[Paper Review] The first stages of the evolution of Globular Clusters

F. D’Antona, P. Ventura+1 more|arXiv (Cornell University)|Dec 21, 2006

Stellar, planetary, and galactic studies3 references3 citations

TL;DR

This paper reviews the asymptotic giant branch (AGB) self-enrichment model for globular cluster (GC) chemical anomalies, proposing that massive AGB stars in the first ~100 Myr enrich the cluster with CNO-processed material, leading to second-generation stars with anomalous abundances. Key evidence comes from horizontal branch morphology, particularly in NGC 6441 and NGC 2808, where helium-rich populations (Y > 0.35) explain extended red clumps and long-period RR Lyrae variables.

ABSTRACT

The majority of the inhomogeneities in the chemical composition of Globular Cluster (GC) stars appear due to primordial enrichment. The most studied model today claims that the ejecta of Asymptotic Giant Branch (AGB) stars of high mass -those evolving during the first ~100Myr of the Clusters life- directly form a second generation of stars with abundance anomalies. In this talk, we review the status of the art with regard to this model, whose major problems are i) the modelling of the chemical anomalies is still not fully complete, and ii) it requires an IMF peculiarly enhanced in the intermediate mass stars. The model predicts enhanced helium abundance in the stars showing chemical anomalies, and the helium abundance distribution can be roughly derived from the morphology of the horizontal branch. Such distribution may possibly help to falsify the model for the first phases of evolution of GCs. As an illustration, we compare the results of the analysis of the HB morphology of some clusters.

Motivation & Objective

• To evaluate the AGB self-enrichment model as the primary mechanism for chemical anomalies in globular cluster stars.
• To assess whether the observed helium abundance distribution in horizontal branch stars can falsify or support the AGB model.
• To reconcile discrepancies in helium-rich star fractions with observed CN abundance patterns in clusters like 47 Tuc.
• To explore the implications of variable initial mass functions and primordial cluster mass for the survival of first-generation stars.

Proposed method

• Synthetic horizontal branch (HB) modeling using helium-rich stellar populations to match observed HB morphologies in metal-rich GCs.
• Comparison of observed HB luminosity functions (e.g., NGC 6441, NGC 6388, 47 Tuc) with simulated distributions to infer helium abundance distributions.
• Use of helium history analysis based on photometric data and synthetic models to derive the fraction of stars with Y > 0.25, Y > 0.33, and Y > 0.35.
• Analysis of abundance anomalies (O/Fe, CNO, Na) in stars across different evolutionary phases (TO, subgiants) to constrain nucleosynthesis sources.
• Incorporation of hot bottom burning (HBB) and third dredge-up effects in AGB star models to predict envelope compositions.
• Evaluation of alternative models, including fast-rotating massive stars and mixed-cloud scenarios, against observational constraints.

Experimental results

Research questions

• RQ1 Can the observed chemical anomalies in globular cluster stars be explained by self-enrichment from massive AGB stars?
• RQ2 What is the required initial mass function (IMF) to produce sufficient AGB ejecta for second-generation star formation?
• RQ3 Does the helium abundance distribution inferred from horizontal branch morphology support the AGB self-enrichment model?
• RQ4 Why do some clusters like NGC 6441 and NGC 2808 show a significant fraction of stars with Y > 0.35, and how does this relate to observed HB morphology?
• RQ5 Can the observed fraction of CN-strong and CN-weak stars in 47 Tuc be reconciled with the helium history derived from HB modeling?

Key findings

• The AGB self-enrichment model predicts enhanced helium abundance in chemically anomalous stars, with helium-rich populations (Y > 0.35) reaching ~14% in NGC 6441 and ~20% in NGC 6388.
• The extended red clump and long-period RR Lyrae variables in NGC 6441 are best explained by a helium discontinuity from Y = 0.25 to Y ≈ 0.27, consistent with AGB enrichment.
• The observed HB morphology of NGC 6441 shows a significant excess of luminous red clump stars at high magnitudes, indicating a population of high-luminosity, helium-rich stars.
• The fraction of stars with Y > 0.35 in NGC 6441 (14%) is comparable to the fraction of stars with Y ≈ 0.4 in NGC 2808 (15%), suggesting a common origin for extreme helium enrichment.
• The discrepancy in 47 Tuc—where only 25% of stars have Y > 0.25 but CN-strong and CN-weak stars are roughly equal—suggests a need for a higher initial helium content or alternative mechanisms.
• The model implies that primordial globular clusters were likely much more massive than today’s clusters, with significant loss of first-generation stars, resolving IMF anomalies.