Impact of selection criteria on the structural parameters of the Galactic thin and thick discs

TL;DR

This study examines how different star classification methods affect the derived structural parameters of the Milky Way's thin and thick discs, revealing that abundance and age-based methods yield cleaner separations and consistent structural trends.

Contribution

It systematically compares five classification methods to assess their impact on the inferred structural properties of Galactic discs, highlighting the advantages of chemical and age-based selections.

Findings

Abundance and age-based methods produce cleaner star separations.

Thin disc shows flaring with increasing radius.

Thick disc remains approximately constant at around 1 kpc height.

Abstract

Context: The Milky Way contains a thick and a thin disc that differ in chemical, kinematic, structural, and spatial properties. There is significant overlap in the distributions of these properties, especially so at higher metallicities. Distinguishing between these major structural components is crucial for understanding the formation and evolution of the Galaxy. Multiple selection methods exist to classify stars as thin or thick disc stars, each with its own advantages and limitations. Aims: We investigate how different classification methods for categorising stars into the thick and thin disc populations influence the determination of structural properties of the two discs. Methods: We apply five different selection methods. Two methods use cuts in the [$\alpha$/Fe]-[Fe/H] and [Mg/Mn]-[Al/Fe] planes; one uses a dynamical separation in $J_\phi$ -$J_Z$ space; one uses an age-based cut; and the last one uses a kinematic likelihood method. For each method, we derive relative density profiles of each component as functions of height above the Galactic plane and Galactocentric radius, and fit these to a simple two-exponential disc model. We use red giant stars from APOGEE DR17 and stellar ages from astroNN. Results: Methods based on abundance or age data produce the cleanest separations, while kinematic and dynamical methods suffer higher contamination due to difficulties in separating well-mixed populations. The thin disc scale heights show a clear flaring as they increase with radius, while the thick disc stays approximately constant at around 1 kpc over most radii for all methods. All methods find the thin disc to have a longer scale length than the thick disc, with the difference being greatest for the chemical selection methods. A scale length of the thick disc of 2.0 kpc leads to one of between 2.3 and 3.0 kpc for the thin disc.