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

¹ University of Birmingham, School of Physics and Astronomy, Edgbaston, Birmingham B15 2TT, UK e-mail: saskia@bison.ph.bham.ac.uk
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁴ Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁵ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
⁶ Institute d'Astrophysique Spatiale, UMR 8617, Université Paris XI, Bâtiment 121, 91405 Orsay Cedex, France
⁷ LESIA, UMR8109, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France
⁸ Thüringer Landessternwarte, 07778 Tautenburg, Germany

Received: 16 February 2009
Accepted: 22 June 2009

Abstract

Context. 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 () in red giants and to search for a possible correlation between  and the large separation ().

Methods. Red giants with detectable solar-like oscillations are identified using both semi-automatic and manual procedures. For these stars, we determine  as the centre of a Gaussian fit to the oscillation power excess. For the determination of , we use the autocorrelation of the Fourier spectra, the comb response function and the power spectrum of the power spectrum.

Results. The resulting distribution shows a pronounced peak between 20-40 μHz. For about half of the stars we obtain  with at least two methods. The correlation between  and  follows the same scaling relation as inferred for solar-like stars.

Conclusions. The shape of the 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  and  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  does not correlate with  in the investigated frequency range, which confirms theoretical predictions.