Chemical abundances of stars with brown-dwarf companions

D. Mata Sánchez, J. I. González Hernández, G. Israelian, N. C. Santos, J. Sahlmann, S. Udry

Abstract
Context. It is well known that stars with giant planets are, on average, more metal-rich than stars without giant planets, whereas stars with detected low-mass planets do not require to be metal-rich.
Aims. With the aim of studying the weak boundary that separates giant planets and brown dwarfs (BDs) and their formation mechanism, we analyze the spectra of a sample of stars with already confirmed BD companions both by radial velocity and astrometry.
Methods. We employ standard and automatic tools to perform an equivalent width (EW) based analysis and to derive chemical abundances from the CORALIE spectra of stars with BD companions.
Results. We compare these abundances with those of stars without detected planets and with low-mass and giant-mass planets. We find that stars with BDs do not have metallicities and chemical abundances similar to those of giant-planet hosts, but they resemble the composition of stars with low-mass planets. The distribution of mean abundances of α-elements and iron peak elements of stars with BDs exhibit a peak at about solar abundance, whereas for stars with low-mass and high-mass planets the [X_α/H] and [X_Fe/H] peak abundances remain at ~−0.1 dex and ~+0.15 dex, respectively. We display these element abundances for stars with low-mass and high-mass planets, and BDs versus the minimum mass, m_C sin i, of the most massive substellar companion in each system, and we find a maximum in α-element as well as Fe-peak abundances at m_C sin i ~ 1.35 ± 0.20 Jupiter masses.
Conclusions. We discuss the implications of these results in the context of the formation scenario of BDs compared with that of giant planets.

Keywords
brown dwarfs - stars: abundances - planets and satellites: formation - planetary systems - stars: atmospheres

Astronomy and Astrophysics
Volume 566, Page A83_1
June 2014

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