similarity in O abundance between H II regions and PNe suggests that NGC 4449' s interstellar medium has been poorly enriched in $\alpha$ elements since the progenitors of the PNe were formed. Finally, our data reveal the presence of a negative oxygen gradient for both H II regions and PNe, whilst nitrogen does not exhibit any significant radial trend. We ascribe the (unexpected) nitrogen behaviour as due to local N enrichment by the conspicuous Wolf-Rayet population in NGC 4449., Comment: Accepted for publication on ApJ

We explore the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the Milky Way have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones observed in Galactic halo stars. Oxygen abundances deviate substantially from the observed abundances in the Galactic halo stars for [Fe/H] values larger than -2 dex, while they overlap for lower metallicities. On the other hand, for the [Ba/Fe] ratio the discrepancy is extended at all [Fe/H] values, suggesting that the majority of stars in the halo are likely to have been formed in situ. Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [O/Fe] ratios. Moreover, we show the effects of an enriched infall of gas with the same chemical abundances as the matter ejected and/or stripped from dwarf satellites of the Milky Way on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the assumed infall time scale, and the presence of a threshold in the gas for star formation., Comment: To appear in Proceeding of Science: Frontier Research in Astrophysics - II 23-28 May 2016 Mondello (Palermo), Italy

We present Iron, Magnesium, Calcium, and Titanium abundances for 235 stars in the central region of the Sagittarius dwarf spheroidal galaxy (within 9.0 arcmin ~70 pc from the center) from medium-resolution Keck/DEIMOS spectra. All the considered stars belong to the massive globular cluster M54 or to the central nucleus of the galaxy (Sgr,N). In particular we provide abundances for 109 stars with [Fe/H] > -1.0, more than doubling the available sample of spectroscopic metallicity and alpha-elements abundance estimates for Sgr dSph stars in this metallicity regime. Also, we find the first confirmed member of the Sagittarius dwarf spheroidal with [Fe/H]< -2.0 based on analysis of iron lines. We find for the first time a metallicity gradient in the Sgr,N population, whose peak iron abundance goes from [Fe/H]=-0.38 for R < 2.5 arcmin to [Fe/H]=-0.57 for 5.0 < R < 9.0 arcmin. On the other hand the trends of [Mg/Fe], [Ca/Fe], and [Ti/Fe] with [Fe/H] are the same over the entire region explored by our study. We reproduce the observed chemical patterns of the Sagittarius dwarf spheroidal as a whole with a chemical evolution model implying a high mass progenitor ( M_(DM)=6 X 10^{10} Msun ) and a significant event of mass-stripping occurred a few Gyr ago, presumably starting at the first peri-Galactic passage after infall., Comment: Accepted for publication to A&A, 12 pages, 14 figures, 1 table

We briefly present the spectroscopic evolution of the recent outburst of the classical nova V1369 Cen, and the presence of a narrow absorption line identified as due to the resonance of neutral lithium at 6708 \AA. We also discuss the consequences for the chemical evolution of lithium in the Galaxy., Comment: 8 pages, 5 figures, 2 tables. To appear in the proceedings of the conference "The Golden Age of Cataclysmic Variables and Related Objects - III" held in Palermo, Italy, 7-12 September 2015

We use state-of-the-art chemical models to track the cosmic evolution of the CNO isotopes in the interstellar medium (ISM) of galaxies, yielding powerful constraints on their stellar initial mass function (IMF). We re-assess the relative roles of massive stars, asymptotic giant branch (AGB) stars and novae in the production of rare isotopes such as 13C, 15N, 17O and 18O, along with 12C, 14N and 16O. The CNO isotope yields of super-AGB stars, novae and fast-rotating massive stars are included. Having reproduced the available isotope enrichment data in the solar neighbourhood, and across the Galaxy, and having assessed the sensitivity of our models to the remaining uncertainties, e.g. nova yields and star-formation history, we show that we can meaningfully constrain the stellar IMF in galaxies using C, O and N isotope abundance ratios. In starburst galaxies, where data for multiple isotopologue lines are available, we find compelling new evidence for a top-heavy stellar IMF, with profound implications for their star-formation rates and efficiencies, perhaps also their stellar masses. Neither chemical fractionation nor selective photodissociation can significantly perturb globally-averaged isotopologue abundance ratios away from the corresponding isotope ones, as both these processes will typically affect only small mass fractions of molecular clouds in galaxies. Thus the Atacama Large Millimetre Array now stands ready to probe the stellar IMF, and even the ages of specific starburst events in star-forming galaxies across cosmic time unaffected by the dust obscuration effects that plague optical/near-infrared studies., Comment: 15 pages, 7 figures, MNRAS in press (the accepted version match the one already available from the arXiv)

