We will discuss some highlights concerning the chemical evolution of our Galaxy, the Milky Way. First we will describe the main ingredients necessary to build a model for the chemical evolution of the Milky Way. Then we will illustrate some Milky Way models which includes detailed stellar nucleosynthesis and compute the evolution of a large number of chemical elements, including C, N, O, $α$-elements, Fe and heavier. The main observables and in particular the chemical abundances in stars and gas will be considered. A comparison theory-observations will follow and finally some conclusions from this astroarchaeological approach will be derived.
In this review I will discuss the comparison between model results and observational data for the Milky Way, the predictive power of such models as well as their limits. Such a comparison, known as Galactic archaeology, allows us to impose constraints on stellar nucleosynthesis and timescales of formation of the various Galactic components (halo, bulge, thick disk and thin disk).
I would like to review recent efforts of detailed chemical abundance measurements for field Milky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a several kpc from the Sun, large samples of field halo stars with detailed chemical measurements are continuously expanding. Combination of the chemical information and full six dimensional phase-space information is now recognized as a powerful tool to identify cosmological accretion events that have built a sizable fraction of the present-day stellar halo. Future observational prospects with wide-field spectroscopic …
The Gaia-ESO Survey (GES) is a large public spectroscopic survey that aims at observing with FLAMES@VLT the main stellar components of our Galaxy. The study of the population of open clusters is one of the main objectives of GES. We present some results from the first 18 months of observations, among them, a preliminary view of the radial metallicity gradient as traced by open cluster data and a comparison of the chemical patterns of clusters located in different parts of the Galactic disk.
We present detailed chemical evolution models for the Milky Way and M31 in presence of radial gas flows. These models follow in detail the evolution of several chemical elements (H, He, CNO, $α$ elements, Fe-peak elements) in space and time. The contribution of supernovae of different type to chemical enrichment is taken into account. We find that an inside-out formation of the disks coupled with radial gas inflows of variable speed can reproduce very well the observed abundance gradients in both galaxies. We also discuss the effects of other parameters, such as a threshold in the gas density …
Early-type dwarfs (ETDs) are the end points of the evolution of low-mass galaxies whose gas supply has been extinguished. The cessation of star-formation lays bare the ancient stellar populations. A wealth of information is stored in the colours, magnitudes, metallicities and abundances of resolved stars of the dwarf spheroidal and ultra-faint galaxies around the Milky Way, allowing their chemistry and stellar populations to be studied in great detail. Here, we summarize our current understanding, which has advanced rapidly over the last decade thanks to the flourishing of large-scale astromet…
In this paper, we place the Milky Way (MW) in the context of similar-looking galaxies in terms of their star-formation and chemical evolution histories. We select a sample of 138 Milky-Way analogues (MWAs) from the SDSS-IV/MaNGA survey based on their masses, Hubble types, and bulge-to-total ratios. To compare their chemical properties to the detailed spatially-resolved information available for the MW, we use a semi-analytic spectral fitting approach, which fits a self-consistent chemical-evolution and star-formation model directly to the MaNGA spectra. We model the galaxies' inner and outer r…
The disrupted chemical enrichment history of the Milky Way driven by gas accretion
South-Western Institute for Astronomy Research, Yunnan University（西南天文研究所，云南大学）；Yunnan Key Laboratory of Survey Science, Yunnan University（云南省测绘科学重点实验室，云南大学）；Department of Astronomy, Xiamen University（天文学系，厦门大学）；Institute of Astronomy, University of Cambridge（天文学研究所，剑桥大学）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（天文与空间科学学院，中国科学院大学）；National Astronomical Observatories, Chinese Academy of Sciences（国家天文台，中国科学院）；Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University（天文与天体物理前沿研究院，北京师范大学）；Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences（射电天文与技术重点实验室，中国科学院）
The aim of Galactic Archaeology is to recover the evolutionary history of the Milky Way from its present day kinematical and chemical state. Because stars move away from their birth sites, the current dynamical information alone is not sufficient for this task. The chemical composition of stellar atmospheres, on the other hand, is largely preserved over the stellar lifetime and, together with accurate ages, can be used to recover the birthplaces of stars currently found at the same Galactic radius. In addition to the availability of large stellar samples with accurate 6D kinematics and chemica…
