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 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 a(1) sin i = 0.0254 +/- 0.0001 au, a(2) sin i = 0.0228 +/- 0.0001 au, M-1 sin i = 0.902 +/- 0.005 M-circle dot and M-2 sin i = 1.008 +/- 0.006 M-circle dot. The cooler component of the system shows H alpha and Ca ii H&K emissions. Using simultaneous spectroscopic and photometric observations, an anticorrelation between the H alpha 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: M-1 = 1.11 +/- 0.01 M-circle dot, M-2 = 1.23 +/- 0.01 M-circle dot, R-1 = 1.63 +/- 0.02 R-circle dot, R-2 = 3.60 +/- 0.02 R-circle dot, L-1 = 3.32 +/- 0.51 L-circle dot and L-2 = 3.91 +/- 0.84 L-circle dot. 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.