Zeeman Doppler Imaging of ksi Boo A and B

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

Representative PEPSI spectra used in the study (panel a). Zeeman-Doppler images of ξ Boo A (panel b) and ξ Boo B (panel c) in orthographic projection. φ is the rotational phase. Magnetic field strength is color coded and identified in the right bars in units of Gauss. Positive polarity is depicted in red, negative polarity in blue. The length of the surface dashes is proportional to field strength.

Read more: K. G. Strassmeier, T. A. Carroll, & I. V. Ilyin 2023, A&A, in press (http://arxiv.org/abs/2305.07470)