Lithium abundances in exoplanet-host stars : modelling

TL;DR

This study models stellar evolution to understand why exoplanet-host stars show more lithium depletion than stars without planets, suggesting extra mixing processes influenced by planetary migration.

Contribution

It demonstrates that overmetallicity alone cannot explain lithium depletion, highlighting the role of additional mixing possibly caused by planetary migration.

Findings

Lithium depletion in EHS is not solely due to overmetallicity.

Extra mixing below the convective zone is necessary to explain observations.

Planetary migration may trigger shear instabilities enhancing mixing.

Abstract

Aims. Exoplanet-host stars (EHS) are known to present superficial chemical abundances different from those of stars without any detected planet (NEHS). EHS are, on the average, overmetallic compared to the Sun. The observations also show that, for cool stars, lithium is more depleted in EHS than in NEHS. The aim of this paper is to obtain constraints on possible models able to explain this difference, in the framework of overmetallic models compared to models with solar abundances. Methods. We have computed main sequence stellar models with various masses and metallicities. The results show different behaviour for the lithium destruction according to those parameters. We compare these results to the spectroscopic observations of lithium. Results. Our models show that the observed lithium differences between EHS and NEHS are not directly due to the overmetallicity of the EHS: some extra mixing is needed below the convective zones. We discuss possible explanations for the needed extra mixing, in particular an increase of the mixing efficiency associated with the development of shear instabilities below the convective zone, triggered by angular momentum transfer due to the planetary migration.