The metallicity signature of evolved stars with planets

TL;DR

This study analyzes the metallicity and chemical abundances of evolved stars with and without planets, revealing that metallicity correlates with stellar mass and challenging the pollution hypothesis for low-mass giants.

Contribution

It provides a homogeneous analysis of metallicity in evolved stars, highlighting the relation between metallicity, stellar mass, and planetary presence, and comparing abundance patterns across different stellar types.

Findings

Giant stars with planets are not universally metal-rich.

Metallicity correlates strongly with stellar mass.

Differences in element abundances are observed in high-mass giants with planets.

Abstract

We determine in a homogeneous way the metallicity and individual abundances of a large sample of evolved stars, with and without known planetary companions. Our methodology is based on the analysis of high-resolution echelle spectra. The metallicity distributions show that giant stars hosting planets are not preferentially metal-rich having similar abundance patterns to giant stars without known planetary companions. We have found, however, a very strong relation between the metallicity distribution and the stellar mass within this sample. We show that the less massive giant stars with planets (M < 1.5 Msun) are not metal rich, but, the metallicity of the sample of massive (M > 1.5 Msun), young (age < 2 Gyr) giant stars with planets is higher than that of a similar sample of stars without planets. Regarding other chemical elements, giant stars with and without planets in the mass domain M < 1.5 Msun show similar abundance patterns. However, planet and non-planet hosts with masses M > 1.5 Msun show differences in the abundances of some elements, specially Na, Co, and Ni. In addition, we find the sample of subgiant stars with planets to be metal rich showing similar metallicities to main-sequence planet hosts. The fact that giant planet hosts in the mass domain M < 1.5 Msun do not show metal-enrichment is difficult to explain. Given that these stars have similar stellar parameters to subgiants and main-sequence planet hosts, the lack of the metal-rich signature in low-mass giants could be explained if originated from a pollution scenario in the main sequence that gets erased as the star become fully convective. However, there is no physical reason why it should play a role for giants with masses M < 1.5 Msun but is not observed for giants with M > 1.5 Msun.