The HARPS search for southern extrasolar planets XXV. Results from the metal-poor sample

TL;DR

This study uses HARPS to search for exoplanets around 104 metal-poor stars, finding some known planets, potential new candidates, and suggesting a higher-than-expected planet frequency in metal-poor environments.

Contribution

First comprehensive HARPS survey targeting metal-poor stars, revealing a higher planet occurrence rate and evidence of a steep metallicity-planet correlation at low metallicities.

Findings

Confirmed giant planets around HD171028, HD181720, and HD190984.

Identified a promising new giant planet candidate around HD107094.

Indications that planet frequency increases with stellar metallicity even at low [Fe/H].

Abstract

Searching for extrasolar planets around stars of different metallicity may provide strong constraints to the models of planet formation and evolution. In this paper we present the overall results of a HARPS (a high-precision spectrograph mostly dedicated to deriving precise radial velocities) program to search for planets orbiting a sample of 104 metal-poor stars (selected [Fe/H] below -0.5). Radial velocity time series of each star are presented and searched for signals using several statistical diagnostics. Stars with detected signals are presented, including 3 attributed to the presence of previously announced giant planets orbiting the stars HD171028, HD181720, and HD190984. Several binary stars and at least one case of a coherent signal caused by activity-related phenomena are presented. One very promising new, possible giant planet orbiting the star HD107094 is discussed, and the results are analyzed in light of the metallicity-giant planet correlation. We conclude that the frequency of giant planets orbiting metal-poor stars may be higher than previously thought, probably reflecting the higher precision of the HARPS survey. In the metallicity domain of our sample, we also find evidence that the frequency of planets is a steeply rising function of the stellar metal content, as found for higher metallicity stars.