* Astronomy

Title: Long-term magnetic field monitoring of the Sun-like star Ksi Boo A
Authors: A. Morgenthaler, P. Petit, S. Saar, S.K. Solanki, M. Auriere, B. Dintrans, R. Fares, T. Gastine, J. Lanoux, F. Lignieres, S.C. Marsden, J. Morin, F. Paletou, J.C. Ramirez Velez, S. Theado, V. Van Grootel

Aims. We aim at investigating the long-term temporal evolution of the magnetic field of the solar-type star Ksi Boo A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods. We use 7 time-series of high-resolution, circularly-polarised spectra obtained with the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using about 6,100 photospheric spectral lines covering the visible domain, we employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile. We model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler Imaging and follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitor the width of several magnetically-sensitive spectral lines, the radial velocity and line asymmetry of intensity line profiles and the chromospheric emission in the cores of the Ca II H and Halpha lines.
Results. During the highest observed activity states, in 2007 and 2011, the large-scale field is almost axisymmetric and is strongly dominated by its toroidal component. This component persists with a constant polarity and carrying a significant fraction of the magnetic energy of the large-scale surface field at all observing epochs. The magnetic topologies reconstructed for these activity maxima are very similar, suggesting a form of short cyclicity in the large-scale field distribution. Correlated temporal evolutions, due to both rotational modulation and seasonal variability, are observed between the Ca II emission, the Halpha emission and the width of magnetically-sensitive lines. Whenever available, differential rotation measurements reveal a strong latitudinal shear in excess of 0.2 rad/d.

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