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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.
| Characterization of Kepler early-type targets *
Context. Stellar pulsation offers a unique opportunity to constrain theintrinsic parameters of stars and unveil their inner structure. TheKepler satellite is collecting an enormous amount of data of unprecedentphotometric precision, which will allow us to test theory and obtain avery precise tomography of stellar interiors. Aims: We attempt todetermine the stars' fundamental parameters (Teff, log g, vsin i, and luminosity) needed for computing asteroseismic models andinterpreting Kepler data. We report spectroscopic observations of 23early-type Kepler asteroseismic targets, 13 other stars in the Keplerfield, that had not been selected to be observed. Methods: Wemeasured the radial velocity by performing a cross-correlation withtemplate spectra to help us identify non-single stars. Spectralsynthesis was performed to derive the stellar parameters of our targetstars, and the state-of-the-art LTE atmospheric models were computed.For all the stars of our sample, we derived the radial velocity,Teff, log g, v sin i, and luminosities. For 12 stars, weperformed a detailed abundance analysis of 20 species, for 16, we couldderive only the [Fe/H] ratio. A spectral classification was alsoperformed for 17 stars in the sample. Results: We identify twodouble-lined spectroscopic binaries, HIP 96299 and HIP 98551, the formerof which is an already known eclipsing binary, and two single-linedspectroscopic binaries, HIP 97254 and HIP 97724. We also report twosuspected spectroscopic binaries, HIP 92637 and HIP 96762, and thedetection of a possible variability in the radial velocity of HIP 96277.Two of our program stars are chemically peculiar, namely HIP 93941,which we classify as B2 He-weak, and HIP 96210, which we classify as B6Mn. Finally, we find that HIP 93522, HIP 93941, HIP 93943, HIP 96210 andHIP 96762, are very slow rotators (v sin i < 20 km s-1)which makes them very interesting and promising targets forasteroseismic modeling.Based on observations collected with the telescope at the M.G.Fracastoro station of the INAF - Osservatorio Astrofisico diCatania.Reduced spectra are only available in electronic form atthe CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/517/A3
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Observation and Astrometry data
| Constellation: | Cygnus |
| Right ascension: | 19h34m36.46s |
| Declination: | +47°50'14.1" |
| Apparent magnitude: | 9.933 |
| Proper motion RA: | -3.3 |
| Proper motion Dec: | -17.3 |
| B-T magnitude: | 10.282 |
| V-T magnitude: | 9.962 |
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