Characterizing and Understanding the Planet Population around Intermediate Mass Stars

近年来，许多行星被发现围绕着比我们的太阳稍大的恒星运行，主要是通过对演化恒星的多普勒调查。这些恒星周围的行星数量与晚型主序恒星周围的行星数量有一些显著的差异，最明显的是在周期分布上。我们的目标是开发一个场景，在这个场景中，这些差异的起源可以被理解。持续的观测是更好地统计行星轨道参数和物理特性的基础。过去，我们已经对围绕巨大恒星运行的行星进行了广泛的多普勒调查，并计划在海德堡Königstuhl上的华尔兹望远镜上新建的光谱仪继续进行这项调查。我们的目标是寻找周期为几十年的行星，我们的样本中有很好的候选行星。我们将与陶滕堡的研究小组合作，制定一套观测标准，围绕演化恒星运行的行星必须满足这些标准，才能被认为是安全的。这些标准将适用于所有已公布的围绕演化恒星运行的行星，从而产生一个只有最安全探测的列表。这个经过清理的行星样本将成为后续统计调查的基础。造成观测到的行星数量差异的可能机制包括：行星形成过程一方面依赖于恒星质量，另一方面依赖于恒星演化和潮汐效应。我们将在时间上进行模拟，目的是推导出行星系统的结构，因为它曾经在主序上。初步结果表明，仅靠恒星演化无法再现观测到的差异。这暗示了与恒星质量相关的行星形成过程的差异，这一定是观测到的行星数量差异的原因。然而，这与观测到的亚巨星的大行星发生率不一致，亚巨星的质量范围与更进化的巨星基本相同。因此，对于大质量演化恒星和小质量主序恒星周围观测到的行星数量的差异，目前还没有令人信服的解释。我们的模拟就是为了解决这个难题。

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.