More pieces of the puzzle: Chemistry and substructures in the Galactic thick disk

TL;DR

This study investigates the chemical and dynamical properties of stars in the Milky Way's thick disk, revealing distinct populations with different metallicities, chemical abundances, and possible substructures related to past cluster disruption.

Contribution

It identifies a chemical and dynamical transition at [Fe/H] ~ -0.4, highlighting heterogeneity in the thick disk and evidence of substructure from disrupted star clusters.

Findings

Stars with [Fe/H] > -0.4 are alpha-enhanced and form a narrow sequence.

Stars with [Fe/H] < -0.4 show a wider range of eccentricities and chemical abundances.

Evidence suggests possible substructure linked to disrupted star clusters.

Abstract

We present a study of the chemical abundances of Solar neighbourhood stars associated to dynamical structures in the Milky Way's (thick) disk. These stars were identified as overdensity in the eccentricity range 0.3< ecc < 0.5 in the Copenhagen-Geneva Survey by Helmi et al. (2006). We find that the stars with these dynamical characteristics do not constitute a homogeneous population. A relatively sharp transition in dynamical and chemical properties appears to occur at a metallicity of [Fe/H] ~ -0.4. Stars with [Fe/H] > -0.4 have mostly lower eccentricities, smaller vertical velocity dispersions, are alpha-enhanced and define a rather narrow sequence in [alpha/Fe] vs [Fe/H], clearly distinct from that of the thin disk. Stars with [Fe/H] < -0.4 have a range of eccentricities, are hotter vertically, and depict a larger spread in [alpha/Fe]. We have also found tentative evidence of substructure possibly associated to the disruption of a metal-rich star cluster. The differences between these populations of stars is also present in e.g. [Zn/Fe], [Ni/Fe] and [SmII/Fe], suggesting a real physical distinction.