Characterizing and Understanding the Planet Population around Intermediate Mass Stars

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In recent years, many planets have been discovered orbiting stars slightly more massive than our Sun, mostly via Doppler surveys of evolved stars. The planet population around these stars shows some remarkable differences to the one around late-type main-sequence stars, most notably in the period distribution. Our goal is to develop a scenario in which the origin of these differences can be understood.The basis for better statistics of orbital parameters and physical characteristics of planets are continued observations. We have carried out an extensive Doppler survey for planets orbiting giant stars in the past, and plan to continue this survey with a newly built spectrograph for the Waltz Telescope on the Königstuhl in Heidelberg. The goal is in particular to search for planets with periods of the order of decades, for which we have excellent candidates in our sample.In collaboration with the group in Tautenburg, we will develop a set of observational criteria that planets around evolved stars have to fulfill in order to be regarded secure. These criteria will be applied to all published planets around evolved stars, resulting in a list with only the most secure detections. This cleaned planet sample will form the basis for subsequent statistical investigations.Possible mechanisms responsible for the observed differences in planet populations include the dependence of the planet formation process on stellar mass on one side and stellar evolution and tidal effects on the other side. We will carry out simulations backward in time with the goal to derive the planetary system configuration as it was once on the main-sequence. Preliminary results indicate that stellar evolution alone cannot reproduce the observed differences. This hints towards differences in the planet formation process related to stellar mass, which must be responsible for the observed differences in planet populations. However, this is not in line with the largeplanet occurrence rates observed for subgiant stars, which represent basically the same stellar mass range as the more evolved giant stars. Thus, there is currently no compelling explanation for the differences in observed planet populations around more massive evolved stars and less massive main-sequence stars. Our simulations are designed to address and resolve this puzzle.