Suppression of lithium depletion in young low-mass stars from fast rotation

Abstract

We compute rotating 1D stellar evolution models that include a modified temperature gradient in convection zones and criterion for convective instability inspired by rotating 3D hydrodynamical simulations performed with the music code. In those 3D simulations we found that convective properties strongly depend on the Solberg-Høiland criterion for stability. We therefore incorporated this into 1D stellar evolution models by replacing the usual Schwarzschild criterion for stability and also modifying the temperature gradient in convection zones. We computed a grid of 1D models between 0.55 and 1.2 stellar masses from the pre-main sequence to the end of main sequence in order to study the problem of lithium depletion in low-mass main sequence stars. This is an ideal test case because many of those stars are born as fast rotators and the rate of lithium depletion is very sensitive to the changes in the stellar structure. Additionally, observations show a correlation between slow rotation and lithium depletion, contrary to expectations from standard models of rotationally driven mixing. By suppressing convection, and therefore decreasing the temperature at the base of the convective envelope, lithium burning is strongly quenched in our rapidly rotating models to an extent sufficient to account for the lithium spread observed in young open clusters.

A GALAH View of the Chemical Homogeneity and Ages of Stellar Strings Identified in Gaia

“…Other structures, such as Theias 97, 371, 381, 452, 1200, and 1350 also show signs of a disagreement between their chemical and isochronal ages as indicated by their Li and/or chemical clock abundances. One possible explanation for the disagreement between isochronal ages and Li depletion ages could be due to the effect that rapid stellar rotation has on suppressing Li depletion (e.g., Constantino et al 2021). This may particularly affect Theia 452, for the mean 𝑣𝑠𝑖𝑛𝑖 of its GALAH-sampled members is 15 kms −1 , with some members displaying 𝑣𝑠𝑖𝑛𝑖 up to 25 kms −1 .…”

Impact of radial truncation on global 2D hydrodynamic simulations for a Sun-like model

“…Note however that the convection zone is not precisely adiabatically stratified with super-adiabaticity reaching 𝑂 (10 −2 ) close to the surface. A close analogue of this initial model has been used in Baraffe et al (2021); Le Saux et al (2021) to study the effects of luminosity boosting. This requires minimising the super-adiabaticity in the convection zone (so that stratification is not affected by the luminosity boosting).…”

“…In general, the dynamics at the lower convective boundary are central to a number of open questions of stellar physics relating to mixing and transport of material, angular momentum, and magnetic fields. For instance, the enhanced mixing of material from the well-mixed cooler convective zone across the lower convective boundary can help deplete the surface lithium abundance in pre-main-sequence and main-sequence stars (Baraffe et al 2017;Constantino et al 2021). Convective motions along and across the boundary excite internal gravity waves (IGWs) in the stably-stratified radiative zone (RZ) (Press 1981;Goldreich & Kumar 1990;Dintrans et al 2005;Pinçon et al 2016).…”

“…The relaxation time scales may be shortened by artificially boosting the stellar luminosity and thermal diffusivity. However, running such simulations self-consistently requires that the convection zone be adiabatic, to avoid changes in the temperature stratification due to the boosting (Baraffe et al 2021). It has also been shown that boosting can have a significant impact on the dynamics throughout the stellar interior (Baraffe et al 2021;Le Saux et al 2021).…”

“…However, running such simulations self-consistently requires that the convection zone be adiabatic, to avoid changes in the temperature stratification due to the boosting (Baraffe et al 2021). It has also been shown that boosting can have a significant impact on the dynamics throughout the stellar interior (Baraffe et al 2021;Le Saux et al 2021). Therefore, in this study, we do not resort to boosted simulations and consider instead twodimensional (2D) models with realistic stellar luminosity covering a wide selection of radial truncations.…”