Abundance of chemical elements in the stars provides important clues regarding galaxy formation. The most powerful diagnostics is the relative abundance of α-elements (O, Mg, Si, S, Ca, and Ti) with respect to iron (Fe), [α/Fe], each of which is produced by different kinds of supernovae. The existence of two distinct groups of stars in the solar neighbourhood, one with high [α/Fe] and another with low [α/Fe], suggests that the stars in the solar vicinity have two different origins. However, the specific mechanism of the realization of this bimodality is unknown. Here, we show that the cold flo…
Chemical bimodality of the Milky Way (MW) disc stars constitutes one of the most remarkable properties of MW. The cold accretion theory for the cosmological gas accretion provides one viable explanation to this phenomenon. In this scenario, the rapid cold-mode accretion in the early epoch creates the first generation stars relatively rich in $α$-elements(O,Mg,Si,S,Ca,etc) and later cooling flow produces iron-rich second generation stars, creating the bimodality in the [$α$/Fe] ratio. We employ a cosmologically motivated chemical evolution model for disc galaxies to elucidate the role played by…
The double red clump (RC) observed in color-magnitude diagrams of the Milky Way bulge is at the heart of the current debate on the structure and formation origin of the bulge. This feature can be explained by the difference between the two RCs either in distance ("X-shaped scenario") or in chemical composition ("multiple-population scenario"). Here we report our high-resolution spectroscopy for the RC and red giant branch stars in a high-latitude field (b ~ -8.5$°$) of the bulge. We find a difference in [Fe/H] between the stars in the bright and faint RC regimes, in the sense that the bright s…
We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify nearby halo stars in the 14th data release from the APOGEE survey that have been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a `blob' of 856 likely accreted stars, with a low disc contamination rate of ~7%. Cross-matching the sample with the second data release from Gaia gives us access to parallaxes and apparent magnitudes, which place constraints on distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this enables us to estimate new ages for the accreted stars, with typ…
Co-evolution of the Milky Way high- and low-α sequences with chemical evolution models
INAF, Osservatorio Astronomico di Trieste（意大利国家天体物理研究所，的里雅斯特天文台）；IFPU, Institute for Fundamental Physics of the Universe（宇宙基础物理研究所）；Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste（的里雅斯特大学，物理系，天文学部）；INFN Sezione di Trieste（意大利国家核物理研究所，的里雅斯特分部）
本文通过改进的化学演化模型探讨银河盘内高α和低α序列的共同演化及古老低α星的存在，解释了观测到的[α/Fe] vs [Fe/H]图谱。
In the era of large-scale spectroscopic surveys in the Local Group (LG), we can explore using chemical abundances of halo stars to study the star formation and chemical enrichment histories of the dwarf galaxy progenitors of the Milky Way (MW) and M31 stellar halos. In this paper, we investigate using the Chemical Abundance Ratio Distributions (CARDs) of seven stellar halos from the Latte suite of FIRE-2 simulations. We attempt to infer galaxies' assembly histories by modelling the CARDs of the stellar halos of the Latte galaxies as a linear combination of template CARDs from disrupted dwarfs,…
We use data from the 17th data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE 2) to contrast the chemical composition of the recently discovered Gaia Enceladus/Sausage system (GE/S) to those of ten Milky Way (MW) dwarf satellite galaxies: LMC, SMC, Boötes I, Carina, Draco, Fornax, Sagittarius, Sculptor, Sextans and Ursa Minor. Our main focus is on the distributions of the stellar populations of those systems in the [Mg/Fe]-[Fe/H] and [Mg/Mn]-[Al/Fe] planes, which are commonly employed in the literature for chemical diagnosis and where dwarf galaxies can be distin…
Albeit radial migration must be a ubiquitous process in disc galaxies, its significance in the evolution of stellar discs is not always reflected through global trends. However, there are other key observables, such as the metallicity distribution function (MDF), that may shed some light in this matter. We argue that the shape of the MDF not only tells us whether the stellar disc experienced radial migration, but it also contains important clues on the structure that triggered it. Specifically, the MDF contains information about the dynamics and morphology of the spiral pattern. To constrain t…
The chemical evolution of neutron capture elements in the Milky Way disc is still a matter of debate. We aim to understand the chemical evolution of r-process elements in Milky Way disc. We focus on three pure r-process elements Eu, Gd, and Dy. Using high-resolution FEROS, HARPS, and UVES spectra from the ESO archive, we perform a homogeneous analysis on 6500 FGK Milky Way stars, thanks to the automatic optimization pipeline GAUGUIN. We present abundances of Ba (5057 stars), Eu (6268 stars), Gd (5431 stars), and Dy (5479 stars). We chemically characterize the thin and the thick discs, and a me…
