Examinando por Autor "Carraro, G."

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    Infrared photometry and CaT spectroscopy of globular cluster M28 (NGC 6626)
    (European Southern Observatory (ESO), 2021) Moni Bidin, C.; Mauro, F.; Contreras Ramos, R.; Zoccali, M.; Reinarz, Y.; Moyano, M.; González-Díaz, D.; Villanova, S.; Carraro, G.; Borissova, J.; Chené, A.-N.; Cohen, R. E.; Geisler, D.; Kurtev, R.; Minniti, D.
    Context. Recent studies show that the inner Galactic regions host genuine bulge globular clusters, but also halo intruders, complex remnants of primordial building blocks, and objects likely accreted during major merging events. Aims. In this study we focus on the properties of M 28, a very old and massive cluster currently located in the Galactic bulge. Methods. We analysed wide-field infrared photometry collected by the VVV survey, VVV proper motions, and intermediate-resolution spectra in the calcium triplet range for 113 targets in the cluster area. Results. Our results in general confirm previous estimates of the cluster properties available in the literature. We find no evidence of differences in metallicity between cluster stars, setting an upper limit of Δ[Fe/H] < 0.08 dex to any internal inhomogeneity. We confirm that M 28 is one of the oldest objects in the Galactic bulge (13–14 Gyr). From this result and the literature data, we find evidence of a weak age–metallicity relation among bulge globular clusters that suggests formation and chemical enrichment. In addition, wide-field density maps show that M 28 is tidally stressed and that it is losing mass into the general bulge field. Conclusions. Our study indicates that M 28 is a genuine bulge globular cluster, but its very old age and its mass loss suggest that this cluster could be the remnant of a larger structure, possibly a primeval bulge building block.
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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.