Analysis of Doppler-Broadened X-ray Emission Line Profiles from Hot Stars

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Analysis of Doppler-Broadened X-ray Emission Line
Profiles from Hot Stars
David Cohen - Swarthmore College with Roban
Kramer - Swarthmore College Stanley Owocki -
Bartol Research Institute
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Outline
0. The astrophysical context I.
Introduction What line profiles can tell us II.
The basic model III. Fitting Chandra data from
hot stars - z Pup Constraining parameters IV.
What the data are telling us Integration with
other X-ray spectral diagnostics
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What produces hot-star X-rays?
Hot stars are thought not to have convective
envelopes, magnetic activity, or coronae
Hot stars have massive radiation-driven winds,
with a significant amount of continuum opacity
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What Line Profiles Can Tell Us
The wavelength of an emitted photon is
proportional to the line-of-sight velocity Line
shape maps emission measure at each
velocity/wavelength interval Continuum absorption
by the cold stellar wind affects the line
shape Correlation between line-of-sight velocity
and absorption optical depth will cause
asymmetries in emission lines
X-ray line profiles can provide the most direct
observational constraints on the X-ray production
mechanism in hot stars
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Emission Profiles from a Spherically Symmetric,
Expanding Medium
A spherically-symmetric, X-ray emitting wind can
be built up from a series of concentric shells.
Occultation by the star removes red photons,
making the profile asymmetric
A uniform shell gives a rectangular profile.
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Continuum Absorption Acts Like Occultation
Red photons are preferentially absorbed, making
the line asymmetric The peak is shifted to the
blue, and the red wing becomes much less steep.
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We calculate line profiles using a 4-parameter
model
3 parameters describe the spatial and velocity
distribution of the emission Ro is the minimum
radius of X-ray emission b describes the
acceleration of the wind q parameterizes the
radial dependence of the filling factor. 1
parameter, t, describes the level of continuum
absorption in the overlying wind.
A wind terminal velocity is assumed based on UV
observations, and the calculated line profile is
convolved with the appropriate instrument-response
function for each line.
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In addition to the wind-shock model,
our empirical line profile model can also
describe a corona
With most of the emission concentrated near the
photosphere and with very little acceleration,
the coronal line profiles are very narrow.
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t?1,2,8
A wide variety of wind-shock characteristics can
be modeled
Ro1.5
Line profiles change in characteristic ways with
t and Ro, becoming broader and more skewed with
increasing t and broader and more flat-topped
with increasing Ro.
Ro3
Ro10
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The X-ray lines in O stars are observed to be
broad z Pup is the prototypical O supergiant
with a strong wind
Ne X
Fe XVII
N VII
O VIII
We fit six lines in the Chandra MEG spectrum of z
Pup
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For each line, we are able to achieve a good fit
with reasonable model parameters
blend
Best-fit model t?1.0, Ro1.4, q-0.4, with b1
fixed
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We also determine the extent of the confidence
limits within the model parameter space Note
how the line profile changes with increasing wind
opacity
68 95 99
t? increasing ?
t? increasing ?
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The fitted lines span a range of wind optical
depth and X-ray temperature
The Fe XVII line at 15 Å (left) has a more
typical profile, while the N VII (right) is more
flat-topped and broad. And despite having a
longer wavelength, it doesnt suffer a lot of
attenuation.
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The confidence regions define the widest possible
variation among acceptable models
best fit model
lowest t?
highest t?
The best fit and two other acceptable (at the 95
confidence level) fits
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The best-fit parameters and 95 confidence limits
are derived for all six lines
?The formation radii for all lines are close to
the surface of the star
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? very little radial dependence of the X-ray
filling factor
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?Wind optical depth is only moderate, and ? only
varies weakly with wavelength
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• Discussion
• A spherically symmetric, distributed wind X-ray
  source (i.e. wind shock model) can account for
  the line profiles in z Pup in a reasonable way
• The X-ray formation zone begins close to the
  photosphere (within 3 R? for all lines)
• Continuum absorption by the overlying cool wind
  is important, but not as strong as models (and UV
  observations of the wind) would seem to suggest
  (t? is between 8 and 20 according to models
  calculated by Hillier et al. (1993)).

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• more Discussion
• Above Ro, the amount of X-ray emitting gas
  scales close to density-squared (i.e. the filling
  factor has very little radial dependence)
• The lower-than-expected absorption could have to
  do with overestimation of the wind opacity, or
  possibly with overestimation of the mass-loss
  ratebut, it could also be due to clumping in the
  wind (which might also be associated with the
  wind-shock process itself)
• Other O stars observed with Chandra do not seem
  to have wind absorption signatures (broad but
  symmetric lines) and B stars have basically
  narrow lines could this have to do with
  clumping too? Or non-spherical winds? (see
  Owockis poster on MHD simulations of magnetic
  hot star winds)

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Extra Slides
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Rad-hydro simulations of the line-force
instability copius shock-heated material
distributed throughout the wind
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The Basic Model
Described in Owocki Cohen (2001, ApJ, 559,
1108), the model assumes a smoothly and
spherically symmetrically distributed
accelerating X-ray emitting plasma subject to
continuum attenuation by the cold stellar wind.
which dictates the density of the wind as well.
The wind velocity is assumed to have the form
The optical depth of the wind along a ray with
impact parameter p is given by
where
Note that while spherical symmetry is natural for
the emission, cylindrical symmetry is natural for
the absorption Combining expressions in these
two sets of variables requires the transformation
Ro parameterizes the lower radius of X-ray
emission
The delta function picks out the resonance
velocity, mapping m into l.
q parameterizes the radial fall-off of the
emissivity.
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