Magnetic Fields in Massive Stars, their Winds, and their Nebulae

Rolf Walder (1), Doris Folini (1), Georges Meynet (2)

(1) CRAL, ENS-Lyon, France
(2) Observatoire de Genève, Switzerland

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.

Reference: Accepted to Space Science Review
Status: Manuscript has been accepted