Four new planets around giant stars and the mass-metallicity correlation of planet-hosting stars

TL;DR

This study reports four new planets around giant stars and confirms that giant planet occurrence correlates positively with stellar metallicity and mass, supporting the core-accretion formation model.

Contribution

It presents four newly discovered planetary systems around giant stars and analyzes the correlation between stellar properties and planet occurrence rates, expanding understanding of planet formation around evolved stars.

Findings

Four new planetary systems around giant stars.

Giant planets are more common around metal-rich stars.

Planet occurrence peaks at stellar mass ~2.1 M_sun.

Abstract

CONTEXT. Exoplanet searches have demonstrated that giant planets are preferentially found around metal-rich stars and that their fraction increases with the stellar mass. AIMS. During the past six years, we have conducted a radial velocity follow-up program of 166 giant stars, to detect substellar companions, and characterizing their orbital properties. Using this information, we aim to study the role of the stellar evolution in the orbital parameters of the companions, and to unveil possible correlations between the stellar properties and the occurrence rate of giant planets. METHODS. Using FEROS and CHIRON spectra, we have computed precision radial velocities and we have derived atmospheric and physical parameters for all of our targets. Additionally, velocities computed from UCLES spectra are presented here. By studying the periodic radial velocity signals, we have detected the presence of several substellar companions. RESULTS. We present four new planetary systems around the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally, we find that giant planets are more frequent around metal-rich stars, reaching a peak in the detection of $f$ = 16.7$^{+15.5}_{-5.9}$% around stars with [Fe/H] $\sim$ 0.35 dex. Similarly, we observe a positive correlation of the planet occurrence rate with the stellar mass, between M$_\star$ $\sim$ 1.0 -2.1 M$_\odot$, with a maximum of $f$ = 13.0$^{+10.1}_{-4.2}$%, at M$_\star$ = 2.1 M$_\odot$. CONCLUSIONS. We conclude that giant planets are preferentially formed around metal-rich stars. Also, we conclude that they are more efficiently formed around more massive stars, in the mass range of M$_\star$ $\sim$ 1.0 - 2.1 M$_\odot$. These observational results confirm previous findings for solar-type and post-MS hosting stars, and provide further support to the core-accretion formation model.