Characteristics of solar-like oscillations in red giants observed in the CoRoT exoplanet field

International audienceContext: Observations during the first long run (~150 days) in the exo-planet field of CoRoT increase the number of G-K giant stars for which solar-like oscillations are observed by a factor of 100. This opens the possibility to study the characteristics of their oscillations in a statistical sense. Aims: We aim to understand the statistical distribution of the frequencies of maximum oscillation power (nu_max) in red giants and to search for a possible correlation between nu_max and the large separation (Delta nu). Methods: Red giants with detectable solar-like oscillations are identified using both semi-automatic and manual procedures. For these stars, we determine nu_max as the centre of a Gaussian fit to the oscillation power excess. For the determination of Delta nu, we use the autocorrelation of the Fourier spectra, the comb response function and the power spectrum of the power spectrum. Results: The resulting nu_max distribution shows a pronounced peak between 20-40 muHz. For about half of the stars we obtain Delta nu with at least two methods. The correlation between nu_max and Delta nu follows the same scaling relation as inferred for solar-like stars. Conclusions: The shape of the nu_max distribution can partly be explained by granulation at low frequencies and by white noise at high frequencies, but the population density of the observed stars turns out to be also an important factor. From the fact that the correlation between Delta nu and nu_max for red giants follows the same scaling relation as obtained for sun-like stars, we conclude that the sound travel time over the pressure scale height of the atmosphere scales with the sound travel time through the whole star irrespective of evolution. The fraction of stars for which we determine Delta nu does not correlate with nu_max in the investigated frequency range, which confirms theoretical predictions