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Post Info TOPIC: 16 Cygni
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Title: Asteroseismic modelling of 16 Cyg A & B using the complete Kepler data set
Author: Travis S. Metcalfe, Orlagh L. Creevey, Guy R. Davies

Asteroseismology of bright stars with well-determined properties from parallax measurements and interferometry can yield precise stellar ages and meaningful constraints on the composition. We substantiate this claim with an updated asteroseismic analysis of the solar-analogue binary system 16 Cyg A & B using the complete 30-month data sets from the Kepler space telescope. An analysis with the Asteroseismic Modelling Portal (AMP), using all of the available constraints to model each star independently, yields the same age (t=7.0±0.3 Gyr) and composition (Z=0.021±0.002, Y_i=0.25±0.01) for both stars, as expected for a binary system. We quantify the accuracy of the derived stellar properties by conducting a similar analysis of a Kepler-like data set for the Sun, and we investigate how the reliability of asteroseismic inference changes when fewer observational constraints are available or when different fitting methods are employed. We find that our estimates of the initial helium mass fraction are probably biased low by 0.02-0.03 from neglecting diffusion and settling of heavy elements, and we identify changes to our fitting method as the likely source of small shifts from our initial results in 2012. We conclude that in the best cases reliable stellar properties can be determined from asteroseismic analysis even without independent constraints on the radius and luminosity.

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Title: Asteroseismology of the solar analogs 16 Cyg A & B from Kepler observations
Authors: T.S. Metcalfe, W.J. Chaplin, T. Appourchaux, R.A. Garcia, S. Basu, I. Brandao, O.L. Creevey, S. Deheuvels, G. Dogan, P. Eggenberger, C. Karoff, A. Miglio, D. Stello, M. Yildiz, Z. Celik, H.M. Antia, O. Benomar, R. Howe, C. Regulo, D. Salabert, T. Stahn, T.R. Bedding, G.R. Davies, Y. Elsworth, L. Gizon, S. Hekker, S. Mathur, B. Mosser, S.T. Bryson, M.D. Still, J. Christensen-Dalsgaard, R.L. Gilliland, S.D. Kawaler, H. Kjeldsen, K.A. Ibrahim, T.C. Klaus, J. Li

The evolved solar-type stars 16 Cyg A & B have long been studied as solar analogues, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar properties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A & B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components respectively, including a clear detection of octupole (l=3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modelling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age (t=6.8±0.4 Gyr) and initial composition (Z_i=0.024±0.002, Y_i=0.25±0.01) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles.

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Title: Detailed Abundances of the Solar Twins 16 Cygni A and B: Constraining Planet Formation Models
Authors: S.C. Schuler, K. Cunha, V.V. Smith, L. Ghezzi, J.R. King, C.P. Deliyannis, A.M. Boesgaard

Results of a detailed abundance analysis of the solar twins 16 Cyg A and 16 Cyg B based on high-resolution, high signal-to-noise ratio echelle spectroscopy are presented. 16 Cyg B is known to host a giant planet while no planets have yet been detected around 16 Cyg A. Stellar parameters are derived directly from our high-quality spectra, and the stars are found to be physically similar, with Delta_T_eff = +43 K, Delta_logg = -0.02 dex, and Delta_xi = +0.10 km/s (in the sense of A - B), consistent with previous findings. Abundances of 15 elements are derived and are found to be indistinguishable between the two stars. The abundances of each element differ by <= 0.026 dex, and the mean difference is +0.003 ±0.015 (sigma) dex. Aside from Li, which has been previously shown to be depleted by a factor of at least 4.5 in 16 Cyg B relative to 16 Cyg A, the two stars appear to be chemically identical. The abundances of each star demonstrate a positive correlation with the condensation temperature of the elements (T_c); the slopes of the trends are also indistinguishable. In accordance with recent suggestions, the positive slopes of the [m/H]-T_c relations may imply that terrestrial planets have not formed around either 16 Cyg A or 16 Cyg B. The physical characteristics of the 16 Cyg system are discussed in terms of planet formation models, and plausible mechanisms that can account for the lack of detected planets around 16 Cyg A, the disparate Li abundances of 16 Cyg A and B, and the eccentricity of the planet 16 Cyg B b are suggested.

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