RESEARCH
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The HARPS search for southern extra-solar planets
XXXIX. HD 175607, the most metal-poor G dwarf with an orbiting sub-Neptune

A. Mortier, J. P. S. Faria, N. C. Santos, V. Rajpaul, P. Figueira, I. Boisse, A. Collier Cameron, X. Dumusque, G. Lo Curto, C. Lovis, M. Mayor, C. Melo, F. Pepe, D. Queloz, A. Santerne, D. Sťgransan, S. G. Sousa, A. Sozzetti, S. Udry

Abstract

Context. The presence of a small-mass planet (Mp < 0.1 MJup) 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 (Mpsini = 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.

Keywords
planetary systems – stars: individual: HD 175607 – techniques: radial velocities – stars: solar-type – stars: activity – stars: abundances

Notes
Based on observations taken with the HARPS spectrograph (ESO 3.6-m telescope at La Silla) under programmes 072.C-0488(E), 082.C-0212(B), 085.C-0063(A), 086.C-0284(A), and 190.C-0027(A).

Radial velocity and stellar activity data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/585/A135

Astronomy and Astrophysics
Volume 585, Article Number A135, Number of pages 11
2016 January

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia