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    Abundances Of C, N, Sr, And Ba On The Red Giant Branch Of Omega Centauri

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    2010_05_omegacentauri.pdf (976.8Kb)
    Date
    2010-05
    Author
    Stanford, Laura M.
    Da Costa, G. S.
    Norris, John E.
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    Abstract
    Abundances relative to iron for carbon, nitrogen, strontium, and barium are presented for 33 stars on the red giant branch (RGB) of the globular cluster omega Centauri. They are based on intermediate-resolution spectroscopic data covering the blue spectral region analyzed using spectrum synthesis techniques. The data reveal the existence of a broad range in the abundances of these elements, and a comparison with similar data for main-sequence stars enables insight into the evolutionary history of the cluster. The majority of the RGB stars were found to be depleted in carbon, i.e., [C/Fe] < 0, while [N/Fe] for the same stars shows a range of similar to 1 dex, from [N/Fe] approximate to 0.7 to 1.7 dex. The strontium-to-iron abundance ratios varied from solar to mildly enhanced (0.0 <= [Sr/Fe] <= 0.8), with [Ba/Fe] generally equal to or greater than [Sr/Fe]. The carbon and nitrogen abundance ratios for the one known CH star in the sample, ROA 279, are [C/Fe] = 0.6 and [N/Fe] = 0.5 dex. Evidence for evolutionary mixing on the RGB is found from the fact that the relative carbon abundances on the main sequence are generally higher than those on the RGB. However, comparison of the RGB and main-sequence samples shows that the upper level of nitrogen enhancement is similar in both sets at [N/Fe] approximate to 2.0 dex. This is most likely the result of primordial rather than evolutionary mixing processes. One RGB star, ROA 276, was found to have Sr and Ba abundance ratios similar to the anomalous Sr-rich main-sequence star S2015448. High-resolution spectra of ROA 276 were obtained with the Magellan Telescope/mike spectrograph combination to confirm this result, revealing that ROA 276 is indeed an unusual star. For this star, calculations of the depletion effect, the potential change in surface abundance that results from the increased depth of the convective envelope as a star moves from the main sequence to the RGB, strongly suggest that the observed Sr enhancement in ROA 276 is of primordial origin, rather than originating from a surface accretion event.
    Department
    McDonald Observatory
    Subject
    globular clusters: general
    globular clusters: individual (omega
    centauri)
    metal-poor stars
    main-sequence
    resolution spectroscopy
    metallicity
    relation
    stellar populations
    globular-clusters
    neutron-capture
    carbon
    evolution
    subgiant
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/35223
    Citation
    Stanford, Laura M., Gary S. Da Costa, and John E. Norris. "Abundances of C, N, Sr, and Ba on the Red Giant Branch of ? Centauri." The Astrophysical Journal, Vol. 714, No. 2 (May., 2010): 1001.
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