[Paper Review] Heavy elements and chemical enrichment in globular clusters

G. James, P. François+4 more|arXiv (Cornell University)|Aug 18, 2004

Stellar, planetary, and galactic studies71 references61 citations

TL;DR

This study presents the first high-resolution abundance measurements of neutron-capture elements (Sr, Y, Ba, Eu) in unevolved main-sequence turnoff and subgiant stars in three globular clusters (NGC 6397, NGC 6752, 47 Tuc) at varying metallicities. Using VLT/UVES data, it finds no correlation between light-element anomalies and heavy-element abundances, rejecting the classical self-enrichment scenario and indicating that globular clusters were uniformly enriched by r- and s-processes, similar to field halo stars.

ABSTRACT

High resolution (R > 40 000) and high S/N spectra have been acquired with UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for several main sequence turnoff stars (V ~ 17 mag) and subgiants at the base of the Red Giant Branch (V ~ 16 mag) in three globular clusters (NGC 6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities (respectively [Fe/H] = -2.0; -1.5; -0.7$). Spectra for a sample of 25 field halo subdwarves have also been taken with equal resolution, but higher S/N. These data have been used to determine the abundances of several neutron-capture elements in these three clusters: strontium, yttrium, barium and europium. This is the first abundance determination of these heavy elements for such unevolved stars in these three globular clusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundance ratios, have been used to test the self-enrichment scenario. A comparison is done with field halo stars and other well known Galactic globular clusters in which heavy elements have already been measured in the past, at least in bright giants (V > 11-12 mag). Our results show clearly that globular clusters have been uniformly enriched by r- and s-process syntheses, and that most of them seem to follow exactly the same abundance patterns as field halo stars, which discards the ``classical'' self-enrichment scenario for the origin of metallicities and heavy elements in globular clusters.

Motivation & Objective

• To determine neutron-capture element abundances in unevolved stars (turnoff and subgiants) in globular clusters to test self-enrichment scenarios.
• To compare heavy-element abundance patterns in globular clusters with those in field halo stars across different metallicities.
• To investigate whether the observed light-element anomalies (O-Na, Mg-Al) correlate with s- and r-process element abundances in the same stars.
• To assess the validity of the classical self-enrichment model in explaining the chemical evolution of globular clusters.
• To evaluate whether globular clusters were pre-enriched or formed from already metal-enriched material, rather than self-polluting via internal processes.

Proposed method

• High-resolution (R > 40,000) and high signal-to-noise (S/N) spectroscopy using the UVES instrument on the VLT-Kueyen telescope.
• Observation of 25 field halo subdwarfs and 25 cluster stars (turnoff and subgiants) in three globular clusters with metallicities [Fe/H] ≈ −2.0, −1.5, and −0.7.
• Spectral analysis using equivalent width and synthesis techniques to derive abundances of Sr, Y, Ba, and Eu.
• Comparison of [Ba/Eu] and [Sr/Ba] abundance ratios across evolutionary phases and between clusters and field stars.
• Use of non-LTE corrections where applicable to improve accuracy of line profile fitting.
• Cross-calibration with field halo stars to assess consistency of abundance patterns across stellar populations.

Experimental results

Research questions

• RQ1 Do neutron-capture element abundances (Sr, Y, Ba, Eu) vary systematically between turnoff stars and subgiants in globular clusters?
• RQ2 Is there a correlation between light-element anomalies (e.g., O-Na anticorrelation) and the abundances of s- and r-process elements in unevolved cluster stars?
• RQ3 Do the [Ba/Eu] and [Sr/Ba] abundance ratios in globular clusters match those of field halo stars at similar metallicities?
• RQ4 Can the classical self-enrichment scenario—where AGB stars pollute the cluster—explain the observed homogeneous heavy-element abundances in unevolved stars?
• RQ5 Is the observed dispersion in [Ba/Fe] ratios between turnoff and subgiant stars real, or due to systematic errors?

Key findings

• No significant correlation or anticorrelation is observed between neutron-capture element abundances and light-element anomalies (e.g., O-Na) in unevolved stars of globular clusters.
• Abundances of s-process elements (Ba, Sr) and r-process elements (Eu) are homogeneous across different evolutionary phases (turnoff and subgiant) in the same cluster, indicating no phase-dependent enrichment.
• The [Ba/Eu] and [Sr/Ba] abundance ratios in globular clusters are consistent with those of field halo stars at the same metallicities, supporting a common chemical enrichment history.
• The observed abundance patterns of neutron-capture elements in globular clusters are best explained by a uniform, external enrichment process rather than internal self-pollution from AGB stars.
• The classical self-enrichment scenario—where AGB stars produce and pollute the cluster with s-process elements—cannot explain the data, as it would produce detectable correlations between light-element anomalies and s-process abundances, which are absent.
• The small dispersion in [Ba/Fe] between turnoff and subgiant stars, if real, contradicts expectations from dilution in growing convective zones, further weakening the self-pollution hypothesis.