Publications (181)216.64 Total impact
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ABSTRACT: 3-D astrophysical atmospheres will have random velocity fields. We seek to
combine the methods we have developed for solving the 1-D problem with
arbitrary flows to those that we have developed for solving the fully 3-D
relativistic radiative transfer problem in the case of monotonic flows. The
methods developed in the case of 3-D atmospheres with monotonic flows, solving
the fully relativistic problem along curves defined by an affine parameter, are
very flexible and can be extended to the case of arbitrary velocity fields in
3-D. Simultaneously, the techniques we developed for treating the 1-D problem
with arbitrary velocity fields are easily adapted to the 3-D problem. The
algorithm we present allows the solution of 3-D radiative transfer problems
that include arbitrary wavelength couplings. We use a quasi-analytic formal
solution of the radiative transfer equation that significantly improves the
overall computation speed. We show that the approximate lambda operator
developed in previous work gives good convergence, even neglecting wavelength
coupling. Ng acceleration also gives good results. We present tests that are of
similar resolution to what has been presented using Monte-Carlo techniques,
thus our methods will be applicable to problems outside of our test setup.
Additional domain decomposition parallelization strategies will be explored in
future work.
10/2012;
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ABSTRACT: In supernova spectroscopy relatively little attention has been given to the
properties of optically thick spectral lines in epochs following the
photosphere's recession. Most treatments and analyses of post-photospheric
optical spectra of supernovae assume that forbidden-line emission comprises
most if not all spectral features. However, evidence exists which suggests that
some spectra exhibit line profiles formed via optically thick
resonance-scattering even months or years after the supernova explosion. To
explore this possibility we present a geometrical approach to supernova
spectrum formation based on the "Elementary Supernova" model, wherein we
investigate the characteristics of resonance-scattering in optically thick
lines while replacing the photosphere with a transparent central core emitting
non-blackbody continuum radiation, akin to the optical continuum provided by
decaying 56Co formed during the explosion. We develop the mathematical
framework necessary for solving the radiative transfer equation under these
conditions, and calculate spectra for both isolated and blended lines. Our
comparisons with analogous results from the Elementary Supernova code SYNOW
reveal several marked differences in line formation. Most notably, resonance
lines in these conditions form P Cygni-like profiles, but the emission peaks
and absorption troughs shift redward and blueward, respectively, from the
line's rest wavelength by a significant amount, despite the spherically
symmetric distribution of the line optical depth in the ejecta. These
properties and others that we find in this work could lead to misidentification
of lines or misattribution of properties of line-forming material at
post-photospheric times in supernova optical spectra.
09/2012;
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S. Hachinger,
P. A. Mazzali,
M. Sullivan,
R. Ellis,
K. Maguire,
A. Gal-Yam,
D. A. Howell,
P. E. Nugent, E. Baron,
J. Cooke, [......],
B. Dilday,
P. A. James,
M. M. Kasliwal,
S. R. Kulkarni,
E. O. Ofek,
R. R. Laher,
J. Parrent,
J. Surace,
O. Yaron,
E. S. Walker
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ABSTRACT: Radiative transfer studies of Type Ia supernovae (SNe Ia) hold the promise of
constraining both the time-dependent density profile of the SN ejecta and its
stratification by element abundance which, in turn, may discriminate between
different explosion mechanisms and progenitor classes. Here we present a
detailed analysis of Hubble Space Telescope ultraviolet (UV) and ground-based
optical spectra and light curves of the SN Ia SN 2010jn (PTF10ygu). SN 2010jn
was discovered by the Palomar Transient Factory (PTF) 15 days before maximum
light, allowing us to secure a time-series of four UV spectra at epochs from
-11 to +5 days relative to B-band maximum. The photospheric UV spectra are
excellent diagnostics of the iron-group abundances in the outer layers of the
ejecta, particularly those at very early times. Using the method of 'Abundance
Tomography' we have derived iron-group abundances in SN 2010jn with a precision
better than in any previously studied SN Ia. Optimum fits to the data can be
obtained if burned material is present even at high velocities, including
significant mass fractions of iron-group elements. This is consistent with the
slow decline rate (or high 'stretch') of the light curve of SN 2010jn, and
consistent with the results of delayed-detonation models. Early-phase UV
spectra and detailed time-dependent series of further SNe Ia offer a promising
probe of the nature of the SN Ia mechanism.
