The Galactic Disk-Halo transition - Evidence from Stellar Abundances

TL;DR

This study investigates the elemental abundances of stars in the Galactic disk and halo, revealing two distinct halo star groups and exploring their origins through chemical and kinematic analysis, supported by cosmological simulations.

Contribution

It provides new observational evidence of two halo star populations and compares their chemical signatures with hierarchical galaxy formation models.

Findings

Halo stars split into two groups based on alpha-element ratios.

One group shares chemical properties with thick-disk stars.

The other group shows lower alpha-element enhancement at similar metallicities.

Abstract

New information on the relations between the Galactic disks, the halo, and satellite galaxies is being obtained from elemental abundances of stars having metallicities in the range -1.5 < [Fe/H] < -0.5. The first results for a sample of 26 halo stars and 13 thick-disk stars observed with the ESO VLT/UVES spectrograph are presented. The halo stars fall in two distinct groups: One group (9 stars) has [alpha/Fe]= 0.30 +-0.03 like the thick-disk stars. The other group (17 stars) shows a clearly deviating trend ranging from [alpha/Fe]= 0.20 at [Fe/H]= -1.3 to [alpha/Fe]= 0.08 at [Fe/H]= -0.8. The kinematics of the stars are discussed and the abundance ratios Na/Fe, Ni/Fe, Cu/Fe and Ba/Y are applied to see if the low-alpha stars are connected to the thin disk or to Milky Way satellite galaxies. Furthermore, we compare our data with simulations of chemical abundance distributions in hierarchically formed stellar halos in a LambdaCDM Universe.