Astron. J., 154, 155-155 (2017/October-0)
Light and heavy element abundance variations in the outer halo globular cluster NGC 6229.
JOHNSON C.I., CALDWELL N., RICH R.M. and WALKER M.G.
Abstract (from CDS):
NGC 6229 is a relatively massive outer halo globular cluster that is primarily known for exhibiting a peculiar bimodal horizontal branch morphology. Given the paucity of spectroscopic data on this cluster, we present a detailed chemical composition analysis of 11 red giant branch members based on high resolution (R ≃ 38,000), high S/N (>100) spectra obtained with the MMT-Hectochelle instrument. We find the cluster to have a mean heliocentric radial velocity of -138.1–1.0+1.0 km s–1, a small dispersion of 3.8–0.7+1.0 km s–1, and a relatively low (M/LV)☉=0.82–0.28+0.49. The cluster is moderately metal-poor with <[Fe/H]>=-1.13 dex and a modest dispersion of 0.06 dex. However, 18% (2/11) of the stars in our sample have strongly enhanced [La, Nd/Fe] ratios that are correlated with a small (∼0.05 dex) increase in [Fe/H]. NGC 6229 shares several chemical signatures with M75, NGC 1851, and the intermediate metallicity populations of ω Cen, which lead us to conclude that NGC 6229 is a lower mass iron-complex cluster. The light elements exhibit the classical (anti-)correlations that extend up to Si, but the cluster possesses a large gap in the O-Na plane that separates first and second generation stars. NGC 6229 also has unusually low [Na, Al/Fe] abundances that are consistent with an accretion origin. A comparison with M54 and other Sagittarius clusters suggests that NGC 6229 could also be the remnant core of a former dwarf spheroidal galaxy.
© 2017. The American Astronomical Society. All rights reserved.
globular clusters: general - globular clusters: individual: NGC 6229 - stars: abundances - stars: abundances
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<Available at CDS (J/AJ/154/155): table1.dat table2.dat table3.dat table4.dat table5.dat>
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