With now more than 20 exoplanets discovered by CoRoT, it has often been considered strange that so many of them are orbiting F-stars, and so few of them K or M-stars. Although transit search programs are mostly sensitive to short-period planets, they
are ideal for verifying these results. To determine the frequency of planets as a function of stellar mass, we also have to characterize the sample of stars that was observed. We study the stellar content of the CoRoT-fields IRa01, LRa01 (=LRa06), and LRa02 by determining the spectral types of 11466 stars. We used spectra obtained with the multi-object spectrograph AAOmega and derived the spectral types by using template spectra with well-known parameters. We find that 34.8+/-0.7% of the stars observed by CoRoT in these fields are F-dwarfs, 15.1+/-0.5% G-dwarfs, and 5.0+/-0.3% K-dwarfs. We conclude that the apparent lack of exoplanets of K- and M-stars is explained by the relatively small number of these stars in the observed sample. We also show that the apparently large number of planets orbiting F-stars is similarly explained by the large number of such stars in these fields. Our study also shows that the difference between the sample of stars that CoRoT observes and a sample of randomly selected stars is relatively small, and that the yield of CoRoT specifically is the detection one hot Jupiter amongst 2100+/-700 stars. We conclude that transit search programs can be used to study the relation between the frequency of planets and the mass of the host stars, and that the results obtained so far generally agree with those of radial velocity programs.
Observations of transiting extrasolar planets are of key importance to our understanding of planets because their mass, radius, and mass density can be determined. The CoRoT space mission allows us to achieve a very high photometric accuracy. By comb
ining CoRoT data with high-precision radial velocity measurements, we derive precise planetary radii and masses. We report the discovery of CoRoT-19b, a gas-giant planet transiting an old, inactive F9V-type star with a period of four days. After excluding alternative physical configurations mimicking a planetary transit signal, we determine the radius and mass of the planet by combining CoRoT photometry with high-resolution spectroscopy obtained with the echelle spectrographs SOPHIE, HARPS, FIES, and SANDIFORD. To improve the precision of its ephemeris and the epoch, we observed additional transits with the TRAPPIST and Euler telescopes. Using HARPS spectra obtained during the transit, we then determine the projected angle between the spin of the star and the orbit of the planet. We find that the host star of CoRoT-19b is an inactive F9V-type star close to the end of its main-sequence life. The host star has a mass M*=1.21+/-0.05 Msun and radius R*=1.65+/-0.04 Rsun. The planet has a mass of Mp=1.11+/-0.06 Mjup and radius of Rp=1.29+/-0.03 Rjup. The resulting bulk density is only rho=0.71+/-0.06 gcm-3, which is much lower than that for Jupiter. The exoplanet CoRoT-19b is an example of a giant planet of almost the same mass as Jupiter but a 30% larger radius.
Evolutionary tracks are of key importance for the understanding of star formation. Unfortunately, tracks published by various groups differ so that it is fundamental to have observational tests. In order to do this, we intend to measure the masses of
the two components of the Pre-Main Sequence binary HD113449 by combining radial velocity measurements taken with HARPS, with infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit that has already been determined, combined with the first AMBER measurement, allows us to obtain a very first estimation of the inclination of the binary system and from this the masses of the two stars. More AMBER measurements of HD 113449 are needed to improve the precision on the masses: in the ESO period P82 two new measurements are scheduled.
