[Paper Review] Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra

E. Carretta, A. Bragaglia+13 more|HAL (Le Centre pour la Communication Scientifique Directe)|Sep 16, 2009

Stellar, planetary, and galactic studiesPhysics and Astronomy60 references492 citations

TL;DR

This study presents a homogeneous analysis of Fe, Na, and O abundances in 1,409 red giant stars across 15 globular clusters using FLAMES/GIRAFFE spectra, revealing a bimodal Na-O anticorrelation that defines three stellar populations: primordial (P), intermediate (I), and extreme (E) second-generation stars. The key finding is that the slope of the Na-O anticorrelation varies with cluster metallicity and luminosity, indicating a correlation between the average mass of polluting stars and cluster mass, with E populations being more prominent in massive clusters.

ABSTRACT

We present abundances of Fe, Na, and O for 1409 red giant stars in 15 galactic globular clusters, derived from the homogeneous analysis of high resolution FLAMES/GIRAFFE spectra. Combining the present data with previous results, we obtained a total sample of 1958 stars in 19 clusters, the largest and most homogeneous database of this kind to date. Our GCs have [Fe/H] from -2.4 to -0.4, with a wide variety of global parameters (morphology of the horizontal branch, mass, concentration, etc). For all clusters we find the Na-O anticorrelation, the classical signature of proton-capture reactions in H-burning at high temperature in a previous generation of more massive stars, now extinct. Using quantitative criteria (from the morphology and extension of the Na-O anticorrelation), we can define 3 components of the stellar population in GCs: a primordial component (P) of first-generation stars, and 2 components of second-generation stars (intermediate I and extreme E populations from their different chemical composition). The P component is present in all GCs, and its fraction is almost constant at about one third. The I component represents the bulk of the cluster population. The E component is not present in all GCs, and it is more conspicuous in some (but not in all) of the most massive ones. We discuss the fractions and spatial distributions of these components in our sample and in two additional clusters (M3 and M13) from the literature. We also find that the slope of the anti-correlation (defined by the minimum O and maximum Na abundances) changes from cluster-to-cluster, a change that is represented well by a bilinear relation on cluster metallicity and luminosity. This second dependence suggests a correlation between average mass of polluters and cluster mass.

Motivation & Objective

• To determine the chemical composition of first- and second-generation stars in globular clusters using high-resolution spectroscopy.
• To resolve the long-standing puzzle of star-to-star abundance variations in light elements (Na, O, C, N) in cluster stars.
• To quantify the relative fractions of primordial (P), intermediate (I), and extreme (E) populations across a diverse sample of clusters.
• To investigate the dependence of the Na-O anticorrelation slope on cluster metallicity and luminosity (proxy for mass).
• To test nucleosynthesis models by comparing observed abundance patterns with predictions for polluting AGB stars.

Proposed method

• High-resolution FLAMES/GIRAFFE spectroscopy was used to measure Fe, Na, and O abundances in 1,409 red giant stars across 15 globular clusters.
• Abundances were derived using a homogeneous spectroscopic analysis pipeline, ensuring consistency across clusters.
• The Na-O anticorrelation was quantified using the minimum [O/Fe] and maximum [Na/Fe] values to define the anticorrelation slope.
• The slope was correlated with cluster metallicity [Fe/H] and absolute V-band magnitude (proxy for cluster mass) to infer polluter star mass.
• Dilution sequences were used to infer original (unpolluted) O and Na abundances, allowing comparison with nucleosynthesis models.
• Strömgren photometry was used to verify nitrogen abundance variations and their impact on color-magnitude diagrams.

Experimental results

Research questions

• RQ1 What is the relative fraction of primordial (P), intermediate (I), and extreme (E) stellar populations in globular clusters across a range of metallicities and masses?
• RQ2 How does the slope of the Na-O anticorrelation vary across different globular clusters, and what does this imply about the mass of polluting stars?
• RQ3 Is the extreme (E) population present in all massive clusters, or is its presence dependent on additional cluster parameters beyond mass?
• RQ4 To what extent do spatial distributions of P, I, and E populations differ, and what does this imply about cluster formation and evolution?
• RQ5 How well do observed Na-O anticorrelation patterns match nucleosynthesis predictions for massive AGB stars?

Key findings

• The primordial (P) population, defined by field-star-like abundances, is present in all 19 clusters studied and constitutes approximately one-third of the total stellar population.
• The intermediate (I) population forms the bulk of the cluster population, accounting for up to 60–70% of observed stars in many clusters.
• The extreme (E) population is not present in all clusters and is more prominent in the most massive clusters, though its presence is not guaranteed even in massive systems like 47 Tuc and M15.
• The slope of the Na-O anticorrelation correlates with both cluster metallicity and absolute V-band magnitude, indicating a systematic dependence on cluster mass.
• The observed anticorrelation slopes are in qualitative but not quantitative agreement with nucleosynthesis predictions for massive AGB stars, suggesting that the average mass of polluters is anticorrelated with total cluster mass.
• In NGC 6752, P stars are N-poor and lie on a tight blue sequence in the color-magnitude diagram, while I stars (N-rich) show a broader distribution due to mixed composition.