The GAPS Programme with HARPS-N at TNG

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 fp. 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 fp = 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.