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Using data from the NASA spacecraft Kepler, we study solar-like oscillationsin red-giant stars in the open cluster NGC6811. We determine oscillationfrequencies, frequency separations, period spacings of mixed modes and modevisibilities for eight cluster giants. The oscillation parameters show thatthese stars are helium-core-burning red giants. The eight stars form two groupswith very different oscillation power spectra; the four stars with lowestDelta_nu-values display rich sets of mixed l=1 modes, while this is not thecase for the four stars with higher Delta_nu. For the four stars with lowestDelta_nu, we determine the asymptotic period spacing of the mixed modes,DeltaP, which together with the masses we derive for all eight stars suggestthat they belong to the so-called secondary clump. Based on the globaloscillation parameters, we present initial theoretical stellar modeling whichindicate that we can constrain convective-core overshoot on the main sequenceand in the helium-burning phase for these ~2M_sun stars. Finally, our resultsindicate less mode suppression than predicted by recent theories for magneticsuppression of certain oscillation modes in red giants.

Convective-core overshoot and suppression of oscillations: Constraints from red giants in NGC6811