The evolution of substellar companions of intermediate mass stars

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In contrast to planets of solar-like stars, very little is known about the planets of intermediate mass stars. Up to now, our knowledge is almost exclusively based on theoretical studies and observations of giants stars. Both imply that the frequency of planets and brown dwarf companions of these stars is much higher than that of low mass stars. However, the fact that no substellar companion orbiting a giant star closer than 0.5 AU has been detected raises the question whether such objects never formed or whether they were engulfed and destroyed when the star became a giant. The fate of substellar companions thus is tied to the evolution of their host stars, in particular in the giant branch phases of the host stars’ evolution. In order to understand the evolution of planets, it is thus essential to follow their evolution from the main sequence to the giant branch and beyond. The two big gaps in our knowledge are planets of intermediate-mass stars on the main-sequence, and stars on the horizontal branch. By studying main sequence A and B stars as well as blue horizontal branch stars in two closely interlinked Ph.D. projects, thereby bracketing the red giant phase, we plan to investigate empirically the impact of host star evolution on the substellar companion. By selecting a well defined sample of stars that has been observed with the CoRoT satellite, using 20000 spectra already taken, we expect to find between 10 and 20 substellar companions of these types of stars. In this way, we will find out what fraction of planets are engulfed when an intermediate-mass star evolves to become a giant, and by studying the BHB stars, we will find out what the fate of substellar companions is after the giant branch phase, and whether substellar companions can effect the evolution of their host stars.