[Paper Review] Shedding Light on Lithium Evolution: The Globular Cluster Perspective

A. J. Korn|arXiv (Cornell University)|Jun 27, 2012

Stellar, planetary, and galactic studies12 citations

TL;DR

This paper reviews 20 years of lithium abundance measurements in metal-poor globular clusters, focusing on unevolved main-sequence, turnoff, and subgiant stars. It concludes that surface lithium evolution is primarily governed by atomic diffusion modulated by extra mixing, with temperature scale uncertainties limiting precise cosmological tests, while no significant deviation from field star lithium abundances is found in clusters.

ABSTRACT

I shall review what has been learnt during 20 years of lithium observations in stars belonging to metal-poor globular clusters. The focus will be on little evolved main-sequence, turnoff-point (TOP) and subgiant-branch (SGB) stars expected to display Spite-plateau lithium abundances like those found in the majority of field stars of similar metallicities. But is the Spite plateau of globular clusters the same as those of field stars? What effect does, e.g., cluster-internal pollution have on lithium abundances in the now dominant second generation of stars? It will be shown that it is primarily our incomplete knowledge of the temperature scale of Population II stars which currently limits the diagnostic power of globular clusters as regards the stellar-surface evolution of lithium.

Motivation & Objective

• To assess whether lithium abundances in globular cluster stars match those of field stars at similar metallicities.
• To investigate the impact of internal cluster pollution (e.g., second-generation stars) on lithium surface abundances.
• To evaluate the role of atomic diffusion and extra mixing in shaping lithium evolution in low-mass Population II stars.
• To identify the main sources of uncertainty in lithium abundance determinations, particularly effective temperature scale errors.
• To determine whether globular clusters can serve as reliable probes for testing Big Bang nucleosynthesis predictions via lithium.

Proposed method

• Analysis of high-resolution spectroscopic data from ESO VLT instruments (UVES, FLAMES-UVES, FLAMES-GIRAFFE) on stars in NGC 6397, NGC 6752, M 4, and M 92.
• Use of non-LTE corrections for lithium lines, particularly adopting Lind et al. (2009b) corrections for 1D models.
• Comparison of lithium abundances across multiple studies with varying temperature scales to isolate systematic effects.
• Application of hydrodynamic (HD) and magneto-hydrodynamic (MHD) models to assess effects on line formation and abundance inference.
• Use of differential spectroscopy techniques to minimize systematic errors, though fully differential analyses remain rare.
• Evaluation of the impact of temperature scale uncertainties on inferred extra-mixing efficiency and lithium depletion.

Experimental results

Research questions

• RQ1 Is the Spite plateau in globular clusters identical to that observed in field stars of similar metallicity?
• RQ2 To what extent does internal cluster pollution in second-generation stars affect lithium surface abundances?
• RQ3 How do uncertainties in the effective temperature scale of Population II stars affect the interpretation of lithium abundances?
• RQ4 Can atomic diffusion and extra mixing fully account for the observed lithium depletion in unevolved cluster stars?
• RQ5 To what extent can globular clusters serve as probes for testing Big Bang nucleosynthesis predictions via lithium?

Key findings

• No significant difference in lithium abundance is found between globular cluster stars and field stars of similar metallicity, with differences not exceeding 0.05 dex.
• The observed lithium abundance in unevolved cluster stars is consistent with predictions from standard stellar evolution when corrected for atomic diffusion and extra mixing.
• Temperature scale uncertainties exceeding 100 K can introduce errors of up to 0.07 dex in lithium abundance, with indirect effects on inferred mixing efficiency reaching 0.4 dex.
• Atomic diffusion corrected for extra mixing (e.g., T6.25 model) can fully reconcile observed lithium with WMAP-calibrated Big Bang nucleosynthesis predictions.
• MHD modelling may reduce the need for high effective temperatures, potentially alleviating current systematic uncertainties.
• There is no convincing evidence for systematic lithium overabundance or underabundance in globular clusters, and the observed dispersion in lithium is not robustly established.