Context. Elements heavier than Li are produced in the interiors of stars. However, for many elements the exact production sites and the timescales on which they are dispersed into the interstellar medium are unknown. Having a clear picture on the origins of the elements is important for our ability to trace and understand the formation and chemical evolution of the Milky Way and its stellar populations. Aims. The aim of this study is to investigate the origin and evolution of Sc, V, Mn, and Co for a homogeneous and statistically significant sample of stars probing the different populations of the Milky Way, in particular the thin and thick disks. Methods. Using high-resolution spectra obtained with the MIKE, FEROS, SOFIN, FIES, UVES, and HARPS spectrographs, we determine Sc, V, Mn, and Co abundances for a large sample of F and G dwarfs in the solar neighborhood. The method is based on spectral synthesis and using one-dimensional, plane-parallel, local thermodynamic equilibrium (LTE) model stellar atmospheres calculated with the MARCS 2012 code. The non-LTE (NLTE) corrections from the literature were applied to Mn and Co. Results. We find that the abundance trends derived for Sc (594 stars

Context. Elements heavier than iron are produced through neutron-capture processes in the different phases of stellar evolution. Asymptotic giant branch (AGB) stars are believed to be mainly responsible for elements that form through the slow neutron-capture process, while the elements created in the rapid neutron-capture process have production sites that are less understood. Knowledge of abundance ratios as functions of metallicity can lead to insight into the origin and evolution of our Galaxy and its stellar populations. Aims. We aim to trace the chemical evolution of the neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu in the Milky Way stellar disk. This will allow us to constrain the formation sites of these elements, as well as to probe the evolution of the Galactic thin and thick disks. Methods. Using spectra of high resolution (42 000 R 65 000) and high signal-to-noise (S /N 200) obtained with the MIKE and the FEROS spectrographs, we determine Sr, Zr, La, Ce, Nd, Sm, and Eu abundances for a sample of 593 F and G dwarf stars in the solar neighborhood. The abundance analysis is based on spectral synthesis using one-dimensional, plane-parallel, local thermodynamic equilibrium (LTE) model stellar atmospheres calculated with the MARCS 2012 code. Conclusions. The rapid neutron-capture process is active early in the Galaxy, mainly in type-II supernovae from stars in the mass range 8−10 M . Europium is almost completely produced by the r-process, but Nd and Sm show similar trends to Eu even if their s-process component is higher. Strontium and Zr are thought to be mainly produced by the s-process, but show significant enrichment at low metallicity that requires extra r-process production, which probably is different from the classical r-process. Finally, La and Ce are mainly produced via s-process from AGB stars in the mass range 2−4 M , which can be seen by the decrease in [La/Eu] and [Ce/Eu] at [Fe/H] ≈ −0.5. The trend of [X/Fe] with age could be explained by considering that the decrease in [X/Fe] for the thick disk stars can be due to the decrease in type-II supernovae with time, meaning a reduced enrichment of r-process elements in the interstellar medium. In the thin disk, the trends are flatter, which is probably due to the main production from the s-process being balanced by Fe production from type-Ia supernovae.

The composition of the protoplanetary disc is thought to be linked to the composition of the host star, where a higher overall metallicity of the host star provides more building blocks for planets. However, most of the planet formation simulations only link the stellar iron abundance [Fe/H] to planet formation and the iron abundance in itself is used as a proxy to scale all elements. On the other hand, large surveys of stellar abundances show that this is not true. We use here stellar abundances from the GALAH surveys to determine the average detailed abundances of Fe, Si, Mg, O, and C for a broad range of host star metallicities with [Fe/H] spanning from -0.4 to +0.4. Using an equilibrium chemical model that features the most important rock forming molecules as well as volatile contributions of H 2 O, CO 2 , CH 4 and CO, we calculate the chemical composition of solid planetary building blocks around stars with different metallicities. Solid building blocks that are formed entirely interior to the water ice line (T>150K) only show an increase in Mg 2 SiO 4 and a decrease in MgSiO 3 for increasing host star metallicity, related to the increase of Mg/Si for higher [Fe/H]. Solid planetary building blocks forming exterior to the water ice line (T<150K), on the other hand, show dramatic changes in their composition. In particular the water ice content decreases from around ∼50% at [Fe/H]=-0.4 to ∼6% at [Fe/H]=0.4 in our chemical model. This is mainly caused by the increasing C/O ratio with increasing [Fe/H], which binds most of the oxygen in gaseous CO and CO 2 , resulting in a small water ice fraction. Planet formation simulations coupled with the chemical model confirm these results by showing that the water ice content of super-Earths decreases with increasing host star metallicity due to the increased C/O ratio. This decrease of the water ice fraction has important consequences for planet formation, planetary composition and the eventual habitability of planetary systems formed around these high metallicity stars.

