Supernova
Explosions and Cosmology
This collaboration brings together the SciDAC Supernova Science
Center and the members of the PHOENIX/SYNPOL collaboration.
The goal is a better understanding of supernovae of all types
through simulation and model validation. Specific objectives
are to clarify the physics of supernova explosions, to improve
the reliability of such explosions as calibrated standard candles,
and to measure fundamental cosmological parameters. Despite
decades of research and modeling, no one understands in detail
how supernovae work. The problem persists largely because, until
recently, computer resources have been inadequate to carry out
credible multi-dimensional calculations.
On June 4, 2002, at the American Astronomical Society meeting
in Albuquerque, N.M., Michael Warren and Chris Fryer from
Los Alamos National Laboratory presented the results of one
of several projects in this collaboration, the first 3D supernova
explosion simulation, based on computation at NERSC (Figure
3). This research eliminates some of the doubts about earlier
2D modeling and paves the way for rapid advances on other
questions about supernovae.
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![High-resolution (1/10 degree) POP ocean model currents](images/sn02a-pr-beginning.jpg) |
![High-resolution (1/10 degree) POP ocean model currents](images/sn02a-pr-middle.jpg) |
![High-resolution (1/10 degree) POP ocean model currents](images/sn02a-pr-end.jpg) |
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Figure
3 Computer visualization shows (left to right)
three stages of a simulated supernova explosion over a
period of 50 milliseconds, starting about 400 milliseconds
after the core begins to collapse. The surfaces show the
material which is flowing outward at a speed of 1,000
kilometers/second. Left is the initial spherical implosion.
Center, as in-falling gas approaches the core, it is exposed
to a higher and higher influx of neutrinos that heat the
gas and make it buoyant. Right, as more cold gas sinks
in, it is heated and rises, resulting in enough convective
energy transfer to create an explosion. |
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Earlier one-dimensional simulations of core-collapse supernovae
almost always failed to explode. Two-dimensional simulations
were qualitatively different from 1D, leading to a robust
explosion without fine-tuning of the star’s physical
properties. They showed that the explosion process is critically
dependent on convection, the mixing of the matter surrounding
the iron core of the collapsing star. It was believed that
the results could again be changed radically by adding a third
dimension, but the 3D simulations turned out to be similar
to the 2D results. The explosion energy, explosion time scale,
and remnant neutron star mass do not differ by more than 10
percent between the 2D and 3D models. With these 3D results,
researchers are ready to attack more exotic problems that
involve rotation and non-symmetric accretion.
The 3D simulation used a parallel smooth particle hydrodynamics
(SPH) code coupled with a flux-limited diffusion radiation
transport. Supernova calculations are computationally demanding
because many processes, involving all four fundamental forces
of physics, must be modeled and followed for more than 100,000
time steps. Typical simulations (1 million particles) took
about three months on the IBM SP at NERSC.
In the next five years, the Supernova Cosmology Project and
the Nearby Supernova Factory experiments will increase both
the quality and quantity of observational supernova data at
low and high redshift by several orders of magnitude. The
purpose of these experiments is to improve the use of supernovae
as tools for cosmology by determining the underlying physics
behind these catastrophic events and to utilize these tools
to help us understand the dark energy that drives the acceleration
of the universe. The only way to fully exploit the power of
this amazing data set is to make a similar order-of-magnitude
improvement in computational studies of supernovae, via spectrum
synthesis and radiation hydrodynamics. The focus of the PHOENIX/SYNPOL
collaboration’s portion of this project is to start
the process of creating 3D spectrum synthesis models of supernovae
(Figure 4) in order to constrain the observations and place
limits on the explosion models and progenitors of supernovae
using the full-physics 1D models as a guide.
