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

Context . The dual nature of the Milky Way disc is well established. It contains a thick and a thin disc that differ in chemical, kinematic, structural, and spatial properties. Simultaneously, there is significant overlap in the distributions of these properties, especially at higher metallicities. Accurately distinguishing between these major structural components is crucial for understanding the formation and evolution of the Milky Way. Aims . We aim to investigate how the various classification methods for categorising stars into the thick and thin disc populations influence the determination of the structural properties of the two discs. Methods . We applied five different selection methods to define samples of stars that are likely members of either the thick or the thin disc. Two methods use cuts in the [ α /Fe]-[Fe/H] and [Mg/Mn]-[Al/Fe] planes; one method uses a dynamical separation in J φ - J Z space; one uses an age-based cut; and the last method uses a kinematic likelihood method. For each method, we derived the relative density profiles of each disc component as functions of height above the Galactic plane and Galactocentric radius, and fitted these to a simple two-exponential disc model. For our analysis, we used 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 thick disc scale length of 2.0 kpc yields a thin disc scale length of between 2.3 and 3.0 kpc. Conclusions . The method chosen to distinguish between Galactic components can significantly impact the view of the Galaxy. Abundance-based approaches offer a clear separation but require spectroscopy, which is not as widely available, while kinematic and dynamical methods work on larger samples but suffer from less clear separations. Every method recovers a longer scale length for the thin disc than the thick disc, unlike the geometric method, used with luminosity profiles in external galaxies.