Exploring the alpha-enhancement of metal-poor planet-hosting stars. The Kepler and HARPS samples

TL;DR

This study investigates the alpha-element enhancement in metal-poor, planet-hosting stars from Kepler and HARPS samples, revealing a significant association with thick disk stars and suggesting early rocky planet formation in this galactic component.

Contribution

It provides new spectroscopic evidence linking alpha-enhancement to planet-hosting stars in the thick disk at low metallicities, highlighting the role of alpha-elements in planet formation.

Findings

12.3% of thick-disk stars are Ti-enhanced with planets

2.2% of thin-disk stars are Ti-enhanced with planets

Alpha-enhanced stars are significantly more likely to host planets

Abstract

Recent studies showed that at low metallicities Doppler-detected planet-hosting stars have preferably high alpha-content and belong to the thick disk. We used the reconnaissance spectra of 87 Kepler planet candidates and data available from the HARPS planet search survey to explore this phenomena. Using the traditional spectroscopic abundance analysis methods we derived Ti, Ca, and Cr abundances for the Kepler stars. In the metallicity region -0.65 < [Fe/H] < -0.3 dex the fraction of Ti-enhanced thick-disk HARPS planet harboring stars is 12.3 +/- 4.1 % and for their thin-disk counterparts this fraction is 2.2 +/- 1.3 %. The binomial statistics gives a probability of 0.008 that this could have occurred by chance. Combining the two samples (HARPS + Kepler) reinforces the significance of this result (P ~ 99.97 %). Since most of these stars are harboring small-mass/size planets we can assume that, although terrestrial planets can be found at low-iron regime, they are mostly enhanced by alpha-elements. This implies that early formation of rocky planets could get started in the Galactic thick disk, where the chemical conditions for their formation were more favorable.