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    X-Shooter Observations of Main-Sequence Stars in the Globular Cluster NGC 2808: First Chemical Tagging of A He-Normal and A He-Rich Dwarf

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    Date
    2010-09
    Author
    Bragaglia, A.
    Carretta, E.
    Gratton, R. G.
    Lucatello, Sara
    Milone, A.
    Piotto, G.
    D'Orazi, V.
    Cassisi, S.
    Sneden, Christopher
    Bedin, L. R.
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    Abstract
    We present the first chemical composition study of two unevolved stars in the globular cluster NGC 2808, obtained with the X-shooter spectrograph at VLT.NGC 2808 shows three discrete, well-separated main sequences. The most accepted explanation for this phenomenon is that their stars have different helium contents. We observed one star on the bluest main sequence (bMS, claimed to have a high helium content, Y similar to 0.4) and the other on the reddest main sequence (rMS, consistent with a canonical helium content, Y = 0.245). We analyzed features of NH, CH, Na, Mg, Al, and Fe. While Fe, Ca, and other elements have the same abundances in the two stars, the bMS star shows a huge enhancement of N, a depletion of C, an enhancement of Na and Al, and a small depletion of Mg with respect to the rMS star. This is exactly what is expected if stars on the bMS formed from the ejecta produced by an earlier stellar generation in the complete CNO and MgAl cycles whose main product is helium. The elemental abundance pattern differences in these two stars are consistent with the differences in the helium content suggested by the color-magnitude diagram positions of the stars.
    Department
    Astronomy
    Subject
    globular clusters: general
    globular clusters: individual (ngc 2808)
    stars: abundances
    stars: evolution
    stars: population ii
    red giant branch
    na-o anticorrelation
    metal-poor stars
    omega-centauri
    abundance
    ngc-2808
    m5
    morphology
    evolution
    spread
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/43047
    Citation
    Bragaglia, Angela, Eugenio Carretta, Raffaele G. Gratton, Sara Lucatello, Antonino Milone, G. Piotto, Valentina D'Orazi, S. Cassisi, Chris Sneden, and L. R. Bedin. "X-SHOOTER OBSERVATIONS OF MAIN-SEQUENCE STARS IN THE GLOBULAR CLUSTER NGC 2808: FIRST CHEMICAL TAGGING OF A He-NORMAL AND A He-RICH DWARFBased on data collected at the ESO telescopes under GTO program 084. D-0070." The Astrophysical Journal Letters, Vol. 720, No. 1 (Sep., 2010): L41.
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