Intermediate-mass stars and the origin of the gas-giant planet-metallicity correlation

TL;DR

This study investigates how the correlation between star metallicity and gas-giant planet presence varies across different evolutionary stages of intermediate-mass stars, revealing a strong correlation on the red giant branch but weaker signals earlier.

Contribution

It demonstrates that the planet-metallicity correlation in intermediate-mass stars develops as stars evolve, supporting core-accretion models and highlighting the role of stellar evolution in metallicity measurements.

Findings

Pre-MS stars with transitional discs show lower metallicities.

Intermediate-mass MS stars tend to follow the planet-metallicity correlation, but weaker than lower-mass stars.

Strong planet-metallicity correlation is observed in red giant branch stars.

Abstract

The number of known planets around intermediate-mass stars (1.5 M$_{\odot}$ < M$_{\star}$ < 3.5 M$_{\odot}$) is rather low. We aim to test whether the correlation between the metallicity of the star and the presence of gas-giant planets found for MS low-mass stars still holds for intermediate-mass stars. In particular, we aim to understand whether or not the planet-metallicity relation changes as stars evolve from the pre-MS to the red giant branch. Our results confirm that pre-MS stars with transitional discs with gaps show lower metallicities than pre-MS with flat discs. We show a tendency of intermediate-mass stars in the MS to follow the gas-giant planet-metallicity correlation, although the differences in metal content between planet and non-planet hosts are rather modest and the strength of the correlation is significantly lower than for the less massive FGK MS stars. For stars in the red giant branch, we find a strong planet-metallicity correlation, compatible with that found for FGK MS stars. We discuss how the evolution of the mass in the convective zone of the star's interior might affect the measured metallicity of the star. The lack of a well-established planet-metallicity correlation in pre-MS and MS intermediate-mass stars can be explained by a scenario in which planet formation leads to an accretion of metal-poor material on the surface of the star. As intermediate-mass stars are mainly radiative the metallicity of the star does not reflect its bulk composition but the composition of the accreted material. When the star leaves the MS and develops a sizeable convective envelope, a strong-planet metallicity correlation is recovered. Thus, our results are in line with core-accretion models of planet formation and the idea that the planet-metallicity correlation reflects a bulk property of the star.