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A Unique Star in the Outer Halo of the Milky Way

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arxiv 0904.4693 v1 pith:IWF6H45M submitted 2009-04-29 astro-ph.SR astro-ph.GA

A Unique Star in the Outer Halo of the Milky Way

classification astro-ph.SR astro-ph.GA
keywords stargalactichalouniquefoundmeasuremetal-poorouter
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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As part of a program to measure abundance ratios in stars beyond 15 kpc from the Galactic center, we have discovered a metal-poor star in the outer halo with a unique chemical signature. We originally identified it in the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey as a distant metal-poor star. We obtained a follow-up spectrum using the Echellette Spectrograph and Imager (ESI) at the Keck 2 telescope, and measure [Fe/H] =-3.17, [Mg/Fe] =-0.10 and [Ca/Fe] =+1.11. This is one of the largest over-abundances of Ca measured in any star to date; the extremely low value of [Mg/Ca]=-1.21 is entirely unique. To have found such an unusual star in our small sample of 27 targets suggests that there may be previously unobserved classes of stars yet to be found in situ in the Galactic halo.

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