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

TL;DR

This study detects and maps the large-scale magnetic field of Proxima Centauri, revealing its strength, topology, and potential implications for its magnetic cycle and habitability of orbiting planets.

Contribution

First detailed magnetic field map of Proxima Centauri's large-scale surface field, linking magnetic topology with stellar activity and planetary environment.

Findings

Magnetic field strength around 200 G with a mainly poloidal topology.

Rotation period estimated at approximately 89.8 days.

Proxima-b orbits outside the Alfvén surface, limiting magnetic interactions.

Abstract

We report the detection of a large-scale magnetic field at the surface of the slowly-rotating fully-convective M dwarf Proxima Centauri. Ten circular polarization spectra, collected from April to July 2017 with the HARPS-Pol spectropolarimeter, exhibit rotationally-modulated Zeeman signatures suggesting a stellar rotation period of $89.8 \pm 4.0$ 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$^{\circ}$ to the rotation axis. The large-scale magnetic flux is roughly 3 times 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 $\sim$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 a fully-convective very-low-mass star. Finally, we find that Proxima Cen's habitable zone planet, Proxima-b, is likely orbiting outside the Alfv\`en surface, where no direct magnetic star-planet interactions occur.