The spatial distribution of elemental abundances in the disc of our Galaxy gives insights both on its assembly process and subsequent evolution, and on the stellar nucleogenesis of the different elements. Gradients can be traced using several types of objects as, for instance, (young and old) stars, open clusters, HII regions, planetary nebulae. We aim at tracing the radial distributions of abundances of elements produced through different nucleosynthetic channels -the alpha-elements O, Mg, Si, Ca and Ti, and the iron-peak elements Fe, Cr, Ni and Sc - by using the Gaia-ESO idr4 results of open clusters and young field stars. From the UVES spectra of member stars, we determine the average composition of clusters with ages >0.1 Gyr. We derive statistical ages and distances of field stars. We trace the abundance gradients using the cluster and field populations and we compare them with a chemo-dynamical Galactic evolutionary model. Results. The adopted chemo-dynamical model, with the new generation of metallicity-dependent stellar yields for massive stars, is able to reproduce the observed spatial distributions of abundance ratios, in particular the abundance ratios of [O/Fe] and [Mg/Fe] in the inner disc (5 kpc

According to the current cosmological cold dark matter paradigm, the Galactic halo could have been the result of the assemblage of smaller structures. Here we explore the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the Milky Way have been the building blocks of the Galactic halo by comparing their [$\alpha$/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones observed in Galactic halo stars. The $\alpha$ elements deviate substantially from the observed abundances in the Galactic halo stars for [Fe/H] values larger than -2 dex, while they overlap for lower metallicities. On the other hand, for the [Ba/Fe] ratio the discrepancy is extended at all [Fe/H] values, suggesting that the majority of stars in the halo are likely to have been formed in situ. Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [$\alpha$/Fe] ratios. Moreover, for the first time we consider the effects of an enriched infall of gas with the same chemical abundances as the matter ejected and/or stripped from dwarf satellites of the Milky Way on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the assumed infall time scale, and the presence of a threshold in the gas for star formation. In particular, in models with an infall timescale for the halo around 0.8 Gyr coupled with a threshold in the surface gas density for the star formation (4 $\mathrm{M}_{\odot}\,\mathrm{pc}^{-2}$), and the enriched infall from dwarf spheroidal satellites, the first halo stars formed show [Fe/H]$>$-2.4 dex. In this case, to explain [$\alpha$/Fe] data for stars with [Fe/H]$<$-2.4 dex we need stars formed in dSph systems., Comment: Accepted for publication in MNRAS

Stellar evolution models predict that internal mixing should cause some sodium overabundance at the surface of red giants more massive than ~ 1.5--2.0 Msun. The surface aluminium abundance should not be affected. Nevertheless, observational results disagree about the presence and/or the degree of the Na and Al overabundances. In addition, Galactic chemical evolution models adopting different stellar yields lead to quite different predictions for the behavior of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these abundances with metallicity are not well reproduced. We readdress both issues, using new Na and Al abundances determined within the Gaia-ESO Survey, using two samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters and ii) low- and intermediate-mass clump giants in six open clusters. Abundances of Na in giants with mass below ~2.0 Msun, and of Al in giants below ~3.0 Msun, seem to be unaffected by internal mixing processes. For more massive giants, the Na overabundance increases with stellar mass. This trend agrees well with predictions of stellar evolutionary models. Chemical evolution models that are able to fit well the observed [Na/Fe] vs. [Fe/H] trend in solar neighborhood dwarfs can not simultaneously explain the run of [Al/Fe] with [Fe/H], and viceversa. The comparison with stellar ages is hampered by severe uncertainties. Indeed, reliable age estimates are available for only a half of the stars of the sample. We conclude that Al is underproduced by the models, except for stellar ages younger than about 7 Gyr. In addition, some significant source of late Na production seems to be missing in the models. Either current Na and Al yields are affected by large uncertainties, and/or some important Galactic source(s) of these elements has not been taken into account up to now. [abridged], Comment: 16 pages, 11 figures, A&A accepted, After language editing

Globular clusters are considerably more complex structures than previously thought, harbouring at least two stellar generations which present clearly distinct chemical abundances. Scenarios explaining the abundance patterns in globular clusters mostly assume that originally the clusters had to be much more massive than today, and that the second generation of stars originates from the gas shed by stars of the first generation (FG). The lack of metallicity spread in most globular clusters further requires that the supernova-enriched gas ejected by the FG is completely lost within ~30 Myr, a hypothesis never tested by means of three-dimensional hydrodynamic simulations. In this paper, we use 3D hydrodynamic simulations including stellar feedback from winds and supernovae, radiative cooling and self-gravity to study whether a realistic distribution of OB associations in a massive proto-GC of initial mass M_tot ~ 10^7 M_sun is sufficient to expel its entire gas content. Our numerical experiment shows that the coherence of different associations plays a fundamental role: as the bubbles interact, distort and merge, they carve narrow tunnels which reach deeper and deeper towards the innermost cluster regions, and through which the gas is able to escape. Our results indicate that after 3 Myr, the feedback from stellar winds is responsible for the removal of ~40% of the pristine gas, and that after 14 Myr, ~ 99% of the initial gas mass has been removed., Comment: ApJ Letters, accepted for publication

We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of different nucleosynthesis sources over time to a not well-mixed interstellar medium. We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure. We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster., Comment: 18 pages, 14 figures, 3 tables. Accepted to A&A