379 research outputs found
Magnetically Confined Wind Shocks in X-rays - a Review
A subset (~ 10%) of massive stars present strong, globally ordered (mostly
dipolar) magnetic fields. The trapping and channeling of their stellar winds in
closed magnetic loops leads to magnetically confined wind shocks (MCWS), with
pre-shock flow speeds that are some fraction of the wind terminal speed. These
shocks generate hot plasma, a source of X-rays. In the last decade, several
developments took place, notably the determination of the hot plasma properties
for a large sample of objects using XMM-Newton and Chandra, as well as fully
self-consistent MHD modelling and the identification of shock retreat effects
in weak winds. Despite a few exceptions, the combination of magnetic
confinement, shock retreat and rotation effects seems to be able to account for
X-ray emission in massive OB stars. Here we review these new observational and
theoretical aspects of this X-ray emission and envisage some perspectives for
the next generation of X-ray observatories.Comment: accepted for publication by Advances in Space Research (special issue
"X-ray emission from hot stars and their winds"
The Effect of Magnetic Field Tilt and Divergence on the Mass Flux and Flow Speed in a Line-Driven Stellar Wind
We carry out an extended analytic study of how the tilt and
faster-than-radial expansion from a magnetic field affect the mass flux and
flow speed of a line-driven stellar wind. A key motivation is to reconcile
results of numerical MHD simulations with previous analyses that had predicted
non-spherical expansion would lead to a strong speed enhancement. By including
finite-disk correction effects, a dynamically more consistent form for the
non-spherical expansion, and a moderate value of the line-driving power index
, we infer more modest speed enhancements that are in good quantitative
agreement with MHD simulations, and also are more consistent with observational
results. Our analysis also explains simulation results that show the
latitudinal variation of the surface mass flux scales with the square of the
cosine of the local tilt angle between the magnetic field and the radial
direction. Finally, we present a perturbation analysis of the effects of a
finite gas pressure on the wind mass loss rate and flow speed in both spherical
and magnetic wind models, showing that these scale with the ratio of the sound
speed to surface escape speed, , and are typically 10-20% compared
to an idealized, zero-gas-pressure model.Comment: Accepted for publication in ApJ, for the full version of the paper go
to: http://www.bartol.udel.edu/~owocki/preprints/btiltdiv-mdotvinf.pd
Magnetically confined wind shock
Many stars across all classes possess strong enough magnetic fields to
influence dynamical flow of material off the stellar surface. For the case of
massive stars (O and B types), about 10\% of them harbour strong, globally
ordered (mostly dipolar) magnetic fields. The trapping and channeling of their
stellar winds in closed magnetic loops leads to {\it magnetically confined wind
shocks} (MCWS), with pre-shock flow speeds that are some fraction of the wind
terminal speed that can be a few thousand km s. These shocks generate
hot plasma, a source of X-rays. In the last decade, several developments took
place, notably the determination of the hot plasma properties for a large
sample of objects using \xmm\ and \ch, as well as fully self-consistent MHD
modelling and the identification of shock retreat effects in weak winds. In
addition, these objects are often sources of H emission which is
controlled by either sufficiently high mass loss rate or centrifugal breakout.
Here we review the theoretical aspects of such magnetic massive star wind
dynamics.Comment: Accepted for publication invited chapter of the Handbook of X-ray and
Gamma-ray Astrophysics published by Nature Springer. arXiv admin note: text
overlap with arXiv:1509.06482, arXiv:1605.0497
A Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars
We introduce a new Rigid-Field Hydrodynamics approach to modeling the
magnetospheres of massive stars in the limit of very-strong magnetic fields.
Treating the field lines as effectively rigid, we develop hydrodynamical
equations describing the 1-dimensional flow along each, subject to pressure,
radiative, gravitational, and centrifugal forces. We solve these equations
numerically for a large ensemble of field lines, to build up a 3-dimensional
time-dependent simulation of a model star with parameters similar to the
archetypal Bp star sigma Ori E. Since the flow along each field line can be
solved for independently of other field lines, the computational cost of this
approach is a fraction of an equivalent magnetohydrodynamical treatment.
The simulations confirm many of the predictions of previous analytical and
numerical studies. Collisions between wind streams from opposing magnetic
hemispheres lead to strong shock heating. The post-shock plasma cools initially
via X-ray emission, and eventually accumulates into a warped, rigidly rotating
disk defined by the locus of minima of the effective (gravitational plus
centrifugal) potential. But a number of novel results also emerge. For field
lines extending far from the star, the rapid area divergence enhances the
radiative acceleration of the wind, resulting in high shock velocities (up to
~3,000 km/s) and hard X-rays. Moreover, the release of centrifugal potential
energy continues to heat the wind plasma after the shocks, up to temperatures
around twice those achieved at the shocks themselves. Finally, in some
circumstances the cool plasma in the accumulating disk can oscillate about its
equilibrium position, possibly due to radiative cooling instabilities in the
adjacent post-shock regions.Comment: 21 pages, 12 figures w/ color, accepted by MNRA
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