The HARPS search for southern extra-solar planets^⋆,⋆⋆

XXXIX. HD 175607, the most metal-poor G dwarf with an orbiting sub-Neptune

¹ SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, UK
e-mail: am352@st-andrews.ac.uk
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, 4099-002 Porto, Portugal
⁴ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
⁵ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ European Southern Observatory, 19001 Casilla, Santiago, Chile
⁸ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
¹⁰ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy

Received: 6 July 2015
Accepted: 2 November 2015

Abstract

Context. The presence of a small-mass planet (M_p < 0.1 M_Jup) seems, to date, not to depend on metallicity, however, theoretical simulations have shown that stars with subsolar metallicities may be favoured for harbouring smaller planets. A large, dedicated survey of metal-poor stars with the HARPS spectrograph has thus been carried out to search for Neptunes and super-Earths.

Aims. In this paper, we present the analysis of HD 175607, an old G6 star with metallicity [Fe/H] =−0.62. We gathered 119 radial velocity measurements in 110 nights over a time span of more than nine years.

Methods. The radial velocities were analysed using Lomb-Scargle periodograms, a genetic algorithm, a Markov chain Monte Carlo analysis, and a Gaussian processes analysis. The spectra were also used to derive stellar properties. Several activity indicators were analysed to study the effect of stellar activity on the radial velocities.

Results. We find evidence for the presence of a small Neptune-mass planet (M_psini = 8.98 ± 1.10 M_⊕) orbiting this star with an orbital period P = 29.01 ± 0.02  days in a slightly eccentric orbit (e = 0.11 ± 0.08). The period of this Neptune is close to the estimated rotational period of the star. However, from a detailed analysis of the radial velocities together with the stellar activity, we conclude that the best explanation of the signal is indeed the presence of a planetary companion rather than stellar related. An additional longer period signal (P ~ 1400 d) is present in the data, for which more measurements are needed to constrain its nature and its properties.

Conclusions. HD 175607 is the most metal-poor FGK dwarf with a detected low-mass planet amongst the currently known planet hosts. This discovery may thus have important consequences for planet formation and evolution theories.