Examinando por Autor "Schiavon, RP"

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  • Miniatura
    Ítem
    DISCOVERY OF A METAL-POOR FIELD GIANT WITH A GLOBULAR CLUSTER SECOND-GENERATION ABUNDANCE PATTERN
    (IOP PUBLISHING, 2016-12) Fernandez-Trincado, JG; Robin, AC; Moreno, E; Schiavon, RP; Perez, AEG; Vieira, K; Cunha, K; Zamora, O; Sneden, C; Souto, D; Carrera, R; Johnson, JA; Shetrone, M; Zasowski, G; Garcia-Hernandez, DA; Majewski, SR; Reyle, C; Blanco-Cuaresma, S; Martinez-Medina, LA; Perez-Villegas, A; Valenzuela, O; Pichardo, B; Meza, A; Meszaros, S; Sobeck, J; Geisler, D; Anders, F; Schultheis, M; Tang, B; Roman-Lopes, A; Mennickent, RE; Pan, K; Nitschelm, C; Allard, F
    We report on the detection, from observations obtained with the Apache Point Observatory Galactic Evolution Experiment spectroscopic survey, of a metal-poor ([Fe/H] = -1.3 dex) field giant star with an extreme Mg-Al abundance ratio ([Mg/Fe] = -0.31 dex; [Al/Fe] = 1.49 dex). Such low Mg/Al ratios are seen only among the second-generation population of globular clusters (GCs) and are not present among Galactic disk field stars. The light-element abundances of this star, 2M16011638-1201525, suggest that it could have been born in a GC. We explore several origin scenarios, studying the orbit of the star in particular to check the probability of its being kinematically related to known GCs. We performed simple orbital integrations assuming the estimated distance of 2M16011638-1201525 and the available six-dimensional phase-space coordinates of 63 GCs, looking for close encounters in the past with a minimum distance approach within the tidal radius of each cluster. We found a very low probability that 2M16011638-1201525 was ejected from most GCs; however, we note that the best progenitor candidate to host this star is GC omega Centauri (NGC 5139). Our dynamical investigation demonstrates that 2M16011638-1201525 reaches a distance vertical bar Z(max)vertical bar < 3 kpc from the Galactic plane and minimum and maximum approaches to the Galactic center of R-min < 0.62 kpc and R-max < 7.26 kpc in an eccentric (e similar to 0.53) and retrograde orbit. Since the extreme chemical anomaly of 2M16011638-1201525 has also been observed in halo field stars, this object could also be considered a halo contaminant, likely to have been ejected into the Milky Way disk from the halo. We conclude that 2M16011638-20152 is also kinematically consistent with the disk but chemically consistent with halo field stars.
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    Ítem
    Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE
    (OXFORD UNIV PRESS, 2017-02) Tang, B; Cohen, RE; Geisler, D; Schiavon, RP; Majewski, SR; Villanova, S; Carrera, R; Zamora, O; Garcia-Hernandez, DA; Shetrone, M; Frinchaboy, P; Meza, A; Fernandez-Trincado, JG; Munoz, RR; Lin, CC; Lane, RR; Nitschelm, C; Pan, K; Bizyaev, D; Oravetz, D; Simmons, A
    Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high-resolution near-infrared (NIR) spectroscopic data from Apache Point Observatory Galactic Evolution Experiment (APOGEE) to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify 10 red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of -0.14 +/- 5.47 km s(-1), land a mean [Fe/H] of -0.15 +/- 0.05. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.