Zeeman Doppler imaging ofξBoo A and B

Abstract

Aims.We present a magnetic-field surface map for both stellar components of the young visual binaryξBoo AB (A: G8V, B: K5V).Methods.We employed high-resolution Stokes-Vspectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT). We inverted StokesVline profiles with ouriMAP 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-Vprofiles. Our mapping is accompanied by a residual bootstrap error analysis.Results.We constructed magnetic flux densities of the radial field component forξBoo A andξBoo B of up to plus or −115±5 G and 55±3 G, respectively. The magnetic morphology ofξBoo 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 ofξBoo B is characterized by four low-to-mid-latitude spots of mixed polarity. No polar magnetic field is reconstructed for the coolerξBoo B star. Both our maps are dominated by the radial field component, containing 86% and 89% of the magnetic energy ofξBoo 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 forξBoo 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 forξBoo B.