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# Kepler observations of Am stars★

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(Impact Factor: 5.23). 06/2011; 414(1):792 - 800. DOI: 10.1111/j.1365-2966.2011.18454.x

ABSTRACT We present an analysis of high-resolution spectra for two pulsating Am stars in the Kepler field. The stellar parameters derived in this way are important because parameters derived from narrow-band photometry may be affected by the strong metal lines in these stars. We analyse the Kepler time series of ten known Am stars and find that six of them clearly show δ Scuti pulsations. The other four appear to be non-pulsating. We derive fundamental parameters for all known pulsating Am stars from ground-based observations and also for the Kepler Am stars to investigate the location of the instability strip for pulsating Am stars. We find that there is not much difference between the Am-star instability strip and the δ Scuti instability strip. We find that the observed location of pulsating Am stars in the HR diagram does not agree with the location predicted from diffusion calculations.

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Available from: L. A. Balona, May 29, 2015
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• ##### Article: Absolute dimensions of detached eclipsing binaries - III. The metallic-lined system YZ Cassiopeiae
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ABSTRACT: The bright binary system YZ Cassiopeiae is a remarkable laboratory for studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2 dwarf on a circular orbit, which undergo total and annular eclipses. We present an analysis of 15 published light curves and 42 new high-quality échelle spectra, resulting in measurements of the masses, radii, effective temperatures and photospheric chemical abundances of the two stars. The masses and radii are measured to 0.5 per cent precision: MA = 2.263 ± 0.012 M⊙, MB = 1.325 ± 0.007 M⊙, RA = 2.525 ± 0.011 R⊙ and RB = 1.331 ± 0.006 R⊙. We determine the abundance of 20 elements for the primary star, of which all except scandium are supersolar by up to 1 dex. The temperature of this star (9520 ± 120 K) makes it one of the hottest Am stars. We also measure the abundances of 25 elements for its companion (Teff = 6880 ± 240 K), finding all to be solar or slightly above solar. The photospheric abundances of the secondary star should be representative of the bulk composition of both stars. Theoretical stellar evolutionary models are unable to match these properties: the masses, radii and temperatures imply a half-solar chemical composition (Z = 0.009 ± 0.003) and an age of 490-550 Myr. YZ Cas therefore presents a challenge to stellar evolutionary theory.
Monthly Notices of the Royal Astronomical Society 12/2013; 438(1):2874-. DOI:10.1093/mnras/stt2229 · 5.23 Impact Factor
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##### Article: Spots on Am stars
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ABSTRACT: We investigate the light variations of 15 Am stars using four years of high-precision photometry from the Kepler spacecraft and an additional 14 Am stars from the K2 Campaign 0 field. We find that most of the Am stars in the Kepler field have light curves characteristic of rotational modulation due to star-spots. Of the 29 Am stars observed, 12 are δ Scuti variables and one is a γ Doradus star. One star is an eclipsing binary and another was found to be a binary from time delay measurements. Two Am stars show evidence for flares which are unlikely to be due to a cool companion. The fact that 10 out of 29 Am stars are rotational variables and that some may even flare strongly suggests that Am stars possess significant magnetic fields. This is contrary to the current understanding that the enhanced metallicity in these stars is due to diffusion in the absence of a magnetic field. The fact that so many stars are δ Scuti variables is also at odds with the prediction of diffusion theory. We suggest that a viable alternative is that the metal enhancement could arise from accretion.
Monthly Notices of the Royal Astronomical Society 03/2015; 448(2):1378-1388. DOI:10.1093/mnras/stv076 · 5.23 Impact Factor
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##### Article: Fundamental parameters of 8 Am stars: comparing observations with theory
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ABSTRACT: In this paper we present a detailed analysis of a sample of eight Am stars, four of them are in the {\it Kepler} field of view. We derive fundamental parameters for all observed stars, effective temperature, gravity, rotational and radial velocities, and chemical abundances by spectral synthesis method. Further, to place these stars in the HR diagram, we computed their luminosity. Two objects among our sample, namely HD\,114839 and HD\,179458 do not present the typical characteristic of Am stars, while for the others six we confirm their nature. The behavior of lithium abundance as a function of the temperature with respect the normal A-type stars has been also investigated, we do not find any difference between metallic and normal A stars. All the pulsating Am stars present in our sample (five out of eight) lies in the $\delta$~Sct instability strip, close to the red edge.
Monthly Notices of the Royal Astronomical Society 04/2014; 441(2). DOI:10.1093/mnras/stu674 · 5.23 Impact Factor