Browsing by Author "Yong, D."

Now showing 1 - 8 of 8
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    A FIRST CONSTRAINT ON THE THICK DISK SCALE LENGTH: DIFFERENTIAL RADIAL ABUNDANCES IN K GIANTS AT GALACTOCENTRIC RADII 4, 8, AND 12 kpc
    (2011) Bensby, T.; Alves-Brito, A.; Oey, M. S.; Yong, D.; Melendez, J.
    Based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan telescopes, we present detailed elemental abundances for 20 red giant stars in the outer Galactic disk, located at Galactocentric distances between 9 and 13 kpc. The outer disk sample is complemented with samples of red giants from the inner Galactic disk and the solar neighborhood, analyzed using identical methods. For Galactocentric distances beyond 10 kpc, we only find chemical patterns associated with the local thin disk, even for stars far above the Galactic plane. Our results show that the relative densities of the thick and thin disks are dramatically different from the solar neighborhood, and we therefore suggest that the radial scale length of the thick disk is much shorter than that of the thin disk. We make a first estimate of the thick disk scale length of L-thick = 2.0 kpc, assuming L-thin = 3.8 kpc for the thin disk. We suggest that radial migration may explain the lack of radial age, metallicity, and abundance gradients in the thick disk, possibly also explaining the link between the thick disk and the metal-poor bulge.
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    CNO and F abundances in the barium star HD 123396
    (2011) Alves-Brito, A.; Karakas, A. I.; Yong, D.; Melendez, J.; Vasquez, S.
    Context. Barium stars are moderately rare, chemically peculiar objects, which are believed to be the result of the pollution of an otherwise normal star by material from an evolved companion on the asymptotic giant branch (AGB).
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    Iron and s-elements abundance variations in NGC 5286: comparison with 'anomalous' globular clusters and Milky Way satellites
    (2015) Marino, A. F.; Milone, A. P.; Karakas, A. I.; Casagrande, L.; Yong, D.; Shingles, L.; Da Costa, G.; Norris, J. E.; Stetson, P. B.; Lind, K.; Asplund, M.; Collet, R.; Jerjen, H.; Sbordone, L.; Aparicio, A.; Cassisi, S.
    We present a high-resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster (GC) NGC 5286. We have determined abundances of representative light proton-capture, a, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s-process element abundance ratios, with two main groups, the s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the s-rich stars having higher Fe, e.g. <[Fe/H]>(s-rich) - <[Fe/H]>(s-poor) similar to 0.2 dex; and (iii) the presence of O-Na-Al (anti) correlations in both stellar groups. We have defined a new photometric index, c(BVI) = (B - V) -(V - I), to maximize the separation in the colour-magnitude diagram between the two stellar groups with different Fe and s-element content, and this index is not significantly affected by variations in light elements (such as the O-Na anticorrelation). The variations in the overall metallicity present in NGC 5286 add this object to the class of anomalous GCs. Furthermore, the chemical abundance pattern of NGC 5286 resembles that observed in some of the anomalous GCs, e.g. M 22, NGC 1851, M 2, and the more extreme omega Centauri, that also show internal variations in s-elements, and in light elements within stars with different Fe and s-elements content. In view of the common variations in s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.
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    The Aquarius comoving group is not a disrupted classical globular cluster
    (2014) Casey, A. R.; Keller, S. C.; Alves-Brito, A.; Frebel, A.; Da Costa, G.; Karakas, A.; Yong, D.; Schlaufman, K. C.; Jacobson, H. R.; Yu, Q.; Fishlock, C.
    We present a detailed analysis of high-resolution, high signal-to-noise ratio spectra for five Aquarius stream stars observed with the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Clay telescope. Our sample represents one-third of the 15 known members in the stream. We find the stream is not monometallic: the metallicity ranges from [Fe/H] = -0.63 to -1.58. No anticorrelation in Na-O abundances is present, and we find a strong positive Mg-Al relationship, similar to that observed in the thick disc. We find no evidence that the stream is a result of a disrupted classical globular cluster, contrary to a previously published claim. High [(Na, Ni, alpha)/Fe] and low [Ba/Y] abundance ratios in the stream suggest that it is not a tidal tail from a disrupted dwarf galaxy, either. The stream is chemically indistinguishable from Milky Way field stars with the exception of one candidate, C222531-145437. From its position, velocity, and detailed chemical abundances, C222531-145437 is likely a star that was tidally disrupted from omega-Centauri. We propose that the Aquarius stream is Galactic in origin, and could be the result of a disc-satellite perturbation in the Milky Way thick disc of the order of a few Gyr ago: derived orbits, UVW velocities, and angular momenta of the Aquarius members offer qualitative support for our hypothesis. Assuming that C222531-145437 is a tidally disrupted member of omega-Centauri, this system is the most likely disc perturber. In the absence of compelling chemical and/or dynamical evidence that the Aquarius stream is the tidal tail of a disrupted satellite, we advocate the 'Aquarius group' as a more appropriate description. Like the Canis Major overdensity, as well as the Hercules and Monoceros groups, the Aquarius group joins the list of kinematically identified substructures that are not actually accreted material: they are simply part of the rich complexity of the Milky Way structure.
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    The chemical composition of red giants in 47 Tucanae I. Fundamental parameters and chemical abundance patterns
    (2014) Thygesen, A. O.; Sbordone, L.; Andrievsky, S.; Korotin, S.; Yong, D.; Zaggia, S.; Ludwig, H. -G.; Collet, R.; Asplund, M.; Ventura, P.; D'Antona, F.; Melendez, J.; D'Ercole, A.
    Context. The study of chemical abundance patterns in globular clusters is key importance to constraining the different candidates for intracluster pollution of light elements.
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    The chemical composition of red giants in 47 Tucanae II. Magnesium isotopes and pollution scenarios
    (EDP SCIENCES S A, 2016) Thygesen, A. O.; Sbordone, L.; Ludwig, H. G.; Ventura, P.; Yong, D.; Collet, R.; Christlieb, N.; Melendez, J.; Zaggia, S.
    Context. The phenomenon of multiple populations in globular clusters is still far from understood, with several proposed mechanisms to explain the observed behaviour. The study of elemental and isotopic abundance patterns are crucial for investigating the differences among candidate pollution mechanisms.
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    The Gaia-ESO Survey : the most metal-poor stars in the Galactic bulge
    (2014) Howes, L.M.; Asplund, M.; Casey, A.R.; Keller, S.C.; Yong, D.; Gilmore, G.; Lind, K.; Worley, C.; Bessell, M.S.; Zoccali, Manuela
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    The halo plus cluster system of the Galactic globular cluster NGC 1851(a similar to...)
    (2014) Marino, A.F.; Milone, A.P.; Yong, D.; Dotter, A.; Da Costa, G.; Asplund, M.; Jerjen, H.; Cassisi, S.; Sbordone, L.; Angeloni, Rodolfo