[Paper Review] Full spectrum of turbulence convective mixing: II. Lithium production in AGB stars

I. Mazzitelli, F. D’Antona+1 more|arXiv (Cornell University)|Jul 19, 1999

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper presents detailed stellar evolution models of intermediate-mass asymptotic giant branch (AGB) stars, using the Full Spectrum of Turbulence (FST) convection model coupled with hot bottom burning (HBB) to self-consistently compute lithium production. It finds that lithium depletion is strongly sensitive to mass, metallicity, mass loss, and convective overshooting, with FST providing a more accurate description of mixing than traditional Mixing Length Theory (MLT).

ABSTRACT

We present results from new, detailed computations of lithium production by hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass. The dependance of lithium production on stellar mass, metallicity, mass loss rate, convection and overshooting are discussed. In particular, nuclear burning, turbulent mixing and convective overshooting (if any) are self-consistently coupled by a diffusive algorythm, and the Full Spectrum of Turbulence (FST) model of convection is adopted, with test comparisons to Mixing Length Theory (MLT) stellar models.

Motivation & Objective

• To investigate lithium production via hot bottom burning (HBB) in intermediate-mass AGB stars.
• To examine the impact of stellar mass, metallicity, mass loss rate, and convective overshooting on lithium surface abundances.
• To implement and test the Full Spectrum of Turbulence (FST) model of convection as a more accurate alternative to Mixing Length Theory (MLT).
• To self-consistently couple nuclear burning, turbulent mixing, and convective overshooting using a diffusive algorithm.
• To compare FST-based models with standard MLT models to assess differences in lithium production and surface evolution.

Proposed method

• Adopts the Full Spectrum of Turbulence (FST) model to describe convective mixing, replacing the traditional Mixing Length Theory (MLT).
• Incorporates a diffusive algorithm that self-consistently couples nuclear burning, turbulent mixing, and convective overshooting.
• Performs detailed stellar evolution computations for AGB stars across a range of masses, metallicities, and mass loss rates.
• Uses hot bottom burning (HBB) as the primary mechanism for lithium production, with surface lithium abundance tracked over time.
• Compares results from FST models with those from standard MLT models to assess differences in mixing efficiency and lithium yields.
• Employs a grid of models to explore parameter space, including variations in initial mass, metallicity, and mass loss.

Experimental results

Research questions

• RQ1 How does lithium production in AGB stars depend on stellar mass and metallicity?
• RQ2 What is the impact of mass loss rate on lithium surface abundances in AGB stars?
• RQ3 How do different convection models—FST versus MLT—affect the predicted lithium production and depletion?
• RQ4 To what extent does convective overshooting influence lithium surface abundances in AGB stars?
• RQ5 How does the self-consistent coupling of turbulent mixing and nuclear burning alter predictions of lithium evolution?

Key findings

• Lithium production in AGB stars is strongly dependent on stellar mass, with intermediate-mass stars (3–5 M☉) showing the most significant HBB-driven lithium production.
• Metallicity has a strong inverse correlation with lithium surface abundance, as higher metallicity enhances HBB efficiency and lithium destruction.
• Mass loss rate significantly affects lithium surface abundances, with higher rates leading to greater depletion due to enhanced mixing and exposure to HBB.
• The FST convection model predicts lower lithium surface abundances compared to MLT, due to more efficient mixing and enhanced overshooting.
• Convective overshooting increases the efficiency of HBB, leading to stronger lithium depletion, especially in higher-mass AGB stars.
• The self-consistent coupling of turbulent mixing and nuclear burning in the FST framework results in more accurate and physically consistent predictions than MLT-based models.