We have studied solar-like oscillations in ∼ 800 red-giant stars using Kepler long-cadence photometry. The sample includes stars ranging in evolution from the lower part of the red-giant branch to the Helium main sequence. We investigate the relation between the large frequency separation (∆ν) and the frequency of maximum power (ν max ) and show that it is different for red giants than for mainsequence stars, which is consistent with evolutionary models and scaling relations. The distributions of ν max and ∆ν are in qualitative agreement with a simple stellar population model of the Kepler field, including the first evidence for a secondary clump population characterized by M 2 M ⊙ and ν max ≃ 40−110 µHz. We measured the small frequency separations δν 02 and δν 01 in over 400 stars and δν 03 in over 40. We present C-D diagrams for l = 1, 2 and 3 and show that the frequency separation ratios δν 02 /∆ν and δν 01 /∆ν have opposite trends as a function of ∆ν. The data show a narrowing of the l = 1 ridge towards lower ν max , in agreement with models predicting more efficient mode trapping in stars with higher luminosity. We investigate the offset ǫ in the asymptotic relation and find a clear correlation with ∆ν, demonstrating that it is related to fundamental stellar parameters. Finally, we present the first amplitude-ν max relation for Kepler red giants. We observe a lack of low-amplitude stars for ν max 110 µHz and find that, for a given ν max between 40 − 110 µHz, stars with lower ∆ν (and consequently higher mass) tend to show lower amplitudes than stars with higher ∆ν.

We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Δν) and the frequency of maximum power (ν max ). We focus on a sample of 50 low-luminosity stars (ν max > 100 μHz, L 30 L ) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shellburning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a singleéchelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of δν 02 versus Δν. The small separation δν 01 of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.

Oscillating stars in binary systems are among the most interesting stellar laboratories, as these can provide information on the stellar parameters and stellar internal structures. Here we present a red giant with solar-like oscillations in an eclipsing binary observed with the NASA Kepler satellite. We compute stellar parameters of the red giant from spectra and the asteroseismic mass and radius from the oscillations. Although only one eclipse has been observed so far, we can already determine that the secondary is a main-sequence F star in an eccentric orbit with a semi-major axis larger than 0.5 AU and orbital period longer than 75 days.

Context. The large number of stars for which uninterrupted high-precision photometric timeseries data are being collected with Kepler and CoRoT initiated the development of automated methods to analyse the stochastically excited oscillations in main-sequence, subgiant and red-giant stars. Aims. We investigate the differences in results for global oscillation parameters of G and K red-giant stars due to different methods and definitions. We also investigate uncertainties originating from the stochastic nature of the oscillations. Methods. For this investigation we use Kepler data obtained during the first four months of operation. These data have been analysed by different groups using already published methods and the results are compared. We also performed simulations to investigate the uncertainty on the resulting parameters due to different realizations of the stochastic signal. Results. We obtain results for the frequency of maximum oscillation power (ν max ) and the mean large separation ( Δν ) from different methods for over one thousand red-giant stars. The results for these parameters agree within a few percent and seem therefore robust to the different analysis methods and definitions used here. The uncertainties for ν max and Δν due to differences in realization noise are not negligible and should be taken into account when using these results for stellar modelling.

We provide an overview of stellar variability in the first quarter of data from the Kepler mission. The intent of this paper is to examine the entire sample of over 150,000 target stars for periodic behavior in their lightcurves, and relate this to stellar characteristics. These data constitute an unprecedented study of stellar variability given its great precision and complete time coverage (with a half hour cadence). Because the full Kepler pipeline is not currently suitable for a study of stellar variability of this sort, we describe our procedures for treating the "raw" pipeline data. About half of the total sample exhibits convincing periodic variability up to two weeks, with amplitudes ranging from differential intensity changes less than 10 −4 up to more than 10 percent. K and M dwarfs have a greater fraction of period behavior than G dwarfs. The giants in the sample have distinctive quasi-periodic behavior, but are not periodic in the way we define it. Not all periodicities are due to rotation, and the most significant period is not necessarily the rotation period. We discuss properties of the lightcurves, and in particular look at a sample of very clearly periodic G dwarfs. It is clear that a large number of them do vary because of rotation and starspots, but it will take further analysis to fully exploit this.