134 research outputs found
The spin rate of pre-collapse stellar cores: wave-driven angular momentum transport in massive stars
The core rotation rates of massive stars have a substantial impact on the
nature of core-collapse supernovae and their compact remnants. We demonstrate
that internal gravity waves (IGW), excited via envelope convection during a red
supergiant phase or during vigorous late time burning phases, can have a
significant impact on the rotation rate of the pre-SN core. In typical () supernova progenitors, IGW may
substantially spin down the core, leading to iron core rotation periods . Angular momentum (AM) conservation during the
supernova would entail minimum NS rotation periods of . In most cases, the combined effects of magnetic torques and IGW
AM transport likely lead to substantially longer rotation periods. However, the
stochastic influx of AM delivered by IGW during shell burning phases inevitably
spin up a slowly rotating stellar core, leading to a maximum possible core
rotation period. We estimate maximum iron core rotation periods of in typical core-collapse supernova
progenitors, and a corresponding spin period of for newborn neutron stars. This is comparable to the typical birth
spin periods of most radio pulsars. Stochastic spin-up via IGW during shell
O/Si burning may thus determine the initial rotation rate of most neutron
stars. For a given progenitor, this theory predicts a Maxwellian distribution
in pre-collapse core rotation frequency that is uncorrelated with the spin of
the overlying envelope.Comment: Published in Ap
Magnetic spots on hot massive stars
Hot luminous stars show a variety of phenomena in their photospheres and
winds which still lack clear physical explanation. Among these phenomena are
photospheric turbulence, line profile variability (LPV), non-thermal emission,
non-radial pulsations, discrete absorption components (DACs) and wind clumping.
Cantiello et al. (2009) argued that a convection zone close to the stellar
surface could be responsible for some of these phenomena. This convective zone
is caused by a peak in the opacity associated with iron-group elements and is
referred to as the "iron convection zone" (FeCZ). Assuming dynamo action
producing magnetic fields at equipartition in the FeCZ, we investigate the
occurrence of subsurface magnetism in OB stars. Then we study the surface
emergence of these magnetic fields and discuss possible observational
signatures of magnetic spots. Simple estimates are made using the subsurface
properties of massive stars, as calculated in 1D stellar evolution models. We
find that magnetic fields of sufficient amplitude to affect the wind could
emerge at the surface via magnetic buoyancy. While at this stage it is
difficult to predict the geometry of these features, we show that magnetic
spots of size comparable to the local pressure scale height can manifest
themselves as hot, bright spots. Localized magnetic fields could be widespread
in those early type stars that have subsurface convection. This type of surface
magnetism could be responsible for photometric variability and play a role in
X-ray emission and wind clumping.Comment: Accepted for publication in A&
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