[Paper Review] Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities

N. Lagarde, T. Decressin+4 more|arXiv (Cornell University)|Apr 23, 2012

Stellar, planetary, and galactic studies83 references141 citations

TL;DR

This study presents a comprehensive grid of low- and intermediate-mass stellar evolution models from the pre-main sequence to the AGB, incorporating rotation-induced mixing and thermohaline instability at four metallicities (Z = 0.0001 to 0.014). It computes global asteroseismic parameters such as large frequency separation (Δν), ν_max, A_max, and asymptotic period spacing, showing that rotation-induced mixing significantly alters these quantities, while thermohaline mixing—though spectroscopically detectable—does not affect the global seismic observables studied here.

ABSTRACT

The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. In this paper, we evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic Giant Branch to distinguish stellar populations. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corresponding to the maximum oscillation power ν_{max}, the maximal amplitude A_{max}, the asymptotic period spacing of g-modes, and different acoustic radii. We discuss the signature of rotation-induced mixing on the global asteroseismic quantities, that can be detected observationally. Thermohaline mixing whose effects can be identified by spectroscopic studies cannot be caracterized with the global seismic parameters studied here. But it is not excluded that individual mode frequencies or other well chosen asteroseismic quantities might help constraining this mixing.

Motivation & Objective

• To evaluate the impact of rotation-induced mixing and thermohaline instability on global asteroseismic parameters across stellar evolution.
• To provide a complete grid of stellar models from pre-main sequence to the AGB for low- and intermediate-mass stars at varying metallicities.
• To enable statistical asteroseismic studies by supplying both classical stellar parameters and global seismic quantities for comparison with CoRoT and Kepler observations.
• To distinguish the observable signatures of non-standard mixing processes in asteroseismic data, particularly for stars with similar luminosity and temperature but different evolutionary stages.
• To assess the limitations of global asteroseismic parameters in detecting thermohaline mixing, despite its known spectroscopic effects.

Proposed method

• Computing a grid of stellar evolution models with initial masses from 0.85 to 6.0 M⊙ and metallicities Z = 0.0001, 0.002, 0.004, and 0.014.
• Including both standard physics and non-standard processes: rotation-induced mixing (via meridional circulation and shear turbulence) and thermohaline convection.
• Calculating global asteroseismic parameters using established scaling relations: Δν (large frequency separation), ν_max (frequency of maximum power), and A_max (maximum oscillation amplitude).
• Computing asymptotic quantities: Δν_asympt (asymptotic large separation), t_BCE (acoustic radius at base of convective envelope), t_He (acoustic radius at HeII ionization zone), T (total acoustic radius), and ΔΠ (ℓ=1 period spacing) for g-modes.
• Tracing the evolution of these parameters from the pre-main sequence through the subgiant and red giant phases up to the thermally pulsing AGB.
• Comparing models with and without non-standard mixing to isolate the effects of rotation and thermohaline processes on observable asteroseismic features.

Experimental results

Research questions

• RQ1 How does rotation-induced mixing affect the large frequency separation (Δν) and frequency of maximum power (ν_max) across the Hertzsprung-Russell diagram?
• RQ2 Can thermohaline mixing be detected through global asteroseismic parameters such as Δν, ν_max, or A_max, despite its known impact on surface abundances?
• RQ3 To what extent can asteroseismic parameters like Δν and A_max distinguish stars with the same effective temperature and luminosity but different evolutionary stages?
• RQ4 How do the asymptotic asteroseismic quantities (e.g., t_BCE, t_He, ΔΠ) evolve during the TP-AGB phase under the influence of rotation and thermohaline mixing?
• RQ5 What is the relative contribution of rotation and thermohaline mixing to the observed spread in asteroseismic properties in stellar populations of different metallicities?

Key findings

• Rotation-induced mixing significantly alters global asteroseismic parameters such as Δν, ν_max, and A_max, particularly during the subgiant and helium-burning phases.
• Thermohaline mixing does not affect the luminosity, effective temperature, or the global asteroseismic quantities studied here, despite its strong impact on surface abundances.
• Stars with identical effective temperatures and luminosities—such as a 1.5 M⊙ star on the HR gap and a 2.0 M⊙ star on the pre-main sequence—can be distinguished by their distinct Δν and A_max values.
• The large frequency separation Δν decreases with increasing stellar mass at the same evolutionary phase, and this difference is enhanced by rotation-induced mixing.
• The maximal amplitude A_max is sensitive to stellar mass and mean density, with rotating models showing higher amplitudes during the subgiant phase due to structural changes.
• Asymptotic quantities like t_BCE, t_He, and ΔΠ (ℓ=1) provide additional constraints on internal structure and are modified by rotation, but not by thermohaline mixing.