2011A&A...529A.100S

Arcturus is the second closest K giant and among the brightest stars in the sky. It has not been found to have a magnetic field, even though CaII H&K lines as activity indicators imply that Arcturus is magnetically active. We measure the mean longitudinal magnetic field strengths and interpret them in terms of an intraseasonal activity modulation. We apply our new Zeeman component decomposition (ZCD) technique to three single sets of Stokes I and V spectra to measure the longitudinal component of the magnetic field responsible for tiny Zeeman signatures detected in spectral line profiles. For two of the spectra, we report the detection of the Zeeman signature of a weak longitudinal magnetic field of 0.65±0.26 G and 0.43±0.16 G. The third measurement is less significant, but all the measurements closely reproduce a rotationally modulated activity cycle with four active longitudes. For the first time, a magnetic field on Arcturus is directly detected. This field can be attributed to a diminishing solar-type αΩ-dynamo acting in the deepening convection zone of Arcturus. We demonstrate that our new method ZCD lowers the detection limit of very weak magnetic fields from spectropolarimetric measurements.