The Application of Forbidden Line X-Ray Diagnostics to the Hot Star Tau Sco
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Transcript of The Application of Forbidden Line X-Ray Diagnostics to the Hot Star Tau Sco
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The Application of Forbidden Line X-Ray Diagnostics to the
Hot Star Tau ScoAuthor: Geneviève de Messières
Swarthmore College ‘04
Advised by: David CohenSwarthmore College
In Collaboration with: Joseph MacFarlane, Prism Computational Sciences
Carolin Cardamone, Wellesley College ‘02Stanley Owocki, University of DelawareAsif Ud-Doula, University of Delaware
Presented at the Keck Northeast Astronomy Consortium, November 3, 2001
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• The processes by which hot stars emit X-rays are not yet fully understood. While dimmer stars like the Sun generate X-rays through magnetic confinement of the corona, it is generally thought that the X-rays from hot stars are created in radiatively driven stellar wind shocks.
• Using a high-resolution spectrum of the B-type star tau Scorpii from the telescope Chandra, we have studied the strength of the ultraviolet field at the location of the X-ray-emitting plasma by examining the forbidden and intercombination lines of helium-like elements in the plasma.
• A stronger UV field, close to the surface of the star, destroys the forbidden line in favor of the intercombination line, so the diagnostic can indicate whether the generation of X-rays is occuring close to the star or far away.
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Magnetic confinement in the corona causes regions of hot, dense material.
This is one way to generate X-
rays. However, hot stars are
typically thought to not have magnetic
fields.
Coronal loops on the surface of the Sun.
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The radiation pressure from luminous stars accelerates the stellar wind to high speeds.
Eta Carina is hidden by the nebula created by its stellar wind.
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This acceleration is not uniform. Fast shells of the wind crash into slower
regions in a typical shock-driven wind.
Time-height simulation of an O-type star.
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The relationship of velocity and density in the previous time-height
simulation.
The collision of fast and slow shells of the stellar wind results in dense, hot X-ray emitting regions
in the radiatively driven wind shock model.
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The Chandra telescope yields unprecedented
spectral resolution.
ROSAT (1993) spectrum of tau Sco
Chandra (2000) spectrum.
How can we study the processes occurring
on tau Sco?
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The sizes and shapes of the lines can be
resolved, distinguishing even closely
spaced groups.
The magnesium XI helium-like triplet, fitted with gaussian models using the
CIAO software package.
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The strength ratio of the forbidden to
intercombination line indicates the strength
of the UV field.*
In a strong UV field, electrons are often excited out of the long-lived upper level of the forbidden line before they spontaneously de-excite, weakening the forbidden line.
* If electron densities are high enough, collisional excitation will destroy the forbidden line in the same manner. However, the effects of the UV field are likely to dominate.
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In the presence of enough UV radiation, the forbidden line can disappear.
The oxygen VII helium-like triplet.
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My work has primarily been to identify the F/I ratio for each helium-like element present in the spectrum by fitting models to the spectral data.
The silicon XIII helium-like triplet.
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The silicon XIII helium-like triplet.
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The neon IX helium-like triplet. Nearby iron lines interfered with the data and
had to be fitted separately to be eliminated from the fit of the neon lines.
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Basic properties of tau Sco:• B0 V• Teff = 31,400 K• L = 4.69 LSun
• Mass loss = 3.1 x 10-8 MSun yr-1
• v∞ = 2400 km s-1
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Assuming reasonable densities, the effects of the UV field dominate and indicate a radius from
the star at which emission is taking place.
1E10 1E11 1E12 1E13 1E14 1E151E-3
0.01
0.1
1
10
T = 6 x 106 K
He-like Ne Ratios for Sco
r / RS = 20
r / RS = 10
r / RS = 5
r / RS = 3
r / RS = 2
r / RS = 1.5
r / RS = 1.2
r / RS = 1.1
r / RS = 1
Ra
tio (
fo
rbid
de
n /
inte
rco
mb
ina
tion
)
Electron Density (cm-3)
F line destruction simulations of neon IX
Observed range for tau Sco Ion Range of radii (r/R)
oxygen VII ~5 - 10neon IX 2.2 - 3magnesium XI 1.8 - 2.5silicon XIII 1.1 - 1.5
Results of the simulations
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Results of the Diagnostic
• The radius of X-ray emission appears to be at about 1.5 - 3 R*. This is closer to the surface than expected for a normal stellar wind but too far for normal coronal activity.
• From Carolin Cardamone’s research, we see that the lines are slightly broadened, but indicate a velocity no greater than 200-300 km s-1. This is much less than the wind’s terminal velocity.
• How can we interpret this? Tau Sco is an unusually young star, and it could retain a primordial magnetic field.
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Y- Velocity
-1000 vy (km/s) 1000
Density
A large-scale magnetic field might channel ionized wind material toward the magnetic equator, where it would crash into other material, generating X-rays.
This would explain both the moderate distance from the star seen in this research and the slow wind velocities discussed by Carolin in her talk.