C and N Abundances in Stars at the Base of the Red Giant Branch in M15
Notice bibliographique
Résumé
We present an analysis of a large sample of moderate-resolution Keck Low Resolution Imaging Spectrometer spectra of subgiants and stars at the base of the red giant branch (RGB) in the Galactic globular cluster (GC) M15 (NGC 7078), most within the range 16.5 < V < 19.5 (1.2 < M_V < 4.2), with the goal of deriving C abundances (from the G band of CH) and N abundances (from the NH band at 3360 Å). Star-to-star stochastic variations with significant range in both [C/Fe] and [N/Fe] are found at all luminosities extending to the subgiants at M_V ~ +3. The C and N abundances appear anticorrelated, as would be expected from the CN-cycle processing of stellar material. Yet these M15 stars are considerably fainter than the RGB bump, the point at which deep mixing is believed to set in. On this basis, while the observed abundance pattern is consistent with proton-capture nucleosynthesis, we infer that the site of the reactions is likely not within the present sample. The range of variation of the N abundances is very large, and the sum of C + N increases as C decreases. To reproduce this requires the incorporation not only of CN but also of ON-processed material. Combining our work with that of Trefzger and coworkers for the brighter giants in M15, we find strong evidence for additional depletion of C among the most luminous giants. This presumably represents the first dredge-up (with enhanced deep mixing) expected for such luminous cluster RGB stars in the course of normal stellar evolution as they cross the RGB bump. \nWe compare the behavior of these patterns for C and N in GCs covering a wide range of metallicity and current mass. While all clusters studied show strong anticorrelated variations of C and N at all luminosities probed, the metal-rich clusters (M71, 47 Tuc, and M5) do not show evidence for the first dredge-up among their most luminous giants, while the metal-poor ones (M13, M92, and M15, plus M5) do. Conversely, the metal-poor clusters do not show evidence for the bimodality in CH and CN line strengths seen in the metal-rich clusters. The collected data on C and N abundances in low-luminosity GC stars cannot be explained by the commonly invoked models for the chemical evolution of GC stars; in particular, "pollution" of existing low-mass stars by ejecta from intermediate-mass asymptotic giant branch (AGB) stars can be ruled out. Pollution of cluster gas by such stars prior to the formation of the lower mass stars we observe today can also be ruled out unless current models of nucleosynthesis and dredge-up into the surface layers of AGB stars are flawed; such models agree qualitatively but disagree quantitatively with our data. We are forced to assume that there was an extended period of star formation in GCs, and that a previous generation of more massive stars evolved, ejected mass, and polluted the GC gas with light elements; the low-mass stars we see today formed afterward. A tentative scenario is developed involving an initial phase of star formation heavily biased toward high-mass stars, with subsequent formation of intermediate-mass, then low-mass stars.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».