Research Note

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

¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: Vardan.Adibekyan@astro.up.pt
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
³ Departamento de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Portugal
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Observatoire de Genève, Université de Genève, 51 Ch. des Mailletes, 1290 Sauverny, Switzerland
⁶ Byurakan Astrophysical Observatory, 0213 Byurakan, Aragatsotn province, Armenia
⁷ Department of General Physics and Astrophysics, Yerevan State University, 1 Alex Manoogian, 0025 Yerevan, Armenia
⁸ Isaac Newton Institute of Chile, Armenian Branch, 0213 Byurakan, Aragatsotn province, Armenia

Received: 4 August 2012
Accepted: 27 September 2012

Abstract

Recent studies have shown that at low metallicities Doppler-detected planet-hosting stars tend to have high α-content and to 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 phenomenon. 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%. Binomial statistics give a probability of 0.008 that this could have occurred by chance. Combining the two samples (HARPS and Kepler) reinforces the significance of this result (P ~ 99.97%). Since most of these stars harbor small sized or low-mass planets we can assume that, although terrestrial planets can be found in a low-iron regime, they are mostly enhanced by α-elements. This implies that early formation of rocky planets could start in the Galactic thick disk, where the chemical conditions for their formation are more favorable.