Incidence of planet candidates in open clusters and a planet confirmation

Context. Detecting exoplanets in clusters of different ages is a powerful tool for understanding a number of open questions, such as how the occurrence rate of planets depends on stellar metallicity, on mass, or on stellar environment.
Aims: We present the first results of our HARPS long-term radial velocity (RV) survey which aims to discover exoplanets around intermediate-mass (between 2 and 6 M_⊙) evolved stars in open clusters.
Methods: We selected 826 bona fide HARPS observations of 114 giants from an initial list of 29 open clusters and computed the half-peak to peak variability of the HARPS RV measurements, namely ΔRV/2, for each target, to search for the best planet-host candidates. We also performed time series analyses for a few targets for which we have enough observations to search for orbital solutions.
Results: Although we attempted to rule out the presence of binaries on the basis of previous surveys, we detected 14 new binary candidates in our sample, most of them identified from a comparison between HARPS and CORAVEL data. We also suggest 11 new planet-host candidates based on a relation between the stellar surface gravity and ΔRV/2. Ten of the candidates are less than 3 M_⊙, showing evidence of a low planet occurrence rate for massive stars. One of the planet-host candidates and one of the binary candidates show very clear RV periodic variations, allowing us to confirm the discovery of a new planet and to compute the orbital solution for the binary. The planet is IC 4651 9122b, with a minimum mass of m sini = 6.3 M_J and a semimajor axis a = 2.0 AU. The binary companion is NGC 5822 201B, with a very low minimum mass of m sini = 0.11 M_⊙ and a semimajor axis a = 6.5 AU, which is comparable to the Jupiter distance to the Sun.

Based on observations collected with the 3.6 m Telescope (La Silla Observatory, ESO, Chile) using the HARPS instrument (programs ID: 091.C-0438, 092.C-0282, and 094.C-0297).The reduced time series data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A139