Spectroscopic surveys of the Milky Way's stars have revealed spatial, chemical and kinematical structures that encode its history. In this work, we study their origins using a cosmological zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at $z\sim 1.5$, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-[$α$/Fe] gas disc around the inner, metal-rich galaxy containing the old high-[$α$/Fe] stars. This event leads to a bimodality in [$α$/Fe] over a range of [Fe/H]. A detailed analysis of how the galax…
Chemical abundances of stars in the Milky Way disk are empirical tracers of its enrichment history. However, they capture joint-information that is valuable to disentangle. In this work, we seek to quantify how individual abundances evolve across the present-day radius of the disk, at fixed supernovae contribution ([Fe/H], [Mg/Fe]). We use 18,135 APOGEE DR17 red clump stars and 7,943 GALAH DR3 main sequence stars to compare the abundance distributions conditioned on ([Fe/H], [Mg/Fe]) across $3-13$ kpc and $6.5-9.5$ kpc, respectively. In total we examine 15 elements: C, N, Al, K (light), O, Si,…
The formation and chemical evolution of the Milky Way Galaxy is numerically simulated by developing a Monte Carlo approach to predict the elemental abundance gradients and other galactic features using the revised solar abundance. The galaxy is accreted gradually by using either a two-infall or a three-infall accretion scenario. The galaxy is chemically enriched by the nucleosynthetic contributions from an evolving ensemble of generations of stars. We analyse the role of star formation efficiency. The influence of the radial gas inflow as well as radial gas mixing on the evolution of galaxy is…
Galactic haloes in a $Λ$-CDM universe are predicted to host today a swarm of debris resulting from cannibalised dwarf galaxies. The chemo-dynamical information recorded in their stellar populations helps elucidate their nature, constraining the assembly history of the Galaxy. Using data from APOGEE and \textit{Gaia}, we examine the chemical properties of various halo substructures, considering elements that sample various nucleosynthetic pathways. The systems studied are Heracles, \textit{Gaia}-Enceladus/Sausage (GES), the Helmi stream, Sequoia, Thamnos, Aleph, LMS-1, Arjuna, I'itoi, Nyx, Icar…
The central (`bulge') region of the Milky Way is teeming with a significant fraction of mildly metal-deficient stars with atmospheres that are strongly enriched in cyanogen ($^{12}$C$^{14}$N). Some of these objects, which are also known as nitrogen-enhanced stars, are hypothesised to be relics of the ancient assembly history of the Milky Way. Although the chemical similarity of nitrogen-enhanced stars to the unique chemical patterns observed in globular clusters has been observed, a direct connection between field stars and globular clusters has not yet been proven. In this work, we report on …
In order to understand the roles of metal flows in galaxy formation and evolution, we analyse our self-consistent cosmological chemo-dynamical simulation of a Milky Way like galaxy during its thin-disc phase. Our simulated galaxy disc qualitatively reproduces the variation of the dichotomy in [$α$/Fe]-[Fe/H] at different Galactocentric distances as derived by APOGEE-DR16, as well as the stellar age distribution in [$α$/Fe]-[Fe/H] from APOKASC-2. The disc grows from the inside out, with a radial gradient in the star-formation rate during the entire phase. Despite the radial dependence, the outf…
Fluorine has many different potential sites and channels of production, making narrowing down a dominant site of fluorine production particularly challenging. In this work, we investigate which sources are the dominant contributors to the galactic fluorine by comparing chemical evolution models to observations of fluorine abundances in Milky Way stars covering a metallicity range -2$<$[Fe/H]$<$0.4 and upper limits in the range -3.4$<$[Fe/H]$<$-2.3. In our models, we use a variety of stellar yield sets in order to explore the impact of varying both AGB and massive star yields on the chemical ev…
The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf (Sgr), Fornax (Fnx), and the now fully disrupted \emph{Gaia} Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C,…
We present our detailed spectroscopic analysis of the chemical composition of four red giant stars in the halo globular cluster NGC 6426. We obtained high-resolution spectra using the Magellan2/MIKE spectrograph, from which we derived equivalent widths and subsequently computed abundances of 24 species of 22 chemical elements. For the purpose of measuring equivalent widths, we developed a new semi-automated tool, called EWCODE. We report a mean Fe content of [Fe/H] = -2.34$\pm$0.05 dex (stat.) in accordance with previous studies. At a mean $α$-abundance of [(Mg,Si,Ca)/3 Fe] = 0.39$\pm$0.03 dex…
We review the state of our chemical evolution models for spiral and low mass galaxies. We analyze the consequences of using different stellar yields, infall rate laws and star formation prescriptions in the time/redshift evolution of the radial distributions of abundances, and other quantities as star formation rate or gas densities, in the Milky Way Galaxy; In particular we will study the evolution of the Oxygen abundance radial gradient analyzing its relation with the ratio SFR/infall. We also compare the results with our old chemical evolution models, cosmological simulations and with the e…
