Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars

TL;DR

This study investigates the metallicity of evolved stars hosting planets, suggesting that the observed metal richness in main sequence stars may be due to pollution rather than intrinsic properties, based on analysis of stellar data.

Contribution

It provides evidence that the metallicity enhancement in planet-hosting stars is likely external and due to pollution, not an intrinsic property, especially in evolved giants.

Findings

Planet-hosting giants do not favor metal-rich systems.

Metallicity differences are explained by stellar mass and dilution effects.

Pollution may be responsible for metallicity enhancement in main sequence stars.

Abstract

Exo-planets are preferentially found around high metallicity main sequence stars. We aim at investigating whether evolved stars share this property, and what this tells about planet formation. Statistical tools and the basic concepts of stellar evolution theory are applied to published results as well as our own radial velocity and chemical analyses of evolved stars. We show that the metal distributions of planet-hosting (P-H) dwarfs and giants are different, and that the latter do not favor metal-rich systems. Rather, these stars follow the same age-metallicity relation as the giants without planets in our sample. The straightforward explanation is to attribute the difference between dwarfs and giants to the much larger masses of giants' convective envelopes. If the metal excess on the main sequence is due to pollution, the effects of dilution naturally explains why it is not observed among evolved stars. Although we cannot exclude other explanations, the lack of any preference for metal-rich systems among P-H giants could be a strong indication of the accretion of metal-rich material. We discuss further tests, as well as some predictions and consequences of this hypothesis.