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Exoplanet hosts reveal lithium depletion
Results from a homogeneous statistical analysis

P. Figueira, J. P. S. Faria, E. Delgado Mena, V. Zh. Adibekyan, S. G. Sousa, N. C. Santos, G. Israelian

Abstract
Aims. We study the impact of the presence of planets on the lithium abundance of host stars and evaluate the previous claim that planet hosts exhibit lithium depletion when compared to their non-host counterparts.
Methods.
Using previously published lithium abundances, we remove the confounding effect of the different fundamental stellar parameters by applying a multivariable regression on our dataset. In doing so, we explicitly make an assumption made implicitly by different authors: that lithium abundance depends linearly on fundamental stellar parameters. Using a moderator variable to distinguish stars with planets from those without, we evaluate the existence of an offset in lithium abundances between the two groups. We perform this analysis first for stars that present a clear lithium detection exclusively and include in a second analysis upper lithium measurements.
Results.
Our analysis shows that under the above-mentioned assumption of linearity, an offset in lithium abundance between planet hosts and non-hosts is recovered. This offset is negative, showing an enhanced depletion for planetary hosts, and is a statistically significant result. By bootstrapping the error bars, we concluded that an inflation on the lithium uncertainty estimations by a factor of larger than 5 is required to render the measured offset compatible with zero at less than 3–4σ and make it non-significant. We demonstrated that the offset as delivered by our method depends on the different nature of the stars in the two samples. We did so by showing that the offset is reduced down to zero if the planet-host stars are replaced by comparison stars in a mock planet-host sample. The offset is also shown to be significant at 3.75σ when compared with that of a population in which planet-host and comparison tags are shuffled, representing a situation in which the tagging is decorrelated from the presence of orbiting planets. Moreover, the measured depletion is still significant when one imposes different constraints on the dataset, such as a limit in planetary mass or constrain the host temperature to around solar value. We conclude then that planet-host stars exhibit enhanced lithium depletion when compared with non-host stars.

Keywords
stars: abundances - planetary systems - methods: data analysis - methods: statistical

Astronomy and Astrophysics
Volume 570, Number of pages A21_1
2014 October

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia