To understand the formation and evolution of solar-type stars in the solar neighborhood, we need to measure their stellar parameters to high accuracy. We present a catalogue of accurate stellar parameters for 451 stars that represent the HARPS Guaranteed Time Observations (GTO) "high precision" sample. Spectroscopic stellar parameters were measured using high signal-to-noise (S /N) spectra acquired with the HARPS spectrograph. The spectroscopic analysis was completed assuming LTE with a grid of Kurucz atmosphere models and the recent ARES code for measuring line equivalent widths. We show that our results agree well with those ones presented in the literature (for stars in common). We present a useful calibration for the effective temperature as a function of the index color B − V and [Fe/H]. We use our results to study the metallicity-planet correlation, namely for very low mass planets. The results presented here suggest that in contrast to their jovian couterparts, neptune-like planets do not form preferentially around metal-rich stars. The ratio of jupiter-to-neptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation.

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We use asteroseismic data obtained by the NASA Kepler mission to estimate the fundamental properties of more than 500 main-sequence and sub-giant stars. Data obtained during the first 10 months of Kepler science operations were used for this work, when these solar-type targets were observed for one month each in survey mode. Stellar properties have been estimated using two global asteroseismic parameters and complementary photometric and spectroscopic data. Homogeneous sets of effective temperatures, T eff , were available for the entire ensemble from complementary photometry; spectroscopic estimates of T eff and [Fe/H] were available from a homogeneous analysis of ground-based data on a subset of 87 stars. We adopt a grid-based analysis, coupling six pipeline codes to 11 stellar evolutionary grids. Through use of these different grid-pipeline combinations we allow implicitly for the impact on the results of stellar model dependencies from commonly used grids, and differences in adopted pipeline methodologies. By using just two global parameters as the seismic inputs we are able to perform a homogenous analysis of all solar-type stars in the asteroseismic cohort, including many targets for which it would not be possible to provide robust estimates of individual oscillation frequencies (due to a combination of low signal-to-noise ratio and short dataset lengths). The median final quoted uncertainties from consolidation of the grid-based analyses are for the full ensemble (spectroscopic subset) approximately 10.8% (5.4%) in mass, 4.4% (2.2%) in radius, 0.017 dex (0.010 dex) in log g, and 4.3% (2.8%) in mean density. Around 36% (57%) of the stars have final age uncertainties smaller than 1 Gyr. These ages will be useful for ensemble studies, but should be treated carefully on a star-bystar basis. Future analyses using individual oscillation frequencies will offer significant improvements on up to 150 stars, in particular for estimates of the ages, where having the individual frequency data is most important.

Context. Stellar noise produced by oscillations, granulation phenomena (granulation, mesogranulation, and supergranulation), and activity affects radial velocity measurements. The signature of the corresponding effect in radial velocity is small, around the meterper-second, but already too large for the detection of Earth-mass planets in habitable zones. Aims. We address the important role played by observational strategies in averaging out the radial velocity signature of stellar noise. We also derive the planetary mass detection limits expected in the presence of stellar noise. Methods. We start with HARPS asteroseismology measurements for four stars (β Hyi, α Cen A, μ Ara, and τ Ceti) available in the ESO archive and very precise measurements of α Cen B. This sample covers different spectral types from G2 to K1 and different evolutionary stages, from subgiant to dwarf stars. Since data span between 5 and 8 days, only stellar noise sources with timescales shorter than this time span will be extracted from these observations. Therefore, we are able to study oscillation modes and granulation phenomena without being significantly affected by activity noise present on longer timescales. For those five stars, we generate synthetic radial velocity measurements after fitting the corresponding models of stellar noise in Fourier space. These measurements allow us to study the radial velocity variation due to stellar noise for different observational strategies as well as the corresponding planetary mass detection limits. Results. Applying three measurements per night of 10 min exposure each, 2 h apart, seems to most efficiently average out the stellar noise considered. For quiet K1V stars such as α Cen B, this strategy allows us to detect planets of about three times the mass of Earth with an orbital period of 200 days, corresponding to the habitable zone of the star. Moreover, our simulations suggest that planets smaller than typically 5 M ⊕ can be detected with HARPS over a wide range of separations around most non-active solar-type dwarfs. Since activity is not yet included in our simulation, these detection limits correspond to a case, which exists, where the host star has few magnetic features and stellar noise is dominated by oscillation modes and granulation phenomena. For our star sample, a trend between spectral type and surface gravity and the level of radial velocity variation is also identified by our simulations.