[Paper Review] Full computation of massive AGB evolution. II. The role of mass loss and cross-sections

P. Ventura, F. D’Antona|ArXiv.org|May 11, 2005

Stellar, planetary, and galactic studies34 references71 citations

TL;DR

This study investigates the impact of nuclear reaction cross-sections (NACRE vs. CF88) and mass loss rates on the nucleosynthesis and evolution of massive AGB stars (3–6.5 M⊙, Z=0.001). Using full 30-element network calculations with the FST convection model, it finds that C+N+O yields remain nearly constant (~within a factor of two), while sodium and magnesium isotopes show strong sensitivity to cross-sections and mass loss, limiting the predictive power of AGB models for globular cluster self-enrichment scenarios.

ABSTRACT

In the course of a systematic exploration of the uncertainties associated to the input micro- and macro-physics in the modeling of the evolution of intermediate mass stars during their Asymptotic Giant Branch (AGB) phase, we focus on the role of the nuclear reactions rates and mass loss. We consider masses 3

Motivation & Objective

• To assess the sensitivity of massive AGB star evolution and nucleosynthesis to nuclear reaction cross-sections (NACRE vs. CF88) and mass loss rates.
• To evaluate the predictive power of AGB models for explaining chemical anomalies (e.g., O-Na, Mg-Al anticorrelations) in globular cluster stars.
• To determine whether massive AGB stars can be the primary source of self-enrichment in globular clusters, given current uncertainties in micro- and macro-physics.
• To examine how convection efficiency (via FST model) and mass loss influence HBB activation and elemental yields.
• To compare model predictions with observed abundance patterns in globular cluster giants and turn-off stars.

Proposed method

• Stellar evolution computed using the ATON2.1 code with a full 30-element nuclear network and time-dependent diffusion equations for convective mixing.
• Convection treated via the FST (Fickian-Style Turbulent) model to accurately capture high-temperature mixing at the base of the convective envelope during quiescent CNO burning.
• Nuclear reaction rates compared between the NACRE compilation (2000) and the older CF88 (1988) values, particularly for 22Ne(p,γ)23Na and 24Mg(p,γ)25Mg reactions.
• Mass loss rates varied systematically (including higher-than-standard values) to assess their impact on HBB efficiency, envelope cooling, and chemical yields.
• Chemical yields calculated as a function of mass and evolutionary phase, with emphasis on C+N+O, 23Na, and 24,25,26Mg isotopic ratios.
• Physical conditions (temperature, luminosity, mass) monitored throughout the AGB phase to link structural evolution to nucleosynthetic outcomes.

Experimental results

Research questions

• RQ1 How do different nuclear cross-sections (NACRE vs. CF88) affect the production and destruction of sodium and magnesium isotopes in massive AGB stars?
• RQ2 To what extent does the mass loss rate influence the efficiency of hot bottom burning and the resulting surface abundances of key elements like oxygen, sodium, and magnesium?
• RQ3 Are the predicted C+N+O yields in AGB ejecta consistent across different cross-section sets and mass loss prescriptions?
• RQ4 Can the model predictions for sodium and magnesium isotopic ratios reconcile with observed abundances in globular cluster stars?
• RQ5 What is the role of convection efficiency (via FST model) in enabling high-temperature HBB and its implications for nucleosynthesis in massive AGB stars?

Key findings

• The total C+N+O abundance in the ejecta remains nearly constant across all models, varying by less than a factor of two, contrary to previous studies.
• With NACRE cross-sections, 23Na is produced in stars with M < 4.5 M⊙ due to efficient 22Ne(p,γ)23Na reactions, while it is destroyed in more massive stars via the Ne-Na cycle.
• Under CF88 cross-sections, 23Na is systematically destroyed due to the extremely low 22Ne(p,γ)23Na reaction rate, leading to negligible sodium production.
• For M > 5 M⊙, 24Mg burning leads to 25Mg/24Mg and 26Mg/24Mg ratios approaching ∼10, indicating extreme magnesium isotopic enrichment.
• Higher mass loss rates reduce envelope mass and shorten the AGB lifetime, suppressing full HBB development and resulting in sodium-rich, magnesium-isotope-poor ejecta.
• For M ≤ 4.5 M⊙, ejecta are sodium-rich and exhibit low 25Mg/24Mg and 26Mg/24Mg ratios regardless of mass loss rate, due to insufficient base convective zone temperatures to trigger strong 24Mg burning.