2011A&A...535A..98S

Multi-wavelength time-series observations with high cadence and long duration are needed to resolve and understand the many variations of magnetically active late-type stars, which is an approach often used to observe the Sun. We present a first and detailed study of the bright and active K0IV-III star HD 123351.We acquired a total of 955 high-resolution STELLA echelle spectra during the years 2006-2010 and a total of 2260 photometric VI_C data points during 1998-2010. These data are complemented by some spectra from CFHT and KPNO.The star is found to be a single-lined spectroscopic binary with a period of 147.8919±0.0003 days and a large eccentricity of e=0.8086±0.0001. The rms of the orbital solution is just 47m/s, making it the most precise orbit ever obtained for an active binary system. The rotation period is constrained from long-term photometry to be 58.32±0.01 days. It shows that HD 123351 is a very asynchronous rotator, rotating five times slower than the expected pseudo-synchronous value. Two spotted regions persisted throughout the 12 years of our observations. We interpret them as active longitudes on a differentially rotating surface with a ΔP/P of 0.076. Four years of Hα, CaII H&K and HeI D3 monitoring identifies the same main periodicity as the photometry but dynamic spectra also indicate that there is an intermittent dependence on the orbital period, in particular for Caii H&K in 2008. Line-profile inversions of a pair of Zeeman sensitive/insensitive iron lines yield an average surface magnetic-flux density of 542±72G. The time series for 2008 is modulated by the stellar rotation as well as the orbital motion, such that the magnetic flux is generally weaker during times of periastron and that the chromospheric emissions vary in anti-phase with the magnetic flux. We also identify a broad and asymmetric lithium line profile and measure an abundance of logn(Li)=1.70±0.05. The star's position in the H-R diagram indicates a mass of 1.2±0.1M_☉ and an age of 6-7Gyr. We interpret the anti-phase relation of the magnetic flux with the chromospheric emissions as evidence that there are two magnetic fields present at the same time, a localized surface magnetic field associated with spots and a global field that is oriented towards the (low-mass) secondary component. We suggest that the inter-binary field is responsible for the magnetic-flux dilution at periastron. It is also likely to be responsible for the unexpected slow and asynchronous rotation of the primary star.