Planetas extrassolares em aglomerados estelares abertos: caracterização de estrelas

Abstract

After the pioneering discovery of a giant planet orbiting 51 Peg by Mayor & Queloz (1995), about two decades ago, the literature reports the discovery of more than 3434 confirmed planets (exoplanet.eu), in about 2568 planetary systems. Solar mass main sequence field stars host the vast majority of these exoplanets. The observation of these stars offers several advantages, including brightness and a large variety of stellar characteristics, such as mass, age, chemical composition and evolutionary status. However, the widely differing characteristics of field stars also represents a drawback for our capability to derive precise conclusions to very basic questions, including the role of stellar environment on planet formation. There is no clear answer for the fact that main-sequence stars hosting giant planets are metal rich (Gonzalez 1997; Santos et al. 2004), while evolved stars hosting giant planets are not (Pasquini et al. 2007). Indeed, different phenomena have been proposed to explain this discrepancy in metallicity, including stellar pollution acting on main-sequence stars (Laughlin & Adams 1997, e.g.), a planet formation mechanism favouring the birth of planets around metal rich stars (Pollack al. 1996) and the stellar environment (Haywood 2009). The observation of stars in open cluster offers the possibility to strictly control the stellar characteristics, because each cluster represents a homogeneous set of stars. Besides, open cluster stars were formed at the same time and in the same circumstances and thus are expected to have the same age, metallicity, and galactocentric distance. From the work of Mermilliod & Mayor 2008 we choose clusters harbouring giants stars to be included in our survey. We used the WEBDA cluster database (Mermilliod 1995) to get information about our sample. The main criteria we focused on were the age of the cluster (between 0.02 and a few Gyr, with TO masses > 1,5 MJ) and the magnitude of its giant stars (brighter than V = 13.5). Then we rejected stars with colour index (B - V) larger than 1.4, because cool, bright giants are known to be RV unstable. The observations were performed using HARPS (Mayor et al. 2003), the planet hunter at the ESO 3.6 m telescope. In high accuracy mode (HAM), it has an aperture on the sky of one arcsecond, and a resolving power of 115000. The spectral range covered is 380 - 680 nm. Our spectral analysis is based on the MARCS models of atmospheres and Turbospectrum spectroscopic tool. We determined stellar parameters and metallicity from LTE analysis of Fe I and Fe II lines. Once we get the high resolution and high S/N spectra, we also computed Li abundances that was obtained using the line at 6707.78 Å, Si I, Na I, Mg I, Al I, Ca I, Ti I, Co I, Ni I, Zr I, La II and Cr I. We presented a spectroscopic characterisation of 42 giants, in 12 open clusters, using high resolution spectroscopy. All these clusters are part of a survey for giant planets orbiting intermediate-mass giant stars and the results show that all the clusters studied have [Fe=H] values close to solar, results that agree with the literature with a small dispersion. These abundances will enable us to perform a comparative analysis of the abundances of stars with and without planets, from which it will be possible to detect differences, anomalies and determine the level of planet-star interactions. The goal of this campaign is to study the formation of giant planets in OCs to understand whether a different environment might affect the planet formation process, the frequency, and the evolution of planetary systems with respect to field stars. In addition, searching for planets in OCs enables us to study the dependency of planet formation on stellar mass and to compare the chemical composition of stars with and without planets in detail.