Transiting Sub-stellar companions on intermediate-mass stars
The aim of the project is to evaluate the predictions from theories of planet formation by determining the absolute frequency of close-in planets of intermediate-mass stars (IMSs). The detection of close-in planets of IMSs can put constraints to the theories of planet formation. First, it is known from infrared observations that the lifetimes of protoplanetary discs of IMSs are half as long as those of solar-mass stars. Second, radial velocity surveys of evolved IMSs show that the frequency of planets at distances larger than 5 AU is higher in comparison to solar-mass stars. However, not much is known about the frequency of close-in planets. An important key to constrain processes of planet formation and evolution is, thus, answering the question: “Is the frequency higher for close-in planets of IMSs than for solar-mass stars?” The transit method is best suited to find such planets. First, it is not affected by the fast rotation of these stars and secondly, it favours the detection of close-in planets. I used the CoRoT database to survey all IMSs observed with this instrument. In contrast to ground- based surveys, the precision of CoRoT allows the detection of all Jupiter-sized transiting objects orbiting IMSs. I first determined the mass-distribution of the stars in the sample. Second I found 14 transit candidates of IMSs in the CoRoT survey and observed them spectroscopically to determine their masses. In result, I detected one planet as well as two very young brown dwarfs with large radii of IMSs by this survey. Among them, the first transiting brown dwarf of a B-type star. The frequency for close-in planets of IMSs derived from this survey is 0.11 ± 0.04 %. This means that close-in sub-stellar companions of IMSs are rare. In the context of disc-migration, this surprising result can be explained by the lifetime of the protostellar discs of IMSs. It is apparently long enough for the planets to form, but on average too short for them to migrate inwards.
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