[Paper Review] Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

R. Gratton, E. Carretta+1 more|arXiv (Cornell University)|Jan 31, 2012

Stellar, planetary, and galactic studiesPhysics and Astronomy375 references441 citations

TL;DR

This paper reviews evidence from Milky Way globular clusters showing they host multiple stellar populations—second-generation stars with anomalous light element abundances (C, N, O, Na, Al)—formed from pollution by first-generation stars. The key contribution is establishing multiple generations as a fundamental feature of globular cluster formation, reshaping models of cluster and galaxy evolution.

ABSTRACT

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.

Motivation & Objective

• To synthesize recent observational and theoretical progress on multiple populations in Milky Way globular clusters.
• To address the paradigm shift from single stellar populations to multiple generations in cluster formation.
• To explore the implications of multiple populations for the second parameter problem and the formation history of the Galactic halo.
• To identify open questions regarding polluters, nucleosynthesis mechanisms, and the role of hydrodynamics in cluster formation.
• To advocate for future observational strategies using high-precision spectroscopy and photometry to resolve remaining uncertainties.

Proposed method

• Analysis of high-resolution spectroscopic data to measure light element abundances (C, N, O, Na, Al, F, Mg) in stars across multiple globular clusters.
• Use of high-precision photometry to identify multiple main sequences and subgiant branches, particularly in clusters like NGC 6397.
• Comparison of spectroscopic and photometric signatures to confirm the link between chemical anomalies and multiple sequences.
• Application of theoretical models involving self-enrichment via intermediate-mass AGB stars or fast-rotating massive stars as polluters.
• Use of kinematic data from surveys (SEGUE, RAVE, Gaia, HERMES, LAMOST) to trace halo star streams and constrain cluster accretion histories.
• Integration of ALMA and integral field spectroscopy data to probe cooling flows and ISM conditions in massive star-forming regions.

Experimental results

Research questions

• RQ1 What causes the observed abundance anomalies (e.g., Na-O anti-correlation) in globular cluster stars, and what are the polluters responsible for them?
• RQ2 How do the timescales of star formation episodes in globular clusters relate to the nucleosynthesis mechanisms involved?
• RQ3 To what extent do multiple populations in globular clusters reflect the formation history of the Galactic halo and its accreted satellites?
• RQ4 Why do some clusters show multiple main sequences while others do not, and what does this imply about cluster mass and formation conditions?
• RQ5 Can the connection between globular clusters and massive star-forming regions in other galaxies be established through observational analogs?

Key findings

• Multiple populations in globular clusters are now well-established through both photometric (multiple main sequences) and spectroscopic (abundance anomalies) evidence.
• The second generation of stars in clusters is chemically distinct, with enhanced Na and Al and depleted C and O, indicating pollution by first-generation stars.
• The first generation of stars in clusters is largely composed of low-mass stars with halo-like composition, while the second generation dominates the current population.
• NGC 6397’s main sequence is resolved into two distinct sequences via high-precision photometry, confirming the presence of multiple populations.
• The formation of multiple populations likely involves self-enrichment via intermediate-mass AGB stars or fast-rotating massive stars, though the exact polluter remains uncertain.
• Future surveys like Gaia, HERMES, and LAMOST will provide kinematic and abundance data for over 10⁵ stars, enabling detailed studies of cluster dynamics and chemical evolution.