Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination

Context. Observations and analysis of solar-type oscillations in red giant stars is an infant field of asteroseismic research with a number of open questions. Although stochastic oscillations have been firmly detected in red giants, both in radial velocity and photometric time series, the mostly too short or incomplete data sets complicate a detailed analysis. The french-led satellite CoRoT induced a breakthrough in observing p-mode oscillations in red giants. We have analyzed continuous photometric time series of about 11 400 relatively faint stars obtained in the exofield of CoRoT during the first 150 days long-run campaign from May to October 2007. Among them, we found more than 300 stars showing a clear power excess in a frequency and amplitude range where it can be expected for red giant pulsators. In this paper we present first results on a sub-sample of these stars. Aims. The knowledge of reliable fundamental parameters is essential for detailed asteroseismic studies of red giant stars. As the CoRoT exofield targets are relatively faint (11–16 mag) no or only weak constraints can be set on their location in the HR-diagram. We therefore aim to extract information about fundamental parameters from the available time series alone. Methods. We model the convective background noise and the power excess hump due to pulsation with a multi-component fit and deduce reliable estimates for the stellar mass and radius from scaling relations for the frequency of maximum power and the characteristic frequency separation. Results. We provide a simple method to estimate stellar mass and radius for stars showing solar-type oscillations and test our approach for a number of well-known solar-type pulsators. We apply the method to our sample of CoRoT red giants and provide their mass and radius as a starting point for a more detailed model analysis.