Long-term magnetic field monitoring of the Sun-like star \xi Bootis A

TL;DR

This study tracks the magnetic field evolution of the Sun-like star  Bootis A over several years using spectropolarimetry, revealing cyclic magnetic patterns, activity correlations, and differential rotation characteristics.

Contribution

It provides the first long-term magnetic topology monitoring of  Bootis A, combining Zeeman-Doppler Imaging with activity indicators to analyze magnetic cycles and differential rotation.

Findings

Magnetic field is axisymmetric with dominant toroidal component during activity maxima.

Magnetic topologies at activity peaks are similar, indicating possible short cycles.

Correlated variability observed between magnetic activity indicators and spectral line widths.

Abstract

Aims. We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star \xi Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods. We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6,100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale photospheric magnetic field of the star was then modelled by means of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitored the width of several magnetically sensitive spectral lines, the radial velocity, the 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 of \xi Boo A is almost completely axisymmetric and is dominated by its toroidal component. 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 evolution, due to both rotational modulation and seasonal variability, is observed between the Ca II emission, the Halpha emission and the width of magnetically sensitive lines. When measurable, the differential rotation reveals a strong latitudinal shear in excess of 0.2 rad/d.