Three New Planetary Systems Orbiting Metal-Poor Thick Disk Stars

We report the detection of Jovian mass planets orbiting three nearby metal-poor thick disk stars. These discoveries were all made using precise radial velocity measurements from the High Resolution Spectrograph of the Hobby-Eberly Telescope. All of the planets are of Jovian mass or larger, with orbital periods ranging from about a year to over six years. HIP 14342 shows two planetary companions with orbital periods near a 2:1 resonance. The other planets detected orbit HIP 13366 and HIP 109384. All three of these stars are kinematic members of the galactic "thick disk", which is a population of stars with a larger vertical scale height and a larger velocity dispersion that the thin disk to which the Sun belongs. The thick disk stars are of lower total metallicity than the Sun, and are also chemically different than thin disk stars, having the abundances of their alpha-capture elements (e.g. O, Ne, Mg, Si, S, Ca, Ti) enhanced by 0.2 to 0.4 dex over those of thin disk stars of the same [Fe/H]. The majority of planets found among stars with [Fe/H] < ~-0.2 orbit thick disk stars, even though thin disk stars significantly outnumber thick disk stars in this metallicity range. Thus, the enhanced abundance of the alpha-capture elements, which are also key elements in the chemistry of planet-forming materials, may be responsible for the large fraction of low-metallicity thick-disk stars with planetary companions.