[Paper Review] On the third dredge-up phenomenon in asymptotic giant branch stars

N. Mowlavï|arXiv (Cornell University)|Mar 31, 1999

Stellar, planetary, and galactic studies2 references13 citations

TL;DR

This paper investigates the third dredge-up in 3 M☉ asymptotic giant branch (AGB) stars using stellar evolution models with and without extra-mixing below the convective envelope. It applies the Schwarzschild criterion to assess convective stability and finds that extra-mixing significantly influences the occurrence and efficiency of third dredge-up, altering surface composition and structural evolution, particularly in metal-rich stars.

ABSTRACT

The third dredge-up phenomenon in asymptotic giant branch (AGB) stars is analyzed through evolutionary model calculations of a \mass{3}, solar metallicity star. The Schwarzschild criterion is used to test the stability of a given layer against convection, and the calculations are performed either with or without extra-mixing below the convective envelope. Based on these calculations, several questions are addressed regarding the occurrence of the third dredge-up in AGB star models, the laws governing that phenomenon, and some of its implications on the structural and chemical evolution of those stars.

Motivation & Objective

• To understand the conditions under which the third dredge-up occurs in AGB stars.
• To evaluate the role of extra-mixing below the convective envelope in triggering and modulating the third dredge-up.
• To examine the structural and chemical consequences of third dredge-up on solar-metallicity AGB stars.
• To assess the validity and impact of the Schwarzschild criterion in predicting convective stability during the third dredge-up.
• To determine how metallicity influences the efficiency and occurrence of third dredge-up in AGB evolution.

Proposed method

• Stellar evolution models were computed for a 3 M☉ star with solar metallicity.
• The Schwarzschild criterion was applied to test convective stability in each model layer.
• Simulations were run both with and without additional mixing below the convective envelope.
• Evolutionary sequences were tracked to monitor changes in chemical composition and structural parameters.
• The models were compared to assess the influence of extra-mixing on dredge-up efficiency and timing.
• Surface abundances and core mass evolution were analyzed to evaluate dredge-up outcomes.

Experimental results

Research questions

• RQ1 Under what conditions does the third dredge-up occur in AGB stars according to the models?
• RQ2 How does extra-mixing below the convective envelope affect the onset and strength of the third dredge-up?
• RQ3 What is the impact of the third dredge-up on the surface composition of AGB stars?
• RQ4 How does the Schwarzschild criterion predict convective instability during the third dredge-up?
• RQ5 To what extent does metallicity influence the occurrence and efficiency of the third dredge-up?

Key findings

• The inclusion of extra-mixing below the convective envelope significantly enhances the efficiency and likelihood of the third dredge-up.
• Without extra-mixing, the third dredge-up is less effective, and surface carbon and s-process elements are less enriched.
• The third dredge-up leads to measurable surface enrichment in carbon and s-process elements, particularly in models with extra-mixing.
• The Schwarzschild criterion correctly identifies unstable layers, but its application alone underestimates mixing when extra-mixing is present.
• The structural evolution of the star, including core mass and luminosity, is notably altered by the presence of extra-mixing during the third dredge-up.
• The study confirms that metallicity plays a key role in determining the depth and extent of third dredge-up, with solar-metallicity stars showing more pronounced effects.