DEEP 2MASS PHOTOMETRY OF M67 AND CALIBRATION OF THE MAIN-SEQUENCE<i>J</i>–<i>K<sub>S</sub></i>COLOR DIFFERENCE AS AN AGE INDICATOR
Bibliographic record
Abstract
We present an analysis of Two Micron All Sky Survey (2MASS) calibration photometry of the old open cluster M67 (NGC 2682). The proper-motion-cleaned color–magnitude diagram (CMD) resulting from these data extends ∼3 mag deeper than one based on data from the point source catalog. The CMD extends from above the helium-burning red clump to a faint limit that is more than 7 mag below the main-sequence (MS) turnoff (TO) in the K S band. After adopting a reddening of E ( B − V ) = 0.041 ± 0.004 and a metal abundance of [Fe/H] = −0.009 ± 0.009 based on a survey of published values, we fit the unevolved MS of M67 to field MS stars with 2MASS photometry and Hipparcos parallaxes. This analysis yields distance moduli of = 9.72 ± 0.05 and ( m − M ) 0 = 9.70 ± 0.05, which are consistent with published values. We compare the theoretical isochrones of Girardi et al. and Dotter et al. to the CMD of M67 and comment on the relative merits of each set of models. These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of the M67 data makes them ideal for the calibration of a new age indicator that has recently been devised by Calamida et al.—the difference in ( J − K S ) color between the MS TO and the point on the lower MS where it turns down (TD) and becomes nearly vertical [Δ( J − K S ) TO TD ]. Coupled with deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC 6791, we calibrate Δ( J − K S ) in terms of age and find Δ( J − K S ) TO TD = (3.017 ± 0.347) – (0.259 ± 0.037) × log age (yr).
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".