08/2012;
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J. T. Parrent,
D. A. Howell,
B. Friesen,
R. C. Thomas,
R. A. Fesen,
D. Milisavljevic,
F. B. Bianco,
B. Dilday,
P. Nugent, E. Baron, [......],
D. Polishook,
D. Poznanski,
R. M. Quimby,
J. M. Silverman,
A. Sternberg,
M. Sullivan,
E. S. Walker,
D. Xu,
C. Buton,
R. Pereira
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ABSTRACT: The nearby Type Ia supernova SN 2011fe in M101 (cz=241 km s^-1) provides a
unique opportunity to study the early evolution of a "normal" Type Ia
supernova, its compositional structure, and its elusive progenitor system. We
present 18 high signal-to-noise spectra of SN 2011fe during its first month
beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This
gives a clear picture of how various line-forming species are distributed
within the outer layers of the ejecta, including that of unburned material
(C+O). We follow the evolution of C II absorption features until they diminish
near maximum light, showing overlapping regions of burned and unburned material
between ejection velocities of 10,000 and 16,000 km s^-1. This supports the
notion that incomplete burning, in addition to progenitor scenarios, is a
relevant source of spectroscopic diversity among SNe Ia. The observed evolution
of the highly Doppler-shifted O I 7774 absorption features detected within five
days post-explosion indicate the presence of O I with expansion velocities from
11,500 to 21,000 km s^-1. The fact that some O I is present above C II suggests
that SN 2011fe may have had an appreciable amount of unburned oxygen within the
outer layers of the ejecta.
05/2012;
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ABSTRACT: We present a study of the peculiar Type Ia supernova 2001ay (SN 2001ay). The
defining features of its peculiarity are: high velocity, broad lines, and a
fast rising light curve, combined with the slowest known rate of decline. It is
one magnitude dimmer than would be predicted from its observed value of
Delta-m15, and shows broad spectral features. We base our analysis on detailed
calculations for the explosion, light curves, and spectra. We demonstrate that
consistency is key for both validating the models and probing the underlying
physics. We show that this SN can be understood within the physics underlying
the Delta-m15 relation, and in the framework of pulsating delayed detonation
models originating from a Chandrasekhar mass, white dwarf, but with a
progenitor core composed of 80% carbon. We suggest a possible scenario for
stellar evolution which leads to such a progenitor. We show that the unusual
light curve decline can be understood with the same physics as has been used to
understand the Delta-m15 relation for normal SNe Ia. The decline relation can
be explained by a combination of the temperature dependence of the opacity and
excess or deficit of the peak luminosity, alpha, measured relative to the
instantaneous rate of radiative decay energy generation. What differentiates SN
2001ay from normal SNe Ia is a higher explosion energy which leads to a shift
of the Ni56 distribution towards higher velocity and alpha < 1. This result is
responsible for the fast rise and slow decline. We define a class of SN
2001ay-like SNe Ia, which will show an anti-Phillips relation.
05/2012;
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ABSTRACT: We compare and analyze a time series of spectral observations obtained during
the first 30 days of evolution of SN 2007od with the non-LTE code PHOENIX.
Despite some spectroscopic particularities in the Balmer features, this
supernova appears to be a normal Type II, and the fits proposed are generally
in good agreement with the observations. As a starting point we have carried
out an analysis with the parameterized synthetic spectrum code SYNOW to confirm
line identifications and to highlight differences between the results of the
two codes. The analysis computed using PHOENIX suggests the presence of a high
velocity feature in H{\beta} and an H{\alpha} profile reproduced with a density
profile steeper than that of the other elements. We also show a detailed
analysis of the ions velocities of the 6 synthetic spectra. The distance is
estimated for each epoch with the Spectral-fitting Expanding Atmosphere Method
(SEAM). Consistent results are found using all the spectra which give the
explosion date of JD 2454403 (29 October, 2007) and a distance modulus \mu =
32.2 \pm 0.3.
02/2012;
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C. Inserra,
M. Turatto,
A. Pastorello,
M. L. Pumo, E. Baron,
S. Benetti,
E. Cappellaro,
S. Taubenberger,
F. Bufano,
N. Elias-Rosa, [......],
A. Harutyunyan,
A. S. Moskvitin,
M. Nissinen,
V. Stanishev,
D. Y. Tsvetkov,
V. P. Hentunen,
V. N. Komarova,
N. N. Pavlyuk,
V. V. Sokolov,
T. N. Sokolova
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ABSTRACT: We present photometry and spectroscopy of the type IIP supernova 2009bw in
UGC 2890 from few days after the outburst to 241 days. The light curve of SN
2009bw during the photospheric phase is similar to that of normal SNe IIP but
with brighter peak and plateau (Mmax R = -17.82 mag, Mplateau R = -17.37 mag).
