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The Kepler characterization of the variability among A- and F-type stars. I. General overview
Context. The Kepler spacecraft is providing time series of photometricdata with micromagnitude precision for hundreds of A-F type stars. Aims: We present a first general characterization of the pulsationalbehaviour of A-F type stars as observed in the Kepler light curves of asample of 750 candidate A-F type stars, and observationally investigatethe relation between ? Doradus (? Dor), ? Scuti(? Sct), and hybrid stars. Methods: We compile a databaseof physical parameters for the sample stars from the literature and newground-based observations. We analyse the Kepler light curve of eachstar and extract the pulsational frequencies using different frequencyanalysis methods. We construct two new observables, "energy" and"efficiency", related to the driving energy of the pulsation mode andthe convective efficiency of the outer convective zone, respectively. Results: We propose three main groups to describe the observedvariety in pulsating A-F type stars: ? Dor, ? Sct, andhybrid stars. We assign 63% of our sample to one of the three groups,and identify the remaining part as rotationally modulated/active stars,binaries, stars of different spectral type, or stars that show no clearperiodic variability. 23% of the stars (171 stars) are hybrid stars,which is a much higher fraction than what has been observed before. Wecharacterize for the first time a large number of A-F type stars (475stars) in terms of number of detected frequencies, frequency range, andtypical pulsation amplitudes. The majority of hybrid stars showfrequencies with all kinds of periodicities within the ? Dor and? Sct range, also between 5 and 10 d-1, which is achallenge for the current models. We find indications for the existenceof ? Sct and ? Dor stars beyond the edges of the currentobservational instability strips. The hybrid stars occupy the entireregion within the ? Sct and ? Dor instability strips andbeyond. Non-variable stars seem to exist within the instability strips.The location of ? Dor and ? Sct classes in the(Teff, log g)-diagram has been extended. We investigate twonewly constructed variables, "efficiency" and "energy", as a means toexplore the relation between ? Dor and ? Sct stars. Conclusions: Our results suggest a revision of the current observationalinstability strips of ? Sct and ? Dor stars and imply aninvestigation of pulsation mechanisms to supplement the ?mechanism and convective blocking effect to drive hybrid pulsations.Accurate physical parameters for all stars are needed to confirm thesefindings.
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