The GAPS Programme with HARPS-N at TNG: XVIII. Two new giant planets around the metal-poor stars HD 220197 and HD 233832

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) MJ(up), 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.