Examinando por Autor "Salaris, M."
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Ítem Discovery of a thin lithium plateau among metal-poor red giant branch stars(EDP Sciences, 2022-05-01) Mucciarelli, A.; Monaco, L.; Bonifacio, P.; Salaris, M.; Deal, M.; Spite, M.; Richard, O.A.; Lallement, R.The surface lithium abundance, A(Li), of warm metal-poor dwarf stars exhibits a narrow plateau down to [Fe/H] ~-2.8 dex, while at lower metallicities the average value drops by 0.3 dex with a significant star-by-star scatter (called 'lithium meltdown'). This behaviour is in conflict with predictions of standard stellar evolution models calculated with the initial A(Li) provided by the standard Big Bang nucleosynthesis. The lower red giant branch (LRGB) stars provide a complementary tool to understand the initial A(Li) distribution in metal-poor stars. We have collected a sample of high-resolution spectra of 58 LRGB stars spanning a range of [Fe/H] between ~-7.0 dex and ~-1.3 dex. The LRGB stars display an A(Li) distribution that is clearly different from that of the dwarfs, without signatures of a meltdown and with two distinct components: (a) a thin A(Li) plateau with an average A(Li) = 1.09 ± 0.01 dex (σ = 0.07 dex) and (b) a small fraction of Li-poor stars with A(Li) lower than ~0.7 dex. The A(Li) distribution observed in LRGB stars can be reconciled with an initial abundance close to the cosmological value by including an additional chemical element transport in stellar evolution models. The required efficiency of this transport allows us to also match the Spite plateau lithium abundance measured in the dwarfs. The emerging scenario is that all metal-poor stars formed with the same initial A(Li), but those that are likely the product of coalescence or that experienced binary mass transfer show lower A(Li). We conclude that the A(Li) in LRGB stars is qualitatively compatible with the cosmological A(Li) value and that the meltdown observed in dwarf stars does not reflect a real drop in the abundance at birth. © 2022 BMJ Publishing Group. All rights reserved.Ítem Lithium abundance in lower red giant branch stars of Omega Centauri(EDP Sciences, 2018-10) Mucciarelli, A.; Salaris, M.; Monaco, L.; Bonifacio, P.; Fu, X.; Villanova, S.We present Li, Na, Al, and Fe abundances of 199 lower red giant branch star members of the stellar system Omega Centauri, using high-resolution spectra acquired with FLAMES at the Very Large Telescope. The A(Li) distribution is peaked at A(Li) ∼ 1 dex with a prominent tail towards lower values. The peak of the distribution well agrees with the lithium abundances measured in lower red giant branch stars in globular clusters and Galactic field stars. Stars with A(Li) ∼ 1 dex are found at metallicities lower than [Fe/H] ∼ -1.3 dex but they disappear at higher metallicities. On the other hand, Li-poor stars are found at all metallicities. The most metal-poor stars exhibit a clear Li-Na anti-correlation, where about 30% of the sample have A(Li) lower than ∼0.8 dex, while these stars represent a small fraction of normal globular clusters. Most of the stars with [Fe/H] > -1.6 dex are Li poor and Na rich. The Li depletion measured in these stars is not observed in globular clusters with similar metallicities and we demonstrate that it is not caused by the proposed helium enhancements and/or young ages. Hence, these stars formed from a gas already depleted in lithium. Finally, we note that Omega Centauri includes all the populations (Li-normal/Na-normal, Li-normal/Na-rich, and Li-poor/Na-rich stars) observed, to a lesser extent, in mono-metallic GCs. © ESO 2018.