D. Barbato, A. Sozzetti, K. Biazzo, L. Malavolta, N. C. Santos, M. Damasso, A. F. Lanza, M. Pinamonti, L. Affer, S. Benatti, A. Bignamini, A. S. Bonomo, F. Borsa, I. Carleo, R. Claudi, R. Cosentino, E. Covino, S. Desidera, M. Esposito, P. Giacobbe, E. González-Álvarez, R. Gratton, A. Harutyunyan, G. Leto, A. Maggio, J. Maldonado, L. Mancini, S. Masiero, G. Micela, E. Molinari, V. Nascimbeni, I. Pagano, G. Piotto, E. Poretti, M. Rainer, G. Scandariato, R. Smareglia, L. S. Colombo, L. Di Fabrizio, J. P. Faria, A. F. Martinez Fiorenziano, M. Molinaro, M. Pedani

Abstract
Context. Statistical studies of exoplanets have shown that giant planets are more commonly hosted by metal-rich dwarf stars than low-metallicity stars, while no such correlation is evident for lower mass planets. The search for giant planets around metal-poor stars and the estimate of their occurrence fp is an important element in providing support to models of planet formation.
Aims. We present results from the HARPS-N search for giant planets orbiting metal-poor (− 1.0 ≤ [Fe/H] ≤−0.5 dex) stars in the northern hemisphere, complementing a previous HARPS survey on southern stars in order to update the estimate of fp.
Methods. High-precision HARPS-N observations of 42 metal-poor stars were used to search for planetary signals to be fitted using differential evolution Markov chain Monte Carlo single-Keplerian models. We then joined our detections to the results of the previous HARPS survey on 88 metal-poor stars to provide a preliminary estimate of the two-hemisphere f_p.
Results. We report the detection of two new giant planets around HD 220197 and HD 233832. The first companion has Msin i = 0.20_−0.04^+0.07 MJup and an orbital period of 1728₋₈₀⁺¹⁶² days, and for the second companion, we find two solutions of equal statistical weight with periods of 2058₋₄₀⁺⁴⁷ and 4047₋₁₁₇⁺⁹¹ days and minimum masses of 1.78_−0.06^+0.08 and 2.72_−0.23^+0.23 MJup, respectively. Joining our two detections with the three from the southern survey, we obtain a preliminary and conservative estimate of the global frequency of f_p = 3.84 _−1.06^+2.45% for giant planets around metal-poor stars.
Conclusions. The two new giant planets orbit dwarf stars at the metal-rich end of the HARPS-N metal-poor sample. This corroborates previous results that suggested that giant planet frequency is still a rising function of the host star [Fe/H]. We also note that all detections in the overall sample are giant long-period planets.

Keywords
techniques: radial velocities / methods: data analysis / planetary systems / stars: abundances / stars: individual: HD 220197 / stars: individual: HD 233832

Notes
^★ Based on observations made with the HARPS-N spectrograph on the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma (Spain) by the INAF – Fundación Galileo Galilei (Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias).
^★★ RV data (Table 5) 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/621/A110

Astronomy & Astrophysics
Volume 621, Article Number A110, Number of pages 12
2019 January

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