Lithium depletion and angular momentum transport in solar-type stars

Abstract

Context. Transport processes occurring in the radiative interior of solar-type stars are evidenced by the surface variation of light elements, in particular 7Li, and the evolution of their rotation rates. For the Sun, inversions of helioseismic data indicate that the radial profile of angular velocity in its radiative zone is nearly uniform, which implies the existence of angular momentum transport mechanisms that are efficient over evolutionary timescales. While there are many independent transport models for angular momentum and chemical species, there is a lack of self-consistent theories that permit stellar evolution models to simultaneously match the present-day observations of solar lithium abundances and radial rotation profiles. Aims. We explore how additional transport processes can improve the agreement between evolutionary models of rotating stars and observations for 7Li depletion, the rotation evolution of solar-type stars, and the solar rotation profile. Methods. Models of solar-type stars are computed including atomic diffusion and rotation-induced mixing with the code STAREVOL. We explore different additional transport processes for chemicals and for angular momentum such as penetrative convection, tachocline mixing, and additional turbulence. We constrain the resulting models by simultaneously using the evolution of the surface rotation rate and 7Li abundance in the solar-type stars of open clusters with different ages, and the solar surface and internal rotation profile as inverted from helioseismology when our models reach the age of the Sun. Results. We show the relevance of penetrative convection for the depletion of 7Li in pre-main sequence and early main sequence stars. The rotational dependence of the depth of penetrative convection yields an anti-correlation between the initial rotation rate and 7Li depletion in our models of solar-type stars that is in agreement with the observed trend. Simultaneously, the addition of an ad hoc vertical viscosity νadd leads to efficient transport of angular momentum between the core and the envelope during the main sequence evolution and to solar-type models that match the observed profile of the Sun. We also self-consistently compute for the first time the thickness of the tachocline and find that it is compatible with helioseismic estimations at the age of the Sun, but we highlight that the associated turbulence does not allow the observed 7Li depletion to be reproduced. The main sequence depletion of 7Li in solar-type stars is only reproduced when adding a parametric turbulent mixing below the convective envelope. Conclusions. The need for additional transport processes in stellar evolution models for both chemicals and angular momentum in addition to atomic diffusion, meridional circulation, and turbulent shear is confirmed. We identify the rotational dependence of the penetrative convection as a key process. Two additional and distinct parametric turbulent mixing processes (one for angular momentum and one for chemicals) are required to simultaneously explain the observed surface 7Li depletion and the solar internal rotation profile. We highlight the need of additional constraints for the internal rotation of young solar-type stars and also for the beryllium abundances of open clusters in order to test our predictions.

The chemical make-up of the Sun: A 2020 vision

“…Quantitative predictions for elemental diffusion are hampered by the possibility of additional mixing below the convection zone due to still poorly understood processes like convective overshooting, rotation, internal gravity waves, and turbulence (e.g., Goldreich & Schubert 1967;Chaboyer et al 1995;Baraffe et al 2017;Aerts et al 2019;Dumont et al 2021), which is required to explain the solar Li abundance and interior rotation profile. Different groups thus require slightly varying proto-solar abundances depending on the ingredients of their stellar evolution codes when calibrating to the solar luminosity, effective temperature and surface composition at the solar age.…”

Stellar convective penetration: parameterized theory and dynamical simulations

“…In the stellar structure literature, "convective overshoot" refers to any convectivelydriven mixing which occurs beyond the boundaries of the Ledoux-unstable zone. This mixing can influence, for example, observed surface lithium abundances in the Sun and solar-type stars, which align poorly with theoretical predictions (Pinsonneault 1997;Carlos et al 2019;Dumont et al 2021). Furthermore, modern spectroscopic observations suggest a lower solar metallicity than previously thought, and models computed with modern metallicity estimates and opacity tables have shallower convection zones than helioseismic observations suggest (Basu & Antia 2004;Bahcall et al 2005;Bergemann & Serenelli 2014;Vinyoles et al 2017;Asplund et al 2021); modeling and observational discrepancies can be reduced with additional mixing below the convective boundary (Christensen-Dalsgaard et al 2011).…”

The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars

“…Among the various considered additional mechanisms, both rotation (see Sestito & Randich 2005) and overshooting mixing (Christensen-Dalsgaard et al 2011;Zhang 2012) have been introduced to reproduce the observational properties of the clusters and at the same time the properties of the Sun. However, for low mass solar-type stars with relatively extended convective envelopes, hydrodynamic processes induced by rotation, as, for instance, meridional circulation and shear mixing, predict large rotation gradients within the interior, needing, e.g., internal gravity waves or other mechanisms, as penetrative convection, tachocline mixing, and additional turbulence, to explain both the rotation profile and the surface abundance of lithium in solar-type stars of various ages (see Talon & Charbonnel 2005;Dumont et al 2021).…”

“…However for the lowest-mass range, corresponding to ages > 4000 Myr, the comparison should be taken with caution. Indeed in the Lagarde et al (2012) models the transport of angular momentum is driven by meridional circulation and turbulence only, while an additional transport is required to explain the internal rotation profile of low-mass stars, both on the MS (see references in Dumont et al 2021, for the case of the Sun and solar-type stars) and along the red giant branch (e.g. Eggenberger et al 2019, and references therein).…”

“…∼2.5 10 6 K or higher. Depending on the mass and metallicity of the star, photospheric Li can be significantly depleted already on the pre-main sequence (PMS), during the proto-stellar accretion phase (Tognelli et al 2020) and along the Hayashi track, and/or on the main sequence (MS), due to several mechanisms that have the potential to transport the photospheric material into hotter layers where Li can be burned: atomic diffusion, overshooting, rotation-induced mixing, internal gravity waves and other types of magneto-hydrodynamical instabilities that are not included in the so-called classical evolution models (Michaud 1986;Charbonneau & Michaud 1990;Schramm et al 1990;Richard et al 1996;Deliyannis et al 2000;Denissenkov & Tout 2003;Talon & Charbonnel 2010;Eggenberger et al 2012;Castro et al 2016;Somers & Pinsonneault 2016;Baraffe et al 2017;Deal et al 2021;Dumont et al 2021).…”