Chemical Compositions of Kinematically Selected Outer Halo Stars

TL;DR

This study analyzes the chemical compositions of 26 outer-halo stars using high-resolution spectra, revealing trends in element abundances that inform our understanding of the Galaxy's outer halo formation.

Contribution

First detailed chemical abundance analysis of outer-halo stars with consistent LTE methods, highlighting unique abundance patterns compared to inner halo stars.

Findings

Decreasing [$ ext{α}$/Fe] with increasing [Fe/H] in outer halo stars.

Most objects show slightly lower [Zn/Fe] than inner halo stars.

Results suggest distinct chemical evolution of the outer halo.

Abstract

Chemical abundances of 26 metal-poor dwarfs and giants are determined from high-resolution and high signal-to-noise ratio spectra obtained with Subaru/HDS. The sample is selected so that most of the objects have outer-halo kinematics. Self-consistent atmospheric parameters were determined by an iterative procedure based on spectroscopic analysis. Abundances of 13 elements, including $\alpha$-elements (Mg, Si, Ca, Ti), odd-Z light elements (Na, Sc), iron-peak elements (Cr, Mn, Fe, Ni, Zn) and neutron-capture elements (Y, Ba), are determined by two independent data reduction and LTE analysis procedures, confirming the consistency of the stellar parameters and abundances results. We find a decreasing trend of [$\alpha$/Fe] with increasing [Fe/H] for the range of $-3.5 <$ [Fe/H]$ < -1$, as found by Stephens and Boesgaard (2002). [Zn/Fe] values of most objects in our sample are slightly lower than the bulk of halo stars previously studied. These results are discussed as possible chemical properties of the outer halo in the Galaxy.