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![High-resolution (1/10 degree) POP ocean model currents](images/nugent_test5.jpg) |
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![High-resolution (1/10 degree) POP ocean model currents](images/nugent_surf.jpg) |
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Figure
4 A spectrum synthesis calculation of a supernova
atmosphere surrounded by a toroid. The layout of the atmosphere
is presented on the left, while at the right is a graph
of the flux vs. wavelength vs. viewing angle. As the viewing
angle shifts towards the toroid, the strength of the absorption
increases dramatically. Data that confirm such a model
would for the first time put strong constraints on the
progenitors of Type Ia supernovae. Such flux features
are seen in the spectrum of SN 2001el. |
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Currently two sets of spectrum synthesis codes, PHOENIX and
SYNPOL, are used at NERSC to study the model atmospheres of
supernovae. PHOENIX models astrophysical plasmas in one dimension
under a variety of conditions, including differential expansion
at relativistic velocities found in supernovae. The current
version solves the fully relativistic radiative transport
equation for a variety of spatial boundary conditions in both
spherical and plane-parallel geometries for both continuum
and line radiation simultaneously and self-consistently using
an operator splitting technique. PHOENIX also solves the full
multi-level non-local thermodynamic equilibrium (NLTE) transfer
and rate equations for a large number of atomic species (with
a total of more than 10,000 energy levels and more than 100,000
primary NLTE lines), including non-thermal processes. PHOENIX
accurately solves the fully relativistic radiation transport
equation along with the non-LTE rate equations (currently
for ~150 ions) while ensuring radiative equilibrium (energy
conservation).
SYNPOL is a 3D radiative transfer code to study the spectropolarimetry
of supernovae. It is based on a Monte Carlo treatment of line
formation via the Sobolev approximation and includes electron
scattering. Because SYNPOL does not solve rate equations and
does not do continuum transfer, it is not used for quantitative
abundance determinations or for absolute flux calculations.
Rather its value lies in establishing line identifications
(the intervals of ejection velocity within which the presence
of particular ions is detected) and in probing the geometry
of the supernova and its ejecta. For a full 3D run, with signal-to-noise
and resolution an order of magnitude greater than the observational
data, approximately 1012 photons are generated within a Cartesian
grid of 300 per side. Due to the size of the atomic data—over
42 million lines whose strengths can vary at each cube in
the grid—the memory requirements and the time it takes
to process the scattering of such a large number of photons
are quite large: 1 million CPU hours for a 3D simulation with
simplified physics, and 10 GB input and 1 GB output per iteration,
with 20 iterations per star model for 20 to 30 models.
NERSC provided a new 24-hour run queue to accommodate this
simulation. Within the next two or three years, 100 times
more CPUs will be needed to run 3D simulations with complex
physics if there are no algorithmic improvements.
INVESTIGATORS
P. Nugent, D. Kasen, and S. Perlmutter, Lawrence Berkeley
National Laboratory; S. Woosley and G. Glatzmaier, University
of California, Santa Cruz; P. Hauschildt, J. Aufdenberg, C.
Shore, A. Schweitz, and T. Barman, University of Georgia;
E. Baron, University of Oklahoma; T. Clune, Goddard Space
Flight Center; A. Burrows, S. Hariri, A. Hungerford, P. Pinto,
H. Sarjoughian, and B. Ziegler, University of Arizona; T.
Evans, C. Fryer, M. Gray, W. Miller, and M. Warren, Los Alamos
National Laboratory; F. Dietrich and R. Hoffman, Lawrence
Livermore National Laboratory.
PUBLICATIONS
C. L. Fryer and M. S. Warren, “Modeling core-collapse
supernovae in three dimensions,” Astrophysical Journal
574, L65 (2002).
C. L. Fryer, D. E. Holz, and S. A. Hughes, “Gravitational
wave emission from core collapse of massive stars,”
Astrophysical Journal 565, 430 (2002).
D. N. Kasen, P. E. Nugent, L. Wang, and D. A. Howell, “Interpreting
supernova polarization spectra,” Bulletin of the American
Astronomical Society 33, 8409.
URLs
http://www.supersci.org/
http://www.lbl.gov/~nugent
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