Magnetic fields of M dwarfs
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REVIEW ARTICLE
Magnetic fields of M dwarfs Oleg Kochukhov1 Received: 22 June 2020 / Accepted: 5 November 2020 Ó The Author(s) 2020
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. Keywords Stars: activity Stars: atmospheres Stars: interiors Stars: low mass Stars: magnetic field Stars: rotation Techniques: polarimetric Techniques: spectroscopic
Contents 1 2
Introduction............................................................................................................................... Methods of magnetic field measurements............................................................................... 2.1 Zeeman effect in spectral lines ....................................................................................... 2.2 Local Stokes parameter spectra ...................................................................................... 2.3 Disk-integrated Stokes parameters....................................................................................
& Oleg Kochukhov [email protected] 1
Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
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O. Kochukhov
2.4 Zeeman broadening and intensification ............................................................................ 2.5 Least-squares deconvolution ............................................................................................. 2.6 Zeeman Doppler imaging.................................................................................................. 2.7 Instrumentation for magnetic field measurements ........................................................... 3 Observations of M-dwarf magnetic fields................................................................................. 3.1 Total magnetic fields from intensity spectra .................................................................... 3.1.1 Results from detailed line
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