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

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<SUB>p</SUB> is an important element in providing support to models of planet formation. <BR /> 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<SUB>p</SUB>. <BR /> 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<SUB>p</SUB>. <BR /> Results: We report the detection of two new giant planets around HD 220197 and HD 233832. The first companion has Msin i = 0.20<SUB>-0.04</SUB><SUP>+0.07</SUP> M<SUB>Jup</SUB> and an orbital period of 1728<SUB>-80</SUB><SUP>+162</SUP> days, and for the second companion, we find two solutions of equal statistical weight with periods of 2058<SUB>-40</SUB><SUP>+47</SUP> and 4047<SUB>-117</SUB><SUP>+91</SUP> days and minimum masses of 1.78<SUB>-0.06</SUB><SUP>+0.08</SUP> and 2.72<SUB>-0.23</SUB><SUP>+0.23</SUP> M<SUB>Jup</SUB>, 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<SUB>p</SUB> = 3.84 <SUB>-1.06</SUB><SUP>+2.45</SUP>% for giant planets around metal-poor stars. <BR /> 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. <P />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 <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A110">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A110