The mass dependence of planet formation : a radial-velocity survey for extrasolar planets around F and Ap stars

Most radial-velocity (RV) planet search programs are focused on solar-type stars with spectral types F8-K0 that cover only a small range in stellar mass (0.8-1.2 solar masses). As a result, our knowledge on how the stellar mass influences the frequency and properties of giant planets is very limited. The present study deals with the question if there is a correlation between the planet occurrence rate and the stellar mass. This will give information on processes that are crucial for the formation of giant planets. The main emphasis of this work was to perform spectroscopic observations to carry out a systematic search for sub-stellar and - as a by-product - stellar companions around stars more massive than the Sun. Therefore, I monitored a sample of 155 F-type main-sequence stars with the 2.0-meter Alfred Jensch Telescope of the Thüringer Landessternwarte Tautenburg (TLS) and its high-resolution Tautenburg Coudé Échelle Spectrograph (TCES). An iodine absorption cell placed in the optical light path provided the wavelength reference for precise stellar RV measurements. In addition, I investigated 62 chemically peculiar A-type (Ap) stars with the HARPS spectrograph at the 3.6-meter telescope at the European Southern Observatory (La Silla, Chile). The RVs were obtained using two different methods: the cross-correlation function (CCF) with a binary mask, which is implemented in the HARPS Data Reduction Software, and a new software tool called HARPS-TERRA, which is based on a least-squares template-matching approach. I find 24 (16%) and 17 (27%) spectroscopic binaries for the F-star and Ap-star survey, respectively. Moreover, the F-star sample contains six stars with planets: Three of them were published by other authors, and three were detected in the framework of this program. The planet frequency of F stars (4%) is less than expected from theoretical studies and contrary to observations based on giant stars.

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