Transcript The Application of Forbidden Line X-Ray Diagnostics to the Hot Star

The Application of Forbidden Line
X-Ray Diagnostics to the Hot Star
Tau Sco
Author: Geneviève de Messières
Swarthmore College ‘04
Advised by: David Cohen
Swarthmore College
In Collaboration with:
Joseph MacFarlane, Prism Computational Sciences
Carolin Cardamone, Wellesley College ‘02
Stanley Owocki, University of Delaware
Asif Ud-Doula, University of Delaware
Presented at the Keck Northeast Astronomy Consortium, November 3,
2001
•
The processes by which hot stars emit X-rays are not yet
fully understood. While dimmer stars like the Sun generate Xrays 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-rayemitting 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 Xrays is occuring close to the star or far away.
Magnetic confinement in the corona
causes regions of hot, dense material.
This is one way
to generate Xrays. However,
hot stars are
typically
thought to not
have magnetic
fields.
Coronal loops on the surface of the Sun.
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.
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.
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.
The relationship of velocity and density
in the previous time-height simulation.
How can we study the
processes occurring
on tau Sco?
ROSAT (1993) spectrum of tau Sco
The Chandra
telescope yields
unprecedented
spectral
resolution.
Chandra (2000) spectrum.
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.
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.
In the presence of enough UV radiation,
the forbidden line can disappear.
The oxygen VII helium-like triplet.
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.
The silicon XIII helium-like triplet.
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.
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
Assuming reasonable densities, the effects of the
UV field dominate and indicate a radius from
the star at which emission is taking place.
He-like Ne Ratios for  Sco
Ratio ( forbidden / intercombination )
10
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
6
T = 6 x 10 K
1
0.1
Observed range
for tau Sco
0.01
1E-3
1E10
1E11
1E12
1E13
1E14
1E15
-3
Electron Density (cm )
F line destruction simulations of neon IX
Results of the simulations
Ion
oxygen VII
neo n IX
mag nesi um XI
sil ico n XIII
Range of radii (r/R)
~5 - 10
2.2 - 3
1.8 - 2.5
1.1 - 1.5
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.
A large-scale magnetic field might channel ionized
wind material toward the magnetic equator, where it
would crash into other material, generating X-rays.
Density
Y- Velocity
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.
-1000
vy (km/s)
1000