[Paper Review] Constraining the original composition of the gas forming first-generation stars in globular clusters

M. V. Legnardi, A. P. Milone+14 more|arXiv (Cornell University)|Jan 1, 2022

Stellar, planetary, and galactic studies65 references46 citations

TL;DR

This study uses high-precision Hubble Space Telescope photometry of NGC 6362 and NGC 6838 to analyze the chromosome map (ChM) of first-generation (1G) stars, demonstrating that metallicity variations—not helium or binarity—are responsible for the extended 1G sequence. The authors find a metallicity spread of 0.05–0.30 dex in 1G stars across 55 globular clusters, mildly correlated with cluster mass, challenging accretion-based formation models that predict iron homogeneity between 1G and 2G stars.

ABSTRACT

Disentangling distinct stellar populations along the red-giant branches (RGBs) of globular clusters (GCs) is possible by using the pseudo-two-colour diagram dubbed chromosome map (ChM). One of the most intriguing findings is that the so-called first-generation (1G) stars, characterized by the same chemical composition of their natal cloud, exhibit extended sequences in the ChM. Unresolved binaries and internal variations in helium or metallicity have been suggested to explain this phenomenon. Here, we derive high-precision Hubble Space Telescope photometry of the GCs NGC 6362 and NGC 6838 and build their ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit wider ChM sequences than those of second-generation (2G). The evidence of this feature even among unevolved 1G MS stars indicates that chemical inhomogeneities are imprinted in the original gas. We introduce a pseudo-two-magnitude diagram to distinguish between helium and metallicity, and demonstrate that star-to-star metallicity variations are responsible for the extended 1G sequence. Conversely, binaries provide a minor contribution to the phenomenon. We estimate that the metallicity variations within 1G stars of 55 GCs range from less than [Fe/H]∼0.05 to ∼0.30 and mildly correlate with cluster mass. We exploit these findings to constrain the formation scenarios of multiple populations showing that they are qualitatively consistent with the occurrence of multiple generations. In contrast, the fact that 2G stars have more homogeneous iron content than the 1G challenges the scenarios based on accretion of material processed in massive 1G stars on to existing protostars.

Motivation & Objective

• To determine the origin of the extended sequence of first-generation (1G) stars in the chromosome map (ChM) of globular clusters.
• To distinguish between competing explanations—helium inhomogeneities, binary stars, and metallicity variations—for the observed ChM spread in 1G stars.
• To assess the implications of 1G metallicity variations for multiple population formation scenarios in globular clusters.
• To compare the internal metallicity spreads of 1G and second-generation (2G) stars and evaluate consistency with accretion-based models.

Proposed method

• High-precision Hubble Space Telescope (HST) photometry was obtained for the globular clusters NGC 6362 and NGC 6838.
• The chromosome map (ChM), a pseudo two-color diagram using F275W, F336W, F438W, and F814W filters, was constructed to analyze stellar populations.
• A new pseudo two-magnitude diagram was introduced to disentangle the effects of helium and metallicity on ChM sequences.
• Statistical analysis of ChM sequences in both red-giant branch (RGB) and main-sequence (MS) stars was performed to isolate the dominant physical cause of the spread.
• The metallicity spread in 1G stars was measured across 55 globular clusters and correlated with cluster mass.
• The results were compared with theoretical models of multiple population formation, particularly those involving accretion of processed material from massive 1G stars.

Experimental results

Research questions

• RQ1 What physical mechanism is responsible for the extended sequence of first-generation (1G) stars in the chromosome map (ChM) of globular clusters?
• RQ2 To what extent do variations in iron abundance ([Fe/H]) contribute to the ChM spread in 1G stars, compared to helium inhomogeneities or binarity?
• RQ3 How do the internal metallicity spreads of 1G and second-generation (2G) stars compare, and what does this imply for accretion-based formation models?
• RQ4 Is there a correlation between the metallicity spread in 1G stars and the mass of the host globular cluster?
• RQ5 Can the observed ChM morphology be reconciled with multi-generation formation scenarios involving self-enrichment or external accretion?

Key findings

• The extended sequence of first-generation (1G) stars in the chromosome map (ChM) is primarily caused by star-to-star variations in metallicity, not helium abundance or binarity.
• The metallicity spread among 1G stars in 55 globular clusters ranges from less than [Fe/H] ∼0.05 to ∼0.30 dex, with a mild positive correlation to cluster mass.
• The metallicity spread is observed not only in evolved red-giant branch (RGB) stars but also in unevolved main-sequence (MS) 1G stars, indicating that chemical inhomogeneities were imprinted in the original gas.
• Second-generation (2G) stars exhibit significantly smaller internal metallicity variations than 1G stars, challenging accretion-based models that predict iron homogeneity between 1G and 2G stars.
• The observed metallicity spread in 1G stars is consistent with self-enrichment from supernova ejecta or incomplete mixing in the interstellar medium, but not with pure helium variations.
• The results support multi-generation formation scenarios in which 1G stars form from chemically inhomogeneous gas, while 2G stars form later in denser regions where chemical inhomogeneities were smoothed out.