Ítem Lithium on the lower red giant branch of five Galactic globular clusters(EDP Sciences, 2022-01-01) Aguilera-Gómez, C.; Monaco, L.; Mucciarelli, A.; Salaris, M.; Villanova, S.; Pancino, E.Context. Lithium is one of the few elements produced during Big Bang nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of various astrophysical investigations. Aims. Stars on the lower red giant branch are key to studies of globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to study signs of extra depletion in these giants. Methods. We used the GIRAFFE spectra to measure the lithium and sodium abundances of lower red giant branch stars in five globular clusters. These cover an extensive range in metallicity, from [Fe/H] ∼-0.7 to [Fe/H] ∼-2.3 dex. Results. We find that the lithium abundance in these lower red giant branch stars forms a plateau, with values from A(Li)NLTE = 0.84 to 1.03 dex, showing no clear correlation with metallicity. When using stellar evolutionary models to calculate the primordial abundance of these clusters, we recover values of A(Li)NLTE = 2.1 - 2.3 dex, consistent with the constant value observed in warm metal-poor halo stars, namely the Spite plateau. Additionally, we find no difference in the lithium abundance of first and second population stars in each cluster. We also report the discovery of a Li-rich giant in the cluster NGC 3201, with A(Li)NLTE = 1.63 ± 0.18 dex, where the enrichment mechanism is probably pollution from external sources.Ítem The discovery of a Li/Na-rich giant star in Omega Centauri: Formed from the pure ejecta of super-AGB stars?(Astronomy and Astrophysics, 2019-03-01) Mucciarelli, A.; Monaco, L.; Bonifacio, P.; Salaris, M.; Fu, X.; Villanova, S.We report the discovery of two Li-rich giant stars (fainter than the red giant branch (RGB) bump) in the stellar system Omega Centauri using GIRAFFE-FLAMES spectra. These two stars have A(Li) = 1.65 and 2.40 dex and they belong to the main population of the system ([Fe/H] = -1.70 and -1.82, respectively). The most Li-rich of them (#25664) has [Na/Fe] = +0.87 dex, which is ~0.5 dex higher than those measured in the most Na-rich stars of Omega Centauri of similar metallicity. The chemical abundances of Li and Na in #25664 can be qualitatively explained by deep extra mixing efficient within the star during its RGB evolution or by super-asymptotic giant branch (AGB) stars with masses between ~7 and 8 M™. In the latter scenario, this Li/Na-rich star could be formed from the pure ejecta of super-AGB stars before the dilution with pristine material occurs, or, alternatively, be part of a binary system having experienced mass transfer from the companion when this latter evolved through the super-AGB phase. In both these cases, the chemical composition of this unique object could allow to look for the first time at the chemical composition of the gas processed in the interior of super-AGB stars.Ítem The Gaia-ESO Survey: Evidence of atomic diffusion in M67?(Oxford University Press, 2018-07) Bertelli Motta, C.; Pasquali, A.; Richer, J.; Michaud, G.; Salaris, M.; Bragaglia, A.; Magrini, L.; Randich, S.; Grebel, E.K.; Adibekyan, V.; Blanco-Cuaresma, S.; Drazdauskas, A.; Fu, X.; Martell, S.; Tautvaišiene, G.; Gilmore, G.; Alfaro, E.J.; Bensby, T.; Flaccomio, E.; Koposov, S.E.; Korn, A.J.; Lanzafame, A.C.; Smiljanic, R.; Bayo, A.; Carraro, G.; Casey, A.R.; Costado, M.T.; Damiani, F.; Franciosini, E.; Heiter, U.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis, J.; Monaco, L.; Morbidelli, L.; Sacco, G.G.; Sousa, S.G.; Worley, C.C.; Zaggia, S.Investigating the chemical homogeneity of stars born from the same molecular cloud at virtually the same time is very important for our understanding of the chemical enrichment of the interstellar medium and with it the chemical evolution of the Galaxy. One major cause of inhomogeneities in the abundances of open clusters is stellar evolution of the cluster members. In this work, we investigate variations in the surface chemical composition of member stars of the old open clusterM67 as a possible consequence of atomic diffusion effects taking place during the main-sequence phase. The abundances used are obtained from high-resolution UVES/FLAMES spectra within the framework of the Gaia-ESO Survey. We find that the surface abundances of stars on the main sequence decrease with increasing mass reaching a minimum at the turn-off. After deepening of the convective envelope in subgiant branch stars, the initial surface abundances are restored.We found themeasured abundances to be consistent with the predictions of stellar evolutionary models for a cluster with the age and metallicity of M67. Our findings indicate that atomic diffusion poses a non-negligible constraint on the achievable precision of chemical tagging methods. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.