An Extremely Carbon-rich, Extremely Metal-poor Star in the Segue 1 System

TL;DR

This study identifies a unique, extremely metal-poor, carbon-rich star in the Segue 1 system, providing insights into early chemical evolution and star formation in ultra-faint dwarf galaxies or debris from larger systems.

Contribution

First detailed chemical analysis of a CEMP-no star in a Milky Way satellite, linking early star formation in dwarf galaxies to the Galactic halo.

Findings

Star has [Fe/H] = -3.52 and high [C/Fe] = +2.3

Chemical signature similar to CEMP-no stars in the halo

Supports the idea that ultra-faint dwarfs contributed to the Milky Way halo

Abstract

We report the analysis of high-resolution, high-S/N spectra of an extremely metal-poor, extremely C-rich red giant, Seg 1-7, in the Segue 1 system - described in the literature alternatively as an unusually extended globular cluster or an ultra-faint dwarf galaxy. The radial velocity of Seg 1-7 coincides precisely with the systemic velocity of Segue 1, and its chemical abundance signature of [Fe/H] = -3.52, [C/Fe] = +2.3, [N/Fe] = +0.8, [Na/Fe] = +0.53, [Mg/Fe] = +0.94, [Al/Fe] = +0.23 and [Ba/Fe] < -1.0 is similar to that of the rare and enigmatic class of Galactic halo objects designated CEMP-no (Carbon-rich, Extremely Metal-Poor and with no enhancement (over solar ratios) of heavy neutron-capture elements). This is the first star in a Milky Way ``satellite'' that unambiguously lies on the metal-poor, C-rich branch of the Aoki et al. (2007) bimodal distribution defined by field halo stars in the ([C/Fe], [Fe/H])-plane. Available data permit us only to identify Seg 1-7 as a member of an ultra-faint dwarf galaxy or as debris from the Sgr dwarf spheroidal galaxy. In either case, this demonstrates that at extremely low abundance, [Fe/H ] < -3.0, star formation and associated chemical evolution proceeded similarly in the progenitors of both the field halo and satellite systems. By extension, this is consistent with other recent suggestions the most metal-poor dwarf spheroidal and ultra-faint dwarf satellites were the building blocks of the Milky Way's outer halo.