Magnetic fields in massive stars, their winds, and their nebulae

TL;DR

This review discusses recent progress in understanding magnetic fields in massive stars, their origins, observational evidence, theoretical models, and implications for stellar evolution and nebulae.

Contribution

It synthesizes observational data, theoretical explanations, and numerical simulations to provide a comprehensive overview of magnetic fields in massive stars.

Findings

Some O- and early B-type stars have confirmed surface magnetic fields.

Indirect evidence suggests many massive stars possess magnetic fields.

Magnetic fields influence stellar evolution, rotation, and mass-loss rates.

Abstract

Massive stars are crucial building blocks of galaxies and the universe, as production sites of heavy elements and as stirring agents and energy providers through stellar winds and supernovae. The field of magnetic massive stars has seen tremendous progress in recent years. Different perspectives -- ranging from direct field measurements over dynamo theory and stellar evolution to colliding winds and the stellar environment -- fruitfully combine into a most interesting and still evolving overall picture, which we attempt to review here. Zeeman signatures leave no doubt that at least some O- and early B-type stars have a surface magnetic field. Indirect evidence, especially non-thermal radio emission from colliding winds, suggests many more. The emerging picture for massive stars shows similarities with results from intermediate mass stars, for which much more data are available. Observations are often compatible with a dipole or low order multi-pole field of about 1 kG (O-stars) or 300 G to 30 kG (Ap / Bp stars). Weak and unordered fields have been detected in the O-star zeta-Ori A and in Vega, the first normal A-type star with a magnetic field. Theory offers essentially two explanations for the origin of the observed surface fields: fossil fields, particularly for strong and ordered fields, or different dynamo mechanisms, preferentially for less ordered fields. Numerical simulations yield the first concrete stable (fossil) field configuration, but give contradictory results as to whether dynamo action in the radiative envelope of massive main sequence stars is possible. Internal magnetic fields, which may not even show up at the stellar surface, affect stellar evolution as they lead to a more uniform rotation, with more slowly rotating cores and faster surface rotation. Surface metallicities may become enhanced, thus affecting the mass-loss rates.