Towards the true planet statistics of intermediate-mass stars inspecting planet-hosting K giants and F stars

This project will continue to address the occurrence rate of extrasolar planets (exoplanets) around intermediate-mass stars (~1.3 solar masses) covering the stellar evolution from the main sequence (MS; F dwarfs) to the red giant branch (RGB; K giants). The planet frequency of evolved giant and subgiant stars ranges from 10% to 25%, while it is considerably lower for MS F-type stars (4%). Our approach for this discrepancy are false planets around giant stars with radii of more than about 21 solar radii because these “planets” have orbital periods in the limited range of 300-800 days, including three examples showing incoherent radial velocity (RV) variations (42 Dra, Gamma Dra, Alpha Tau). We will investigate whether this is due to a detection or selection effect or due to a stellar phenomenon mimicking planets. We will explore the connection between the (non-)existence of planets and further properties of the host stars. Therefore, we will collect spectroscopic data at TLS, McDonald and Calar Alto observatories to measure precise RVs, analyze chromospheric activity (Ca II H & K, He I) and determine rotation periods. For this purpose, we will also exploit archival photometric data from the DIRBE instrument. The modeling of stellar activity, its influence on the detection of exoplanets and the possible removal from RV data will be done in a collaboration with Dr. Sandra Jeffers and Florian Liebing from the University of Göttingen. Additionally, we will derive precise interferometric stellar radii using the CHARA array to refine the radius threshold, above which planet detections become unconfident. Once the stellar rotation periods and radii are available, models of oscillatory convective modes in giant stars can be developed. The comparison of orbital parameters of F-dwarf and K-giant planets reveals that planets around F stars are more eccentric than around K giants. We will investigate possible reasons for this differences.