Abstract
Observations of stable mainly dipolar magnetic fields at the surface of \~7\%
of single hot stars indicate that these fields are of fossil origin, i.e. they
descend from the seed field in the molecular clouds from which the stars were
formed. Recent results confort this theory. First, theoretical work and
numerical simulations confirm that the properties of the observed fields
correspond to those expected from fossil fields. They also showed that rapid
rotation does not modify the surfacic dipolar magnetic configurations, but
hinders the stability of fossil fields. This explains the lack of correlation
between the magnetic field properties and stellar properties in massive stars.
It may also explain the lack of detections of magnetic fields in Be stars,
which rotate close to their break-up velocity. In addition, observations by the
BinaMIcS collaboration of hot stars in binary systems show that the fraction of
those hosting detectable magnetic fields is much smaller than for single hot
stars. This could be related to results obtained in simulations of massive star
formation, which show that the stronger the magnetic field in the original
molecular cloud, the more difficult it is to fragment massive cores to form
several stars. Therefore, more and more arguments support the fossil field
theory.
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