Examinando por Autor "Baratella, M."

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    The Gaia-ESO Survey: a new approach to chemically characterising young open clusters
    (European Southern Observatory (ESO), 2021) Baratella, M.; D’Orazi, V.; Sheminova, V.; Spina, L.; Carraro, G.; Gratton, R.; Magrini, L.; Randich, L.; Lugaro, M.; Pignatari, M.; Romano, D.; Biazzo, K.; Bragaglia, A.; Casali, G.; Desidera, S.; Frasca, A.; De Silva, G.; Melo, C.; Van Der Swaelmen, M.; Tautvaišienė, G.; Jiménez-Esteban, F. M.; Gilmore, G.; Bensby, T.; Smiljanic, R.; Bayo, Amelia; Franciosini, E.; Gonneau, A.; Hourihane, A.; Jofré, P.; Monaco, L.; Morbidelli, L.; Sacco, G.; Sbordone, L.; Worley, C.; Zaggia, S.
    Context. Young open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include 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: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages. Aims. In this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu I, Sr I, Sr II, Y II, Zr II, Ba II, La II, and Ce II. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time. Methods. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the Gaia-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun. Results. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm that [Ba/Fe] is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with [Y/Fe] ratios covering values between 0 and +0.3 dex. For the other s-process elements we find that [X/Fe] ratios are solar at all ages. Conclusions. 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. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) 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 the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of the s-process at young ages, and we strongly encourage further critical observations.
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    Ítem
    The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars
    (European Southern Observatory (ESO), 2021) Magrini, L.; Lagarde, N.; Charbonnel, C.; Franciosini, E.; Randich, S.; Smiljanic, R.; Casali, G.; Viscasillas Vázquez, C.; Spina, L.; Biazzo, K.; Pasquini, L.; Bragaglia, A.; Van Der Swaelmen, M.; Tautvaišienė, G.; Inno, L.; Sanna, N.; Prisinzano, L.; Degl’Innocenti, S.; Prada Moroni, P.; Roccatagliata, V.; Tognelli, E.; Monaco, L.; De Laverny, P.; Delgado-Mena, E.; Baratella, M.; D’Orazi, V.; Vallenari, A.; Gonneau, A.; Worley, C.; Jiménez-Esteban, F.; Jofre, P.; Bensby, T.; François, P.; Guiglion, G.; Bayo, Amelia; Jeffries, R. D.; Binks, A. S.; Gilmore, G.; Damiani F.; Korn, A.; Pancino, E.; Sacco, G. G.; Hourihane, A.; Morbidelli, L.; Zaggia, S.
    Aims. We aim to constrain the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence. We take advantage of the data from the sixth internal data release of Gaia-ESO, IDR6, and from the Gaia Early Data Release 3, EDR3s. Methods. We selected a sample of main-sequence, sub-giant, and giant stars in which the Li abundance is measured by the Gaia-ESO survey. These stars belong to 57 open clusters with ages from 130 Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between ∼​ − 1.0 and ∼​ + 0.5 dex, with few stars between ∼​ − 1.0 and ∼​ − 2.5 dex. We studied the behaviour of the Li abundances as a function of stellar parameters. We inferred the masses of giant stars in clusters from the main-sequence turn-off masses, and for field stars through comparison with stellar evolution models using a maximum likelihood technique. We compared the observed Li behaviour in field giant stars and in giant stars belonging to individual clusters with the predictions of a set of classical models and of models with mixing induced by rotation and thermohaline instability. Results. The comparison with stellar evolution models confirms that classical models cannot reproduce the observed lithium abundances in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to 2 dex), making lithium the best element with which to constrain stellar mixing processes in low-mass stars. We discuss the nature of a sample of Li-rich stars. Conclusions. We demonstrate that the evolution of the surface abundance of Li in giant stars is a powerful tool for constraining theoretical stellar evolution models, allowing us to distinguish the effect of different mixing processes. For stars with well-determined masses, we find a better agreement of observed surface abundances and models with rotation-induced and thermohaline mixing. Rotation effects dominate during the main sequence and the first phases of the post-main-sequence evolution, and the thermohaline induced mixing after the bump in the luminosity function.