Monitoring of the magnetic field topology and activity of the core helium-burning giant β Ceti in the period 2007 - 2013

We present a second spectropolarimetric study dedicated to the single giant β Ceti. Adding new data to our previously published data, we are able to trace the evolution of the magnetic activity of the giant for the period from 2007 to 2013. We apply the same data reduction procedures and methods as in our earlier study, so we could reliably compare the results. We reconstruct a third magnetic map, applying the Zeeman Doppler imaging (ZDI) method. From our new observations we recover a dipolar magnetic topology similar to that obtained previously, supporting the long-term stability of the large-scale magnetic field of β Ceti. To visualise the magnetic field lines that extend above the surface, we perform field extrapolations using a potential field method. In contrast to the large-scale field, the behaviour of the activity indicators Hα, Ca II H&K and Ca II IRT suggests an evolution of the small-scale magnetic structures in the chromosphere. This study presents for the first time a long-term monitoring of the magnetic activity of a probable Ap star descendant, which confirms the stability of its large-scale magnetic field.