How Different Star-Sorting Methods Change Our View of the Milky Way’s Discs

The Milky Way’s thin and thick discs are well-known features of our Galaxy, but the borders between them are blurry. The paper begins by explaining that the two discs differ in their chemical abundances, motions, and spatial distributions, but their properties overlap, especially for stars with higher metallicity. Because of this, astronomers have developed many different ways to decide whether a star belongs to the thin or thick disc. Alinder and collaborators aim to test how these classification choices influence the conclusions scientists draw about the discs’ structure.

Methods for Selecting Thin and Thick Disc Stars

To explore this question, the authors apply five selection methods: two based on chemical abundance diagrams, one based on stellar ages, one using stellar velocities (“kinematic selection”), and one using orbital properties within a Galactic potential (“dynamical selection”). Because each method depends on different aspects of a star’s behavior or history, they can pick out different groups of stars. For example, chemical selections look for distinct sequences in diagrams like the [α/Fe]–[Fe/H] plane, while age-based methods assume thick-disc stars formed earlier in the Galaxy’s history.

Data and Modeling Approach

The team uses high-quality red-giant data from APOGEE DR17, which provides chemical abundances and covers a large volume of the Milky Way. They fit each star group with a simple model describing how stellar density changes with both Galactocentric radius and height above the Galactic plane, restricting the sample to stars between 4 and 14 kpc from the Galactic center to ensure the model remains valid. This allows them to compare the structural parameters, such as scale heights and scale lengths, derived from each selection method.

Key Results

The results show clear differences depending on which method is used. Chemical and age-based methods produce the cleanest separation between the discs, while kinematic and dynamical methods show more “contamination,” meaning stars from the two discs mix more easily when selected based on motions alone. Across all methods, the thin disc’s scale height increases with radius, demonstrating flaring, whereas the thick disc remains roughly constant at about 1 kpc. All methods also find that the thin disc has a longer scale length than the thick disc, with chemical selections giving the strongest difference, up to ~3.0 kpc for the thin disc compared to ~2.0 kpc for the thick disc.

Discussion and Interpretation

In the discussion, the authors emphasize that the choice of selection method strongly shapes the picture of the Galaxy that emerges. Chemical selections offer clarity but require spectroscopy, while kinematic and dynamical approaches can be applied to much larger datasets but blur the boundaries between the disc components. This means that when astronomers talk about “thin” and “thick” discs, they may actually be referring to different populations depending on the selection method used.

Conclusions

Finally, the paper concludes by highlighting that although the specifics vary, all methods agree on some key points: the thin disc flares, the thick disc is roughly constant in height, and the thin disc extends farther in radius. These shared patterns suggest that, despite methodological differences, a consistent broad picture of the Milky Way’s structure continues to emerge.

Source: Alinder