The chromospherically active binary CF Tuc revisited

Abstract

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 analysed 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 a1 sin i= 0.0254 ± 0.0001 au, a2 sin i= 0.0228 ± 0.0001 au, M1 sin i= 0.902 ± 0.005 M⊙ and M2 sin i= 1.008 ± 0.006 M⊙. The cooler component of the system shows Hα and Ca ii H&K emissions. Using simultaneous spectroscopic and photometric observations, an anticorrelation 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 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.