Zeeman Doppler imaging of ξ Boo A and B

Aims: We present a magnetic-field surface map for both stellar components of the young visual binary ξ Boo AB (A: G8V, B: K5V).
Methods: We employed high-resolution Stokes-V spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT). We inverted Stokes V line profiles with our iMAP software and compared them with previous inversions. We employed an iterative regularization scheme without the need for a penalty function and incorporated a three-component description of the surface magnetic-field vector. The spectral resolution of our data is 130 000 (0.040-0.055 Å) and we obtain a signal-to-noise ratio (S/N) of up to 3000 per pixel depending on wavelength. We used a singular-value decomposition (SVD) of a total of 1811 spectral lines to average Stokes-V profiles. Our mapping is accompanied by a residual bootstrap error analysis.
Results: We constructed magnetic flux densities of the radial field component for ξ Boo A and ξ Boo B of up to plus or -115±5 G and 55±3 G, respectively. The magnetic morphology of ξ Boo A is characterized by a very high latitude, nearly polar spot of negative polarity and three low-to-mid-latitude spots of positive polarity, while that of ξ Boo B is characterized by four low-to-mid-latitude spots of mixed polarity. No polar magnetic field is reconstructed for the cooler ξ Boo B star. Both our maps are dominated by the radial field component, containing 86% and 89% of the magnetic energy of ξ Boo A and B, respectively. We find only weak azimuthal and meridional field densities on both stars (plus or -15-30 G), about a factor two weaker than what was seen previously for ξ Boo A. The phase averaged longitudinal field component and dispersion is +4.5±1.5 G for ξ Boo A and -5.0±3.0 G for ξ Boo B.