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Alwin Mao Mentor: Dr. Stephen Reynolds
1. Introduction
We know what Type Ia Supernova are . . .
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Thermonuclear explosions of white dwarfs in binary systems without Hydrogen or Helium spectral lines
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Necessary for distance measurements used to understand Cosmology
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Important to the chemical evolution of galaxies (Fe production)
3. Results
RIGHT: The Hα image of Supernova Remnant
0509-67.5 illustrates an element of the search
for a progenitor star. Clearly, there are
numerous stars contained within the remnant,
making the search difficult. The small circle
indicates a region around the geometric center
with a proposed 99.73% chance of containing
any existent companion star. Examined
carefully, asymmetries in the supernova
remnant can be found. From this picture an
axial ratio of 1.07 and offset of 0.09 can be
determined.
(Schaefer, B. E., & Pagnotta, A. 2012, Nature,
481, 164)
Linear external density case
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(Sedov expansion), with larger change in offset Jump discontinuity external density case
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The supernova offset only changes rapidly in the early stages where the external density discontinuity has the . . . but how do they begin?
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Only begins changing asymmetry and offset in late stages BELOW: These are 5 images of simulations at
select time frames.
The top image is a late-stage (Sedov
expansion) linear case wherein the linear
external density gradient can easily be seen.
Furthermore, the asymmetry in the remnant
and the offset are fairly noticeable.
There are two major theories in contest. Single and Double Degenerate
most impact due to the supernova's smaller size
Density
Hyperbolic tangent external density case
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The bottom 4 images depict frames of a
simulation. Time is increasing from left to right.
Clearly, as time progresses, the remnant
becomes less spherical and more asymmetric.
Furthermore, the offset increases as time
progresses due to a higher density on one
side. An artifact of the grid geometry can be
seen on the horizontal axis—ear-shaped
deformities.
Density
Star denotes supernova center
In the early stages as the supernova passes through the sharpest region of the external density gradient the change Center of late stage linear case
in offset is large for small changes in aspect ratio
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In the later stages as the SNR expands through almost constant density, the rate of change in offset levels off
Star denotes supernova center
Star denotes supernova center
Star denotes supernova center
4. Conclusions
How do we figure out which model is valid?
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Only the single­degenerate model predicts a surviving companion star.
Finding the companion in a Type Ia supernova remnant (SNR) supports the single­degenerate (SD) model
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Possible because survivor has high velocity and unusual spectral lines
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Difficult because an SNR is enormous and contains many stars
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Complete search too resource­intensive; search area must be narrow
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A narrow search area around the geometric center of the remnant has 2. Methods
Data were collected using simulations...
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maintained and provided by Dr. John Blondin. ➢
proved fruitless and a companion star has yet to be found
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cylindrical symmetry
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would have contained any possibly existent companion star
How can we determine a better search area?
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Can we assume the companion star is near the SNR geometric center?
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Irregular interstellar medium (ISM) affects the remnant's expansion
Our goal is to consider the effects of density irregularities surrounding the supernova remnant on offsets in order to discover if they can potentially significantly increase the radius of a search area.
Tested various external density profiles such as linear, jump discontinuity, and hyperbolic tangent
...and then given meaning through analysis
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Asymmetry and offset measured with axial ratio and offset
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Python code imports data files and determines the shape The supernova expands into higher density at a slower rate
This causes asymmetry and apparent geometric shift
Modeled a Type Ia SNR in a nonuniform ISM by placing an exponential ejecta density profile into a 2­D grid assuming If the absence of companion stars is to be used in support of the double­degenerate (DD) model, we must be sure that the search area Used a version of VH­1 (FORTRAN­based PPMLR code) and location of the SNR
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This is done for each of the dozens of data files created at various times during a simulation; plots reveal change in symmetry and offset over time as the supernova evolves
RIGHT: This is a plot of three
different simulations run with
linear external density profiles with
slopes 4, 3, and 2 (i.e. Density =
4x + 7, Density = 3x + 7, Density =
2x + 7). Each point corresponds to
the calculated fractional offset and
axial ratio (Max Radius / Min
Radius) of one time frame of a
simulation. It is visible that in the
early stages there is insignificant
change since the points
corresponding to earlier time are
clustered at a low offset and axial
ratio. However, the late stages
have high offset gain relative to
axial ratio gain. The Cyan point
indicates an axial ratio of 1.07 and
offset of 0.09 corresponding to
SNR 0509-67.5.
RIGHT: This is a plot of two
different simulations in hyperbolic
tangent external density profiles.
Line 1 is a simulation with a ratio
of 6 between the minimum and
maximum densities.
Line 2 is a simulation with a ratio
of 2 between the minimum and
maximum densities.
The magenta points are axial
ratios of 1.07 and 1.09 at offset
0.09 corresponding to
approximate values of the axial
ratio and offset for SNR 0509-67.5
The Cyan diamonds on Line 1
correspond to the four simulation
slices displayed above.
References:
Dohm­Palmer, R. C., & Jones, T. W. 1996, ApJ, 471, 279
Dwarkadas, V. V., & Chevalier, R. A. 1998, ApJ, 497, 807
Hnatyk, B., & Petruk, O. 1999, A&A, 344, 295
4. Conclusions
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SNR 0509­67.5's axial ratio is ~1.07. An offset ~0.09 places the companion star outside of the search area
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Comparisons reveal whether or not, for some level of asymmetry, SNR is significantly offset relative to search area
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Linear ISM profile: significantly offset, but only later
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Jump discontinuity ISM profile: early significant offset
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Tanh ISM profile: significantly offset at SNR 0509­67.5 size
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Thus, considering the effects of an irregular ISM possibly widens the search area appreciably. If SNR 0509­67.5 has a density ratio across of 6, its true center, and a possible surviving companion, could likely be outside the search area. Then, Schaefer and Pagnotta's “absent” companion star would not rule out a single­degenerate origin.
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Future work will examine the brightness variations implied by the varying density in the model SNR to make sure they are consistent with observations.
Ruiz­Lapuente, P. et al., 2004, Nature, 431, 1069
Schaefer, B. E., & Pagnotta, A. 2012, Nature, 481, 164
Wang, B., & Han, Z. 2012, New Astronomy Reviews, 56, 122