We aim to prepare the machine-learning ground for the next generation of spectroscopic surveys, such as 4MOST and WEAVE. Our goal is to show that convolutional neural networks can predict accurate stellar labels from relevant spectral features in a physically meaningful way. We built a neural network and trained it on GIRAFFE spectra with associated stellar labels from the sixth internal Gaia-ESO data release. Our neural network predicts the atmospheric parameters Teff and log(g) as well as the chemical abundances [Mg/Fe], [Al/Fe], and [Fe/H] for 30115 stellar spectra. The scatter of predictio…
Measuring Detailed Chemical Abundances From Co-added Medium Resolution Spectra I. Tests using Milky Way dwarf spheroidal galaxies and globular clusters
We present a scenario of the chemical enrichment of the solar neighborhood that solves the G-dwarf problem by taking into account constraints on a larger scale. We argue that the Milky Way disk within 10 kpc has been enriched to solar metallicity by a massive stellar population: the thick disk, which itself formed from a massive turbulent gaseous disk. The key new consideration is that the pre-enrichment provided by the thick disk is not related to the mass fraction of this stellar population at the solar radius, as is classically assumed in inside-out scenarios, but is actually related to the…
Detailed chemical abundances are presented for seven M31 outer halo globular clusters (with projected distances from M31 greater than 30 kpc), as derived from high resolution integrated light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)---this paper presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), alpha-enhanced (though they are possibly less…
The chemical evolution of lithium in the Milky Way represents a major problem in modern astrophysics. Indeed, lithium is, on the one hand, easily destroyed in stellar interiors, and, on the other hand, produced at some specific stellar evolutionary stages that are still not well constrained. The goal of this paper is to investigate the lithium stellar content of Milky Way stars in order to put constraints on the lithium chemical enrichment in our Galaxy, in particular in both the thin and thick discs. Thanks to high-resolution spectra from the ESO archive and high quality atmospheric parameter…
We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation hi…
We present the first high-resolution chemical abundances of seven stars in the recently discovered high-energy stream Typhon. Typhon stars have apocenters >100 kpc, making this the first detailed chemical picture of the Milky Way's very distant stellar halo. Though the sample size is limited, we find that Typhon's chemical abundances are more like a dwarf galaxy than a globular cluster, showing a metallicity dispersion and no presence of multiple stellar populations. Typhon stars display enhanced $α$-element abundances and increasing r-process abundances with increasing metallicity. The high-$…
In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in-situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in whi…
We present a reassessment of the radial abundance gradients of He, C, N, O, Ne, S, Cl, and Ar in the Milky Way using the deep optical spectra of 42 HII regions presented in Arellano-Córdova et al. (2020, 2021) and Méndez-Delgado et al. (2020) exploring the impact of: (1) new distance determinations based on Gaia EDR3 parallaxes and (2) the use of Peimbert's temperature fluctuations paradigm ($t ^ 2> 0$) for deriving ionic abundances. We find that distances based on Gaia EDR3 data are more consistent with kinematic ones based on Galactic rotation curves calibrated with radio parallaxes, which g…
The Milky Way underwent its last significant merger ten billion years ago, when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and progenitor stars born prior to the merger make up the bulk of the inner halo. Even though these two main populations of halo stars have similar $durations$ of star formation prior to their merger, they differ in [$α$/Fe]-[Fe/H] space, with the GES population bending to lower [$α$/Fe] at a relatively low value of [Fe/H]. We use cosmological simulations of a 'Milky Way' to argue that the different tracks of the halo stars through the [$α$/Fe]-[Fe/H…
We present a set of 144 galactic chemical evolution models applied to a Milky Way analogue, computed using four sets of low and intermediate star nucleosynthetic yields, six massive star yield compilations, and six functional forms for the initial mass function. The integrated or true yields for each combination are derived. A comparison is made between a grid of multiphase chemical evolution models computed with these yield combinations and empirical data drawn from the Milky Way's disc, including the solar neighbourhood. By means of a chi2 methodology, applied to the results of these multiph…
