Magnetic geometry and surface differential rotation of the bright Am star Alhena A
Bibliographic record
Abstract
ABSTRACT Alhena A (γ Gem A) is a bright Am star, with the strongest disc-integrated magnetic field strength reported so far for an Am star. Its spectrum exhibits standard circularly polarized Zeeman signatures, contrary to all previously studied Am stars that display abnormal signatures dominated by a single-signed lobe. We present here the result of follow-up observations of Alhena, using very high signal-to-noise spectropolarimetric data obtained over 25 observing nights with NARVAL at Télescope Bernard Lyot, in the frame of the BRITE (BRIght Target Explorer) spectropolarimetric survey. We confirm that Alhena A is magnetic and we determine its surface magnetic properties using different methods. Inclined dipole models are used to reproduce the longitudinal field measurements, as well as the Stokes V line profiles themselves. In both cases, the model is consistent with a polar field strength of ∼30 G. This is confirmed by a Zeeman-Doppler Imaging (ZDI) model, which also unveils smaller scale magnetic structures. A rotational period of 8.975 d was identified using intensity line profile variations. The ZDI inversion suggests that the surface magnetic field is sheared by differential rotation, with a difference in rotation rate between high and low latitudes at about 15 per cent of the solar value. This result challenges theories of the development of surface differential rotation in intermediate mass main-sequence stars.
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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 | 0.000 |
| 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 | 0.000 |
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".