Spectroscopic analysis of hot binaries. 2: The components of Y Cygni

Astronomy and Astrophysics – Astrophysics

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Early Stars, Eclipsing Binary Stars, Spectroscopic Analysis, Stellar Atmospheres, Stellar Evolution, Stellar Models, Stellar Orbits, Stellar Spectra, Astronomical Spectroscopy, Charge Coupled Devices, Helium, Hydrogen, Light Curve, Reflecting Telescopes, Spectrographs

Scientific paper

The blue spectra of the components of Y Cygni are analysed by means of non-local thermodynamic equilibrium (NLTE) model photospheres. The disentangling technique is used for the reconstruction of the individual spectra of the components of this well detached binary. This method makes it possible to determine the photospheric parameters of the binary components by means of spectral analysis with the same accuracy as for single stars. We obtained the following parameters for the primary/secondary: Teff = 34500/34200 +/- 600, log geff = 4.16/4.18 +/- 0.10, helium abundance by number fraction epsilonHe = 0.08/0.08 +/- 0.01, vr sin i = 142/138 +/- 2 km/s and the light ratio l = 0.928 +/- 0.011. The temperatures are significantly higher than those found in photometric studies. From their position in the log g-log Teff diagram and comparison with theoretical evolutionary tracks we conclude that both components of the system are very close to the zero age main sequence (ZAMS). The spectroscopic orbit is redetermined with improved accuracy. The orbit eccentricity is found to be e = 0.142 +/- 0.002 in agreement with results from light curve analysis. We determined radial velocity amplitudes of 238.9 +/- 0.5 km/s and 246.4 +/- 0.5 km/s corresponding to a mass ratio of q = 0.970 +/- 0.005. The best fit to the blue photometry of Magalashvili & Kumsishvili (1959) yields the inclination i = 86.37 deg +/- 0.12 deg and the relative radii. Combination of these values with the spectroscopic orbit allows the determination of absolute dimensions: M1 = 17.57 +/- 0.27 solar mass, M2 = 17.04 +/- 0.26 solar mass, and the polar radii Rp, 1 = 5.93 +/- 0.07 solar radius, Rp, 2 = 5.78 +/- 0.07 solar radius. The evolutionary age of Y Cygni as found by comparison of temperatures and gravities with evolutionary tracks is bracketed by the time scales for synchronization and circularization predicted by tidal theory.

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