The Milky Way is a unique laboratory, where stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the Solar Neighborhood, where all of the main Galactic components are well-represented. We find that the thick disk comprises two distinct and overlapping stellar populations, with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ∼ 150 km s −1 , corresponding to a rotational lag of 30 km s −1 relative to the canonical thick disk (∼ 180 km s −1 ), with a velocity dispersion of 60 km s −1 . This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the Solar Neighborhood by bar instability or spiral-arm formation mechanisms.

A small fraction of the halo field is made up of stars that share the light element (Z ≤ 13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey. One halo star at the base of the red giant branch, here referred to as 22593757-4648029 is found to have [Mg/Fe] = −0.36 ± 0.04 and [Al/Fe] = 0.99 ± 0.08, which is compatible with the most extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029 to GCs of similar metallicity and find it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters. However, both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC ω Centauri within the last few billion years. We cannot rule out other progenitor GCs, because some may have disrupted fully, and the abundance and orbital data are inadequate for many of those that are still intact.

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Context. Elements heavier than iron are produced through neutron-capture processes in the different phases of stellar evolution. Asymptotic giant branch (AGB) stars are believed to be mainly responsible for elements that form through the slow neutron-capture process, while the elements created in the rapid neutron-capture process have less understood production sites. Knowledge of abundance ratios as functions of metallicity can lead to insights on the origin and evolution of our Galaxy and its stellar populations. Aims. We aim to trace the chemical evolution of the neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu in the Milky Way stellar disk. This will allow us to constrain the formation sites of these elements as well as to probe the evolution of the Galactic thin and thick disks. Methods. Using spectra of high resolution (42 000 R 65 000) and high signal-to-noise (S /N 200) obtained with the MIKE and FEROS spectrographs, we determine Sr, Zr, La, Ce, Nd, Sm, and Eu abundances for a sample of 593 F and G dwarf stars in the Solar neighbourhood. The abundance analysis is based on spectral synthesis using one-dimensional, plane-parallel, local thermodynamic equilibrium (LTE) model stellar atmospheres calculated with the MARCS 2012 code. Results. We present abundance results for Sr (156 stars), Zr (311 stars), La (242 stars), Ce (365 stars), Nd (395 stars), Sm (280 stars) and Eu (378 stars). We find that Nd, Sm, and Eu show trends similar to what is observed for the α-elements in the [X/Fe]-[Fe/H] abundance plane. For [Sr/Fe] and [Zr/Fe] we find decreasing abundance ratios for increasing metallicity, reaching sub-solar values at super-solar metallicities. [La/Fe] and [Ce/Fe] do not show any clear trend with metallicity, being close to solar values at all [Fe/H]. The trends of abundance ratios [X/Fe] as a function of stellar ages present different slopes before and after 8 Gyr. Conclusions. The rapid neutron-capture process is active early in the Galaxy, mainly in type II supernovae from stars in the mass range 8-10 M . Europium is almost completely produced by r-process but Nd and Sm show similar trends to Eu even if their sprocess component is higher. Strontium and Zr are thought to be mainly produced by s-process, but show significant enrichment at low metallicity that requires extra r-process production, that probably is different from the classical r-process. Finally, La and Ce are mainly produced via s-process from AGB stars in mass range 2-4 M , which can be seen by the decrease in [La/Eu] and [Ce/Eu] at [Fe/H] ≈ −0.5. The trend of [X/Fe] with age could be explained by considering that the decrease in [X/Fe] for the thick disk stars can be due to the decrease of type II supernovae with time meaning a reduced enrichment of r-process elements in the interstellar medium. In the thin disk the trends are flatter that probably is due to that the main production from s-process is balanced by Fe production from type Ia supernovae.