92 research outputs found
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
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"
Diving into the magnetosphere of the Of?p star HD108
We analyse optical and X-ray spectroscopy of the Of?p star HD108, known for
its strong dipolar magnetic field and its optical line profile variability with
a timescale of yrs, interpreted as the stellar rotation period.
Optical emission lines have now recovered from their minimum emission state
reached in 2007 - 2008. The variations of the equivalent width of the H
emission provide constraints on the inclination of the rotation axis () and
the obliquity of the magnetic axis (). The best agreement between model
and observations is found for (, ) pairs with and . The Balmer emission lines
display stochastic variability at the % level on timescales of a few
days. TESS photometry unveils transient modulations on similar timescales in
addition to prominent red noise variations. A Chandra X-ray observation of
December 2021, when the star was at a higher emission level, indicates a slight
increase of the flux and a spectral hardening compared to the August 2002
XMM-Newton observation, taken near minimum emission state. Magnetohydrodynamic
simulations are used to compute synthetic X-ray spectra. With our current best
estimate of the mass-loss rate, the simulated X-ray luminosity
and spectral energy distribution agree very well with the observations.
Finally, the radial velocities vary on a period of 8.5 years with a
peak-to-peak amplitude of 10 - 11 km s, suggesting orbital motion with
an unseen companion of at least 4 M.Comment: Accepted for publication in MNRA
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