Magnetic fields of M dwarfs

TL;DR

This paper reviews modern techniques for measuring magnetic fields in M dwarfs, highlighting their importance for stellar and planetary science, and discusses recent findings on magnetic field strength, topology, and underlying dynamo models.

Contribution

It provides a comprehensive overview of observational methods and recent results on M-dwarf magnetic fields, integrating empirical data with theoretical models.

Findings

Mean magnetic field strengths from Zeeman broadening

Global magnetic geometries from spectropolarimetric mapping

Complex, multi-scale magnetic field structures in M dwarfs

Abstract

Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields.