X-ray Line Profiles of Magnetically Confined Hot-Star Winds

1
X-ray Line Profiles of Magnetically Confined
Hot-Star Winds Stephanie K. Tonnesen1, David H.
Cohen1, Stanley P. Owocki2, Asif ud-Doula2,3,
Marc Gagne4, Mary Oksala4 (1) Swarthmore
College, (2) Bartol Research Institute,
University of Delaware, (3) North Carolina State,
(4) West Chester University
Magnetic Hot Stars
Modeling Line Profiles
Line Profile Simulations
The magnetosphere periodically empties as
material falls back onto the star, leading to
more irregular structure in the X-ray emission
region
An 1100 G magnetic field has been detected on the
O7 V star, q1 Ori C (Donati et al. 2002). This
star is a strong X-ray source with very hard
X-rays (Schulz et al. 2000) modulated on the 16
day rotation period (Gagne et al. 1997). Based
on the Zeeman magnetic field detection, UV and
optical line strength variability, and X-ray
variability, the following picture has emerged of
the geometry of this stars circumstellar matter
We first explore line profiles from non-spherical
winds by considering a disk (opening angle 20)
with a purely radial outflow.
Below we show synthetic line profiles
post-processed from the MHD simulation snapshot
shown at the bottom of the previous column. We
show profiles as seen from 0 (pole-on), 45 and
90 (equator-on). For each view we show two
profiles including occultation by the star and
not including occultation. We also show the
corresponding line-of-sight velocity contours
(color plots on right), with the contours of hot
(Tgt106 K) plasma superimposed.
A different snapshot in the MHD simulation shows
a dense disk infall back onto the star
Pole-on view, 0
Density
Speed
Temperature
45 view
Line of Sight Velocities
Emission Line Profiles
Note that due to the tilt of the field and the
viewing inclination angle, there is a phase
dependence of the view with respect to the
magnetosphere. We indicate the four phases for
which we have Chandra observations.
Thin color bar at top and bottom of spectral line
profiles indicate the velocities that correspond
to a given location in the line.
NOTE expect strong viewing-angle dependence of
line-profile.
Scale in terms of UV-based vterm2500 km s-1
Equator-on view, 90
Initial MHD simulations isothermal, but large
shock at equator
New MHD simulations of magnetically confined
winds (ud-Doula and Owocki 2002) go beyond the
Babel and Montmerle model in that they allow for
the relaxation of the magnetic field structure
based on the kinetic energy of the wind flow.
Crucial parameter for magnetic confinement is
h, proportional to the ratio of the magnetic
energy density to wind kinetic energy
density. Note the the equator-ward flow in the
confined loops The shock velocity corresponds to
temperatures above 50 million K
The X-ray and Ha emission maxima, magnetic field
maximum, and UV absorption minimum occur at
phase0.0.
Babel and Montmerle proposed the Magnetically
Confined Wind Shock model (1997) to explain the
hard X-rays seen in q1 Ori C. In this model, a
radiation-driven wind is forced to flow along
magnetic field lines, leading to a collision of
oppositely directed flows at the magnetic
equator, and the associated shock heating of the
confined gas. This leads to an X-ray
magnetosphere of relatively slow moving hot gas.
MHD Simulations of q1 Ori C with energy equation
(radiative as well as adiabatic cooling)
Density
Speed
Temperature
In these calculations, the field is assumed to be
completely rigid. Note also, a post-shock
cooling disk forms in this steady-state model.
This disk may be a significant source of X-ray
opacity.
X-ray Lightcurve

• Conclusions
• Dynamic and quasi-steady hydrodynamics both lead
  to similar X-ray line profiles and overall
  emission properties.
• Emission lines are indeed very narrow, and
• They vary only slightly with phase/viewing angle.

X-ray Lightcurve

• Hot Star X-rays
• The standard model of hot star wind emission
  explains many X-ray observations of O stars (see
  Roban Kramers poster (11305)) Broad and
  asymmetric lines, due to Doppler-shifted emission
  from shock zones embedded in the radiation-driven
  winds, affected by continuum absorption in the
  cooler component of the wind.
• But observations of some hot stars, including q1
  Ori C, cannot be understood in the context of the
  standard wind shock model
• Lines are quite narrow.
• They are relatively unshifted and symmetric.
• The X-ray emission is hard.
• And modulated on the rotation period (such that
  occultation by the star seems to be the cause of
  reduced X-ray emission at certain phases).

Strong magnetic confinement of the wind, but
geometrically thick shock zone and relatively
little cooling disk
Overall X-ray flux synthesized from the same MHD
simulation snapshot. The dip at oblique viewing
angles is due to stellar occultation. Data from
four different Chandra observations is
superimposed green squares represent the
relative fluxes in the strong lines near 5 Å
while purple circles represent the relative
fluxes in the strong lines between 10 and 15 Å,
where absorption is likely to be more important.
These recently calculated models are specific to
q1 Ori C and include post-shock radiative
cooling. We post-process these simulations to
produce line profiles, as seen from arbitrary
viewing angles (corresponding to different
rotation phases), in the next columns to the
right. We calculate the emissivity at each grid
zone based on the simulation temperature and
density. The line-of-sight velocity determines
the wavelength of the emission. We take
occultation by the star into account but not
(currently) X-ray absorption by the cold wind
component. Note that these simulations show that
cooler post-shock gas periodically falls back
onto the star, and so a dense, opaque cooling
disk does not readily form. Furthermore, not all
of the hot gas is in the closed magnetic
structures, but rather in the interface between
the confined equatorial gas and the wind.
0
Poster available at astro.swarthmore.edu/cohen/po
sters/stephanie_aas.ppt, .jpg