The large-scale magnetic field of Proxima Centauri near activity maximum

Abstract

We report the detection of a large-scale magnetic field at the surface of the slowly rotating fully convective (FC) M dwarf Proxima Centauri. 10 circular polarization spectra, collected from 2017 April to July with the HARPS-Pol spectropolarimeter, exhibit rotationally modulated Zeeman signatures suggesting a stellar rotation period of 89.8 ± 4.0 d. Using Zeeman-Doppler Imaging, we invert the circular polarization spectra into a surface distribution of the large-scale magnetic field. We find that Proxima Cen hosts a large-scale magnetic field of typical strength 200 G, whose topology is mainly poloidal, and moderately axisymmetric, featuring, in particular, a dipole component of 135 G tilted at 51° to the rotation axis. The large-scale magnetic flux is roughly 3× smaller than the flux measured from the Zeeman broadening of unpolarized lines, which suggests that the underlying dynamo is efficient at generating a magnetic field at the largest spatial scales. Our observations occur ∼1 yr after the maximum of the reported 7 yr-activity cycle of Proxima Cen, which opens the door for the first long-term study of how the large-scale field evolves with the magnetic cycle in an FC very low mass star. Finally, we find that Proxima Cen's habitable zone planet, Proxima-b, is likely orbiting outside the Alfvèn surface, where no direct magnetic star-planet interactions occur.