Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation, ∆ν, and the frequency of maximum oscillation power, ν max . We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

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We present the first asteroseismic results for δ Scuti and γ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilize the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung–Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, γ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74 per cent for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young δ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

One year of continuous Kepler Space Telescope data for the central object of planetary nebula NGC 6826 (KIC 12071221) were analyzed. KIC 12071221 has long-cadence light curves with stochastic brightness variations and broadband power spectra. The exception is a one-month long period during which variability was detected with a period of 14.8 hr. In search of the origin of this periodicity and in an attempt to better understand the complex NGC 6826 system, long-cadence KIC 12071221 data were analyzed using nonlinear time series methods. Nonlinear projective noise reduction from a phase-space dimension of six lowered the tail of the power spectra at the higher frequencies by factors of 38-133, an unexpected result for light curves with no obvious dominant frequencies. Even more notable is the identification of a structured distribution of trajectories, or coherence, in the reconstructed phase space for vectors spanning an interval of 1.5 hr. Organization weakens as the delay increases and is lost for phase-space vectors spanning times longer than 10 hr. This may be indicative of a binary object which shares a common envelope and/or pulsations but for a complete understanding one or both may have to be combined with wind structure. Thus, an estimate is made of the timescale of the response to what we propose is stochastic driving. This estimate of the timescale of the organized response is made by monitoring the loss of structure in the distribution of trajectories in phase space, a novel application of nonlinear methodology that may be used on data sampled with any sampling time.