The HARPS search for southern extra-solar planets

XXXVIII. Bayesian re-analysis of three systems. New super-Earths, unconfirmed signals, and magnetic cycles ^⋆,⋆⋆

¹ Observatoire astronomique de l’Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: Rodrigo.Diaz@unige.ch
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Dpto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Physikalisches Institut, Universitat Bern, Silderstrasse 5, 3012 Bern, Switzerland
⁶ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁷ School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
⁸ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5C, 4000 Liège, Belgium
¹⁰ European Southern Observatory, 3107 Alonso de Cordova, Vitacura, Casilla 19001, Santiago 19, Chile
¹¹ Canada France Hawaii Telescope Corporation, Kamuela, 96743, USA
¹² Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
¹³ Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK
¹⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 12 June 2015
Accepted: 9 October 2015

Abstract

We present the analysis of the entire HARPS observations of three stars that host planetary systems: HD 1461, HD 40307, and HD 204313. The data set spans eight years and contains more than 200 nightly averaged velocity measurements for each star. This means that it is sensitive to both long-period and low-mass planets and also to the effects induced by stellar activity cycles. We modelled the data using Keplerian functions that correspond to planetary candidates and included the short- and long-term effects of magnetic activity. A Bayesian approach was taken both for the data modelling, which allowed us to include information from activity proxies such as log R'_HK in the velocity modelling, and for the model selection, which permitted determining the number of significant signals in the system. The Bayesian model comparison overcomes the limitations inherent to the traditional periodogram analysis. We report an additional super-Earth planet in the HD 1461 system. Four out of the six planets previously reported for HD 40307 are confirmed and characterised. We discuss the remaining two proposed signals. In particular, we show that when the systematic uncertainty associated with the techniques for estimating model probabilities are taken into account, the current data are not conclusive concerning the existence of the habitable-zone candidate HD 40307 g. We also fully characterise the Neptune-mass planet that orbits HD 204313 in 34.9 days.