[Paper Review] Thermohaline instability and rotation-induced mixing. I - Low- and intermediate-mass solar metallicity stars up to the end of the AGB

C. Charbonnel, N. Lagarde|arXiv (Cornell University)|Jun 28, 2010

Stellar, planetary, and galactic studies90 references139 citations

TL;DR

This study investigates thermohaline mixing driven by 3He burning and rotation-induced mixing in low- and intermediate-mass (1–4 M⊙) solar-metallicity stars, showing that thermohaline mixing explains observed Li, C/N, and 12C/13C trends in evolved giants beyond the RGB bump, while rotation enhances thermohaline onset and accounts for star-to-star abundance variations, particularly in CN-processed material in intermediate-mass stars.

ABSTRACT

(abridged) Numerous spectroscopic observations provide compelling evidence for non-canonical processes that modify the surface abundances of low- and intermediate-mass stars beyond the predictions of standard stellar theory. We study the effects of thermohaline instability and rotation-induced mixing in the 1-4 Msun range at solar metallicity. We present evolutionary models by considering both thermohaline and rotation-induced mixing in stellar interior. We discuss the effects of these processes on the chemical properties of stars from the zero age main sequence up to the end of the second dredge-up on the early-AGB for intermediate-mass stars and up to the AGB tip for low-mass stars. Model predictions are compared to observational data for lithium,12C/13C,[N/C],[Na/Fe],16O/17O, and 16O/18O in Galactic open clusters and in field stars with well-defined evolutionary status,as well as in planetary nebulae. Thermohaline mixing simultaneously accounts for the observed behaviour of 12C/13C,[N/C], and lithium in low-mass stars that are more luminous than the RGB bump, and its efficiency is increasing with decreasing initial stellar mass. On the TP-AGB,thermohaline mixing leads to lithium production, although the 7Li yields remain negative. Although the 3He stellar yields are much reduced thanks to this process, we find that solar-metallicity, low-mass stars remain net 3He producers. Rotation-induced mixing is found to change the stellar structure so that in the mass range between \sim 1.5 and 2.2 Msun the thermohaline instability occurs earlier on the red giant branch than in non-rotating models. Finally rotation accounts for the observed star-to-star abundance variations at a given evolutionary status, and is necessary to explain the features of CN-processed material in intermediate-mass stars.

Motivation & Objective

• To understand non-canonical mixing processes affecting surface abundances in low- and intermediate-mass stars beyond standard stellar theory.
• To investigate the role of thermohaline instability triggered by 3He(3He,2p)4He reaction in altering surface composition post-first dredge-up.
• To assess how rotation-induced mixing modifies stellar structure and influences the onset and efficiency of thermohaline mixing.
• To reconcile model predictions with observational data on Li, 12C/13C, [N/C], [Na/Fe], and oxygen isotopes in open clusters, field giants, and planetary nebulae.
• To determine the net yields of 3He and 7Li under combined mixing processes and assess their impact on Galactic nucleosynthesis.

Proposed method

• Stellar evolution models were computed for 1–4 M⊙ stars at solar metallicity, including both thermohaline and rotation-induced mixing.
• The thermohaline diffusivity was modeled using the prescription of Ulrich (1972), validated by laboratory experiments and previously successful in low-metallicity stars.
• Rotation-induced mixing was implemented via a turbulent diffusivity formalism that alters internal composition and delays the onset of convection-driven mixing.
• Models were evolved from zero-age main sequence to the end of the second dredge-up (early-AGB) and, for selected cases, to the AGB tip.
• Predictions were compared with observational data on surface abundances: Li, 12C/13C, [N/C], [Na/Fe], 16O/17O, and 16O/18O in red giants, clump stars, and planetary nebulae.
• Nuclear reaction rates were taken from NACRE with key exceptions for 14C(p,γ)15N, 14C(p,n)14N, 14C(p,α)11B, 14N(p,γ)15O, and others from specialized literature.

Experimental results

Research questions

• RQ1 To what extent does thermohaline mixing explain the observed decrease in 12C/13C and increase in [N/C] in low-mass giants beyond the RGB bump?
• RQ2 How does rotation-induced mixing affect the timing and efficiency of thermohaline mixing in intermediate-mass stars (1.5–2.2 M⊙)?
• RQ3 Can the observed star-to-star abundance dispersion in stars of similar mass and evolutionary stage be explained by rotation-induced mixing?
• RQ4 What are the net yields of 3He and 7Li in low-mass stars when thermohaline and rotation-induced mixing are included?
• RQ5 How do thermohaline and rotation-induced mixing affect oxygen isotopic ratios (16O/17O and 16O/18O) in evolved stars?

Key findings

• Thermohaline mixing simultaneously reproduces the observed decrease in 12C/13C, increase in [N/C], and depletion of lithium in low-mass giants more luminous than the RGB bump.
• The efficiency of thermohaline mixing increases with decreasing initial stellar mass, with the strongest effects in stars below 2.2 M⊙.
• On the TP-AGB, thermohaline mixing leads to lithium production, but net 7Li yields remain negative, so these stars do not enrich the Galaxy in lithium.
• Despite significant 3He reduction due to thermohaline mixing, solar-metallicity low-mass stars remain net 3He producers over their lifetime.
• Rotation-induced mixing advances the onset of thermohaline mixing on the red giant branch in stars with masses between ~1.5 and 2.2 M⊙ compared to non-rotating models.
• Rotation is necessary to explain the observed dispersion in surface abundances at fixed evolutionary status and is essential for reproducing CN-processed material in intermediate-mass stars.