Examinando por Autor "Gallagher A.J."
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Ítem MINCE: II. Neutron capture elements(EDP Sciences, 2024-06) François P.; Cescutti G.; Bonifacio P.; Caffau E.; Monaco L.; Steffen M.; Puschnig J.; Calura F.; Cristallo S.; Di Marcantonio P.; Dobrovolskas V.; Franchini M.; Gallagher A.J.; Hansen C.J.; Korn A.; Kučinskas A.; Lallement R.; Lombardo L.; Lucertini F.; Magrini L.; Matas Pinto A.M.; Matteucci F.; Mucciarelli A.; Sbordone L.; Spite M.; Spitoni E.; Valentini M.Context. Most of the studies on the determination of the chemical composition of metal-poor stars have been focused on the search of the most pristine stars, searching for the imprints of the ejecta of the first supernovae. Apart from the rare and very interesting r-enriched stars, few elements are measurable in the very metal-poor stars. On the other hand, a lot of work has been done also on the thin-disc and thick-disc abundance ratios in a metallicity range from [Fe/H]> -1.5 dex to solar. In the available literature, the intermediate metal-poor stars (-2.5<[Fe/H]< -1.5) have been frequently overlooked. The MINCE (Measuring at Intermediate metallicity Neutron-Capture Elements) project aims to gather the abundances of neutron-capture elements but also of light elements and iron peak elements in a large sample of giant stars in this metallicity range. The missing information has consequences for the precise study of the chemical enrichment of our Galaxy in particular for what concerns neutron-capture elements and it will be only partially covered by future multi object spectroscopic surveys such as WEAVE and 4MOST. Aims. The aim of this work is to study the chemical evolution of galactic sub-components recently identified (i.e. Gaia Sausage Enceladus (GSE), Sequoia). Methods. We used high signal-to-noise ratios, high-resolution spectra and standard 1D LTE spectrum synthesis to determine the detailed abundances. Results. We could determine the abundances for up to 10 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu) in 33 stars. The general trends of abundance ratios [n-capture element/Fe] versus [Fe/H] are in agreement with the results found in the literature. When our sample is divided in sub-groups depending on their kinematics, we found that the run of [Sr/Ba] versus [Ba/H] for the stars belonging to the GSE accretion event shows a tight anti-correlation. The results for the Sequoia stars, although based on a very limited sample, shows a [Sr/Ba] systematically higher than the [Sr/Ba] found in the GSE stars at a given [Ba/H] hinting at a different nucleosynthetic history. Stochastic chemical evolution models have been computed to understand the evolution of the GSE chemical composition of Sr and Ba. The first conclusions are that the GSE chemical evolution is similar to the evolution of a dwarf galaxy with galactic winds and inefficient star formation. Conclusions. Detailed abundances of neutron-capture elements have been measured in high-resolution, high signal-to-noise spectra of intermediate metal-poor stars, the metallicity range covered by the MINCE project. These abundances have been compared to detailed stochastic models of galactic chemical evolution.Ítem The Gaia-ESO Survey: Galactic evolution of sulphur and zinc(EDP Sciences, 2017-08) Duffau S.; Caffau E.; Babusiaux C.; Damiani F.; Franciosini E.; Jofré P.; Sbordone L.; Salvadori S.; Hourihane A.; Lardo C.; Lewis J.; Morbidelli L.; Sousa S.G.; Worley C.C.; Bonifacio P.; Andrievsky S.; Korotin S.; Monaco L.; François P.; Skúladóttir Á.; Bragaglia A.; Donati P.; Spina L.; Gallagher A.J.; Ludwig H.-G.; Christlieb N.; Hansen C.J.; Mott A.; Steffen M.; Zaggia S.; Blanco-Cuaresma S.; Calura F.; Friel E.; Jiménez-Esteban F.M.; Koch A.; Magrini L.; Pancino E.; Tang B.; Tautvaišiene G.; Vallenari A.; Hawkins K.; Gilmore G.; Randich S.; Feltzing S.; Bensby T.; Flaccomio E.; Smiljanic R.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.Context. Due to their volatile nature, when sulphur and zinc are observed in external galaxies, their determined abundances represent the gas-phase abundances in the interstellar medium. This implies that they can be used as tracers of the chemical enrichment of matter in the Universe at high redshift. Comparable observations in stars are more difficult and, until recently, plagued by small number statistics. Aims. We wish to exploit the Gaia-ESO Survey (GES) data to study the behaviour of sulphur and zinc abundances of a large number of Galactic stars, in a homogeneous way. Methods. By using the UVES spectra of the GES sample, we are able to assemble a sample of 1301 Galactic stars, including stars in open and globular clusters in which both sulphur and zinc were measured. Results. We confirm the results from the literature that sulphur behaves as an α-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The lower ratios are observed in giant stars at Galactocentric distances less than 7.5 kpc. No such effect is observed among dwarf stars, since they do not extend to that radius. Conclusions. Given the sample selection, giants and dwarfs are observed at different Galactic locations, and it is plausible, and compatible with simple calculations, that Zn-poor giants trace a younger population more polluted by SN Ia yields. It is necessary to extend observations in order to observe both giants and dwarfs at the same Galactic location. Further theoretical work on the evolution of zinc is also necessary. © 2017 ESO.Ítem TOPoS: V. Abundance ratios in a sample of very metal-poor turn-off stars(EDP Sciences, 2018-12) François P.; Caffau E.; Bonifacio P.; Spite M.; Spite F.; Cayrel R.; Christlieb N.; Gallagher A.J.; Klessen R.; Koch A.; Ludwig H.; Monaco L.; Plez B.; Steffen M.; Zaggia S.Context. Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars. Aims. Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium. Methods. We analysed medium-resolution spectra (R 10 000) obtained with the ESO X-shooter spectrograph and computed the abundances of several α and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres. Results. Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [α/Fe] ratios with several stars showing subsolar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424-004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex). © ESO 2018.