The chromospherically–active binary CF Tuc revisited

This paper presents results derived from analysis of new spectroscopic and photometric observations of the chromospherically active binary system CF Tuc. New high-resolution spectra, taken at the Mt. John University Observatory in 2007, were analyzed using two methods: cross-correlation and Fourier–based disentangling. As a result, new radial velocity curves of both components were obtained. The resulting orbital elements of CF Tuc are: a1sini=0.0254±0.0001 AU, a2sini=0.0228±0.0001 AU, M1sini=0.902±0.005 M⊙, and M2sini=1.008 ± 0.006 M⊙. The cooler component of the system shows Hα and CaII H & K emissions. Using simultaneous spectroscopic and photometric observations, an anti-correlation between the Hα emission and the BV light curve maculation effects was found. This behaviour indicates a close spatial association between photospheric and chromospheric active regions. Our spectroscopic data and recent BV light curves were solved simultaneously using the Wilson-Devinney code. A dark spot on the surface of the cooler component was assumed to explain large asymmetries observed in the light curves. The following absolute parameters of the components were determined: M1=1.11± 0.01 M⊙, M2=1.23± 0.01 M⊙, R1=1.63± 0.02 R⊙, R2=3.60± 0.02 R⊙, L1=3.32± 0.51 L⊙ and L2=3.91± 0.84 L⊙. The primary component has an age of about 5 Gyr and is approaching its Main Sequence terminal age. The distance to CF Tuc was calculated to be 89 ± 6 pc from the dynamic parallax, neglecting interstellar absorption, in agreement with 2 D. Doğru, A. Erdem, S. S. Doğru, S. Zola the Hipparcos value. The orbital period of the system was studied using the O − C analysis. The O − C diagram could be interpreted in terms of either two abrupt changes or a quasi-sinusoidal form superimposed on a downward parabola. These variations are discussed by reference to the combined effect of mass transfer and mass loss, the Applegate mechanism and also a light-time effect due to the existence of a third body in the system.