A Unique Star in the Outer Halo of the Milky Way

TL;DR

This paper reports the discovery of a unique, extremely metal-poor star in the Milky Way's outer halo with unusual chemical signatures, indicating potential new classes of stars.

Contribution

It presents the identification and detailed chemical analysis of a rare star with unprecedented calcium over-abundance and low magnesium-to-calcium ratio.

Findings

Discovered a star with [Fe/H] = -3.17, [Mg/Fe] = -0.10, [Ca/Fe] = +1.11

Found an extremely low [Mg/Ca] ratio of -1.21

Suggests existence of previously unobserved stellar classes in the Galactic halo

Abstract

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.