Extrasolar planets and brown dwarfs around A-F type stars

Aims. Systematic surveys to search for exoplanets have been mostly dedicated to solar-type stars sofar. We developed in 2004 a method to extend such searches to earlier A-F type dwarfs and started spectroscopic surveys to search for planets and quantify the detection limit achievable when taking the stars properties (Spectral Type, v sin i) and their actual levels of intrinsic variations into account. We give here the first results of our southern survey with HARPS. Methods. We observed 185 A-F (B-V in the range [-0.1; 0.6]) stars with HARPS and analysed them with our dedicated software. We used several criteria to probe different origins for the radial-velocity variations-stellar activity (spots, pulsations) or companions: bisector shape, radial-velocity variations amplitudes, and timescales. Results. 1) Sixty-four percent of the 170 stars with enough data points are found to be variable. Twenty are found to be binaries or candidate binaries (with stars or brown dwarfs). More than 80% of the latest type stars (once binaries are removed), are intrinsically variable at a 2 m s(-1) precision level. Stars with earlier spectral type (B-V = 0.2) are either variable or associated to levels of uncertainties comparable to the RV rms observed on variable stars of the same B-V. 2) We detected one long-period planetary system (presented in another paper) around an F6IV-V star. 3) We quantified the jitter due to stellar activity and we show that when taking this jitter into account in addition to the stellar parameters (spectral type, v sin i), it is still possible to detect planets with HARPS with periods of 3 days (resp. 10 days and 100 days) on 91% (resp. 83%, 61%) of them. We show that even the earliest spectral type stars are accessible to this type of search, provided they have a low projected rotational velocity and low levels of activity. 4) Taking the present data into account, we computed the actually achieved detection limits for 107 targets and discuss the limits as a function of B-V. Given the data at hand, our survey is sensitive to short-period (few days) planets and to longer ones (100 days) to a lower extent (latest type stars). We derive first constrains on the presence of planets around A-F stars for these ranges of periods.