Through Thick and Thin: Kinematic and Chemical Components in the Solar Neighbourhood
Bibliographic record
Abstract
We search for chemically distinct stellar components in the solar neighbourhood using a compilation of published data. Extending earlier work, we show that when the abundances of Fe, α elements and the r-process element Eu are considered together, stars separate neatly into two groups that delineate the traditional thin and thick disc components of the Milky Way. The group akin to the thin disc is traced by stars with [Fe/H] > −0.7 and [α/Fe] < 0.2. The thick disc-like group overlaps the thin disc in [Fe/H] but has higher abundances of α elements and Eu. Stars in the range −1.5 < [Fe/H] < −0.7 with low [α/Fe] ratios, however, seem to belong to a separate, dynamically cold, non-rotating component that we associate with tidal debris. The kinematically hot stellar halo dominates the sample for [Fe/H] < −1.5. These results suggest that it may be possible to define the main dynamical components of the solar neighbourhood using only their chemistry, an approach with a number of interesting consequences. With such definition, the kinematics of thin disc stars is found to be independent of metallicity: their average rotation speed remains roughly constant in the range −0.7 < [Fe/H] < +0.4, a result that argues against radial migration having played a substantial role in the evolution of the thin disc. The velocity dispersion of the thin disc is also independent of [Fe/H], implying that the familiar increase in velocity dispersion with decreasing metallicity is the result of the increasing prevalence of the thick disc at lower metallicities, rather than of the sustained operation of a dynamical heating mechanism. The substantial overlap in [Fe/H] and, likely, stellar age, of the various components might affect other reported trends in the properties of stars in the solar neighbourhood. A purely chemical characterization of these components would enable us to scrutinize these trends critically in order to understand which result from accretion events and which result from secular changes in the properties of the Galaxy.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".