Xray Line Profiles of Magnetically Confined Wind Shock Models

1
X-ray Line Profiles of Magnetically Confined Wind
Shock Models Stephanie K. Tonnesen1, David H.
Cohen1,2, Roban H. Kramer1,2, Stanley P. Owocki3,
Asif ud-Doula3 (1) Swarthmore College, (3)
Bartol Research Institute, University of Delaware
Lightcurves
Hot Star Winds
Modeling Simulation Output
Azimuthally Symmetric Model with Radial Outflow
Now we examine the total emission observed as a
function of rotation phase. Using analytic models
with dipole magnetic field geometry, we
calculated these lightcurves. The addition to
our original model is a cooling disk in the
equatorial plane that absorbs x-rays. This
absorption is due to photoionization of the less
ionized atoms that constitute the cool gas.
To improve on the static field assumption of
Babel and Montmerle, ud-Doula and Owocki (ApJ
2002) calculated numerical magnetohydrodynamic
models. These simulations are time-dependent,
with the magnetic fields and the outflow
interacting dynamically.
These are line profiles of analytic models of
equatorially enhanced x-ray emitting flows. This
model emits only in a region 20o above and below
the magnetic equator.

• Hot stars have massive, highly supersonic
  radiation driven winds
• Observed in UV absorption lines
• Velocities of order few 1000 km s-1
• The standard model of hot star wind emission
  explains many x-ray observations (see Roban
  Kramers poster). However, there are some stars
  that cannot be explained by this model. Their
  anomalous properties include
• X-ray spectra are very hard because of the high
  temperatures produced in the wind.
• The strong x-ray emission levels means that a
  lot of photons are emitted at these short
  wavelengths.
• The x-ray properties change with rotational
  period.
• These facts point to the Magnetically Confined
  Wind Shock Model (MCWS)
• The magnetic field lines channel the fast-moving
  gas towards the equator where the collisions can
  cause a velocity jump large enough for hard
  x-rays.
• The field also confines the wind so that the hot
  material stays dense enough to produce strong
  emission lines.
• For a tilted dipole field, the observer views the
  star from a perspective that changes throughout
  one rotational period.

• The viewing angle affects the appearance of the
  line profile
• As it increases (from the pole to the equator),
  the doppler shift of the photons from the disk
  increases to produce wider profiles.
• Occultation of emitting gas also increases with
  viewing angle, and note that the star always
  occults the gas traveling with the most redshift
  (the gas traveling directly away from the
  observers line of sight).

MHD simulations of magnetic wind shock scenario
90o Equator-on
0o Pole-on
45o
(ud-Doula Owocki 2002)
This model assumes a strong (kG) large-scale
dipole field. The velocity plot shows strong
winds traveling towards each other into a dense
collisional plane at the equator. The velocity
jump is large enough to heat the wind to many 107
K.
The densities and velocities for this profile
were calculated by an MHD simulation. We assumed
that the hot gas in this simulation was confined
to a volume outside a radius of 1.5 Rstar and
within ten degrees above or below the equatorial
plane. There is no stellar occultation in these
profiles.
Models with both a Radial and Azimuthal Velocity
We next considered models with an azimuthal
velocity component. This model has the same
geometry of emission as the previous model, but
with a keplerian azimuthal velocity in addition
to the radial outflow.
In the last few years, magnetic fields have been
directly detected on some hot stars. This
motivated the original MCWS model by Babel and
Montmerle in 1997.
90o Equator-on
80o
The stellar material is accelerating off the star
along the fixed field lines and then colliding at
the equator, forming a strong shock. This heats
the gas enough to emit x-rays (emitting area is
shaded).
0o Pole-on
45o
50o
0o Pole-on
0