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The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models for Active and Inactive Solar-Type Stars
We present spectroscopic and photometric observations of the eclipsingsystem V1061 Cyg (P=2.35 days). A third star is visible in the spectrum,and the system is a hierarchical triple. We combine the radialvelocities for the three stars, times of eclipse, and intermediateastrometric data from the Hipparcos mission (abscissa residuals) toestablish the elements of the outer orbit, which is eccentric and has aperiod of 15.8 yr. We determine accurate values for the masses, radii,and effective temperatures of the binary components:MAa=1.282+/-0.015 Msolar,RAa=1.615+/-0.017 Rsolar, andTAaeff=6180+/-100 K for the primary (star Aa), andMAb=0.9315+/-0.0068 Msolar,RAb=0.974+/-0.020 Rsolar, andTAbeff=5300+/-150 K for the secondary (Ab). Themass of the tertiary is determined to be MB=0.925+/-0.036Msolar and its effective temperature isTBeff=5670+/-150 K. Current stellar evolutionmodels agree well with the properties of the primary but show a verylarge discrepancy in the radius of the secondary, in the sense that thepredicted values are ~10% smaller than observed (a ~5 ? effect).In addition, the temperature is cooler than predicted, by some 200 K.These discrepancies are quite remarkable given that the star is only 7%less massive than the Sun, the calibration point of all stellar models.We identify the chromospheric activity as the likely cause of theeffect. Inactive stars agree very well with the models, while activeones such as V1061 Cyg Ab appear systematically too large and too cool.
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Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1957ApJ...125..195N&db_key=AST
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