Transcript Powerpoint - Astronomy at Swarthmore College

Post Processing of ZEUS MHD Simulations of Young, Hot Stars
Stephen V. St.Vincent and David H. Cohen
Swarthmore College Department of Physics & Astronomy
Introduction/Background
Young, hot stars are very luminous and are
strong sources of X-rays (very shortwavelength, or very blue, light). Recent
detections of strong magnetic fields on
luminous hot stars such as Θ1 Ori C may help
to explain the source of some of these X-rays.
Θ1 Ori C is the brightest star in the
Trapezium (the four stars shown on
the right) which make up the middle
star in Orion’s sword (above). The
right-hand picture is of the famous
Orion Nebula, an active star-forming
region also known as M42.
Visualization and Dynamics
Our colleagues have created computer simulations to model θ1 Ori C and other stars.
The simulations run through time-evolution of the star, and often represent thousands
of kiloseconds of the star’s simulated life.
One important aspect of this project was creating a way to visualize the results of these
simulations. Below are color contour plots of temperature, material density, speed,
and a representation of the magnetic field, all of which are data that are included in the
output files of the simulations. In all of these images, the small white circle is the star
itself, while the remainder of the image is the surrounding stellar wind.
Magnetic field lines quickly become
stretched from their original configuration
(above) to a new and constantly changing
configuration (below).
These contour plots show how the temperature at
different areas around the star change with time.
Below, large amounts of material are being
ejected from of the star.
Post-Processing
We produced some diagnostics by manipulating the data output from the
simulations. Many of these synthesized observables can be compared to actual
data. Below are examples of a few of these synthesized observables that we
created.
Line-of-Sight Velocity. These contour
plots show the speed at which the
material is moving towards or away
from the observer, who is positioned in
the top center of the image. Blue
corresponds to material moving towards
the observer (which would be blueshifted), while red corresponds to
material moving away from the
observer (red-shifted). Black contours
indicate material hotter than 106 K.
Line Profiles. This diagnostic shows
the relative blue- or red-shift of an
emission line the stellar spectrum.
These lines are very narrow,
suggesting that most of the hottest
material is moving very slowly, as is
shown in the contours above. The
observed lines are about as narrow as
these models predict.
Hot stars have strong stellar winds (very fast outflows of material from the star’s
surface). Our group studies how the magnetic fields of the stars interact with these
strong stellar winds. Namely, this interaction occurs via a mechanism called
Magnetically-Channeled Wind-Shock, or MCWS. In this model, the strong
magnetic fields channel the high-velocity winds to the equator from each pole,
where the two fronts of material collide. This causes shock-heating of the material,
which can bring it to temperatures high enough to produce X-rays.
Emission Measure. This is
proportional to the amount of light
emitted from a region. The
equation for EM is:
EM   ne nH dVol
Here, ne is the number of electrons
and nH is the number of hydrogen
atoms in a given volume element.
As an example, typical speeds of channeled winds are around 2000 km/s. We can
relate this kinetic energy to thermal energy with the following equation:
1
3 2
2
mH v  kT
2
2
Using v=2000 km/s, the Boltzmann constant k, and the mass of a hydrogen atom for
mH and solving for T, we get a temperature in the area of 108 K. As a general rule,
any material hotter than 106 K is capable of emitting X-rays.
Schematic of θ1 Ori C. From Earth, we get many
different views of this star as the star rotates. This
is due to the fact that the magnetic and rotational
axes are offset from one another. This diagram
shows the relative position of the observer (at α)
relative to the magnetic axis (solid vertical line).
The values signify the angle of the axis to our lineof-sight. The dashed line represents the magnetic
equator, while the curved solid lines represent
magnetic field lines that have been warped due to
the stellar wind.
The images above and below show the speed of the
material in the stellar wind, with times corresponding
to the temperature contour plots. As you can see, the
hottest material is moving the most slowly.
The images directly above and below are density
contour plots that correspond to the same times as
the temperature contours above. The material may
be hot, but its density is low.
EM vs. Temperature Histogram.
This diagnostic bins the emission
measure as a function of
temperature. Since plasma of a
certain T tends to give off a
particular wavelength of light, this
helps show how much X-ray
radiation will be emitted. The bulk
of the material (at log(T)=4.66) has
been omitted from this graph.
Conclusions
We have found the synthetic data from the simulations that use the MagneticallyChanneled Wind-Shock Model (MCWS) to be in good agreement with what is
actually observed with respect to θ1 Ori C. It is important to remember that this
simulation is still incomplete since it is only a two-dimensional simulation;
however, that being said, the extrapolation into three dimensions performed by this
project appears to indicate that the two-dimensional simulation is in fact a good
approximation for a fully-three-dimensional model, and that it matches the data
well.