2019A&A...621A.110B

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]≤-0.5dex) 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 Msini=0.20_–0.04^+0.07M_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^+0.23M_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.