The GAPS Programme with HARPS-N at TNG

XVIII. Two new giant planets around the metal-poor stars HD 220197 and HD 233832^★,★★

¹ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: domenico.barbato@inaf.it
³ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123, Catania, Italy
⁴ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
⁹ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁰ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹¹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131, Napoli, Italy
¹³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁴ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 23 September 2018
Accepted: 17 November 2018

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 f_p 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 f_p.

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 M_Jup 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 M_Jup, 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.