* Astronomy

Title: A polarity reversal in the large-scale magnetic field of the fast-rotating Sun HD 190771
Authors: P. Petit, B. Dintrans, A. Morgenthaler, V. Van Grootel, J. Morin, J. Lanoux, M. Auriere, R. Konstantinova-Antova

Aims. We investigate the long-term evolution of the large-scale photospheric magnetic field geometry of the solar-type star HD 190771. With fundamental parameters very close to the Sun's except a shorter rotation period of 8.8 d, HD 190771 provides us with a first insight into the specific impact of the rotation rate in the dynamo generation of magnetic fields in 1 Msun stars.
Methods. We use circularly polarised, high-resolution spectra obtained with the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute cross-correlation line profiles with high signal-to-noise ratio in order to detect polarised Zeeman signatures. From three phase-resolved data sets collected during the summers of 2007, 2008 and 2009, we model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler imaging and follow its temporal evolution.
Results. The comparison of the magnetic maps reveals a polarity reversal of the axisymmetric component of the large-scale magnetic field between 2007 and 2008, this evolution being observed both in the poloidal and toroidal magnetic components. Between 2008 and 2009, another type of global evolution occurs, with a sharply decreased fraction of magnetic energy stored in the toroidal component. These changes are not accompanied by a significant evolution of the total photospheric magnetic energy. Using our spectra to perform radial velocity measurements, we also report the detection of a very low-mass stellar companion to HD 190771.

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