Stromgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index
Bibliographic record
Abstract
We present a new calibration of the Strömgren metallicity index m 1 using red giant (RG) stars in four globular clusters (GCs: M92, M13, NGC 1851, 47 Tuc) with metallicity ranging from -2.2 to -0.7, marginally affected by reddening [ E ( B - V ) ≤ 0.04] and with accurate ( u , v , b , y ) photometry. The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we have adopted the u - y and v - y colors instead of b - y . These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71, NGC 288, NGC 362, NGC 6397, and NGC 6752 is 0.04 ± 0.03 dex (σ = 0.11 dex). We also apply the new MIC relations to 85 field RGs with metallicity ranging from -2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is -0.14 ± 0.01 dex (σ = 0.17 dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semiempirical or theoretical color-temperature relations. We apply the semiempirical relations to the nine GCs and find that the difference between photometric and spectroscopic metallicities is 0.04 ± 0.03 dex (σ = 0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09 ± 0.03 dex (with σ = 0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11 ± 0.03 dex (σ = 0.14 dex) for the nine GCs and -0.24 ± 0.03 dex (σ = 0.15 dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2 dex.
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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 | 0.002 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.002 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.006 | 0.004 |
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".