Examinando por Autor "O'Connell, J."
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Ítem Exploring the S-process History in the Galactic Disk: Cerium Abundances and Gradients in Open Clusters from the OCCAM/APOGEE Sample(Astrophysical Journal, 2022-02) Sales-Silva, J.; Daflon, S.; Cunha, K.; Souto, D.; Smith, V.; Chiappini, C.; Donor, J.; Frinchaboy, P.; García-Hernández, D.; Hayes, C.; Majewski, S.; Masseron, T.; Schiavon, R.; Weinberg, D.; Beaton, R.; Fernández-Trincado, J.; Jönsson, H.; Lane, R.; Minniti, D.; Manchado, A.; Moni, B.; Nitschelm, C.; O'Connell, J.; Villanova, S.The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce ii lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/α-element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]-age plane (where X can be H, Fe, or the α-elements O, Mg, Si, or Ca). These metallicity-dependent relations result in [Ce/Fe] and [Ce/α] ratios with ages that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d[Ce/H]/d R GC being negative, while d[Ce/Fe]/d R GC is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by ∼+0.009 dex kpc-1 Gyr-1. Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.Ítem Investigating a predicted metallicity [Fe/H] variation in the Type II Globular Cluster NGC 362(Oxford University Press, 2022-09-01) Vargas, C.; Villanova, S.; Geisler, D.; Muñoz, C.; Monaco, L.; O'Connell, J.; Sarajedini, AtaNGC 362 is a non-common Type II Galactic globular cluster, showing a complex pseudo two-colour diagram or 'chromosome map'. The clear separation of its stellar populations in the colour-magnitude diagram and the distribution of the giant stars in the chromosome map strongly suggests that NGC 362 could host stars with both cluster-nominal, as well as enhanced heavy-element abundances, and one of them could be iron. However, despite previous spectroscopic observations of NGC 362, no such iron variation has been detected. Our main goal is to confirm or disprove this result by searching for any internal variation of [Fe/H], which would give us insight into the formation and evolution of this interesting globular cluster. In this paper, we present the abundance analysis for a sample of 11 red giant branch members based on high-resolution and high S/N spectra obtained with the MIKE echelle spectrograph mounted at the Magellan-Clay telescope. HST and GAIA photometry and astrometry has been used to determine atmospheric parameters and membership. We obtained Teff, log(g), and vt for our target stars and measured the mean iron content of the sample and its dispersion with three different methods, which lead to [Fe/H]1 =-1.10±0.02, [Fe/H]2 =-1.09 ±0.01, and [Fe/H]3 =-1.10 ±0.01, while the internal dispersion turned out to be σ[ Fe/H ]1 = 0.06 ±0.01, σ[Fe/H ]2 = 0.03 ±0.01, and σ[ Fe/H ]3 = 0.05 ±0.01, respectively. The error analysis gives an internal dispersion due to observational error of 0.05 dex. Comparing the observed dispersion with the internal errors, we conclude that NGC 362 does not show any trace of an internal iron spread. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.