The luminosity drop from the photospheric to the nebular phase is one of the
fastest ever observed, ~2.2 mag in about 13 days. The radioactive tail of the
bolometric light curve indicates that the amount of ejected 56 Ni is \approx
0.022 M\odot. The photospheric spectra reveal high velocity lines of H{\alpha}
and H{\beta} until about 105 days after the shock breakout, suggesting a
possible early interaction between the SN ejecta and pre-existent circumstellar
material, and the presence of CNO elements. By modeling the bolometric light
curve, ejecta expansion velocity and photospheric temperature, we estimate a
total ejected mass of 8-12M\odot, a kinetic energy of ~0.3 foe and an initial
radius of ~ 3.6 - 7 \times 10^13 cm.
02/2012;
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ABSTRACT: We present an analysis of high precision V light curves (LC) for 18 local
Type Ia Supernovae, SNe Ia, as obtained with the same telescope and setup at
the Las Campanas Observatory (LCO). This homogeneity provides an intrinsic
accuracy a few hundreds of a magnitude both with respect to individual LCs and
between different objects. Based on the Single Degenerate Scenario, SD, we
identify patterns which have been predicted by model calculations as signatures
of the progenitor and accretion rate which change the explosion energy and the
amount of electron capture, respectively. Using these templates as principle
components and the overdetermined system of SN pairs, we reconstruct the
properties of progenitors and progenitor systems. All LCO SNe Ia follow the
brightness decline relation but 2001ay. After subtraction of the two
components, the remaining scatter is reduced to 0.01-0.03m. Type SNe Ia seem to
originate from progenitors with Main Sequence masses of 3Mo with the exception
of two subluminous SNe Ia with < 2Mo. The component analysis indicates a wide
range of accretion rates in the progenitor systems closing the gap to accretion
induced collapses (AIC). SN1991t-like objects show differences in $dm15$ but no
tracers of our secondary parameters. This may point to a different origin such
as DD-Scenario or the Pulsating Delayed Detonations. SN2001ay does not follow
the decline relation. It can be understood in the framework of C-rich WDs, and
this group may produce an anti-Phillips relation. We suggest that this may be a
result of a common envelope phase and mixing during central He burning as in
SN1987A.
09/2011;
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ABSTRACT: We discuss an implementation of our 3D radiative transfer (3DRT) framework
with the OpenCL paradigm for general GPU computing. We implement the kernel for
solving the 3DRT problem in Cartesian coordinates with periodic boundary
conditions in the horizontal $(x,y)$ plane, including the construction of the
nearest neighbor $\Lstar$ and the operator splitting step. We present the
results of a small and a large test case and compare the timing of the 3DRT
calculations for serial CPUs and various GPUs. The latest available GPUs can
lead to significant speedups for both small and large grids compared to serial
(single core) computations.
07/2011;
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ABSTRACT: A solution of the radiative-transfer problem in 3D with arbitrary velocity fields in the Eulerian frame is presented. The method is implemented in our 3D radiative transfer framework and used in the PHOENIX/3D code. It is tested by comparison to our well- tested 1D co-moving frame radiative transfer code, where the treatment of a monotonic velocity field is implemented in the Lagrangian frame. The Eulerian formulation does not need much additional memory and is useable on state-of-the-art computers, even large-scale applications with 1000's of wavelength points are feasible.
07/2010;
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ABSTRACT: Hydrogen recombination is one of the most important atomic processes in many astrophysical objects such as Type II supernova (SN~II) atmospheres, the high redshift universe during the cosmological recombination era, and H II regions in the interstellar medium. Accurate predictions of the ionization fraction can be quite different from those given by a simple solution if one takes into account many angular momentum sub-states, non-resonant processes, and calculates the rates of all atomic processes from the solution of the radiative transfer equation instead of using a Planck function under the assumption of thermal equilibrium. We use the general purpose model atmosphere code PHOENIX 1D to compare how the fundamental probabilities such as the photo-ionization probability, the escape probability, and the collisional de-excitation probability are affected by the presence of other metals in the environment, multiple angular momentum sub-states, and non-resonant processes. Our comparisons are based on a model of SN 1999em, a SNe Type II, 20 days after its explosion. Comment: 29 pages, 12 figures, MNRAS, in press
05/2010;
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ABSTRACT: High-quality observations of $B$ and $V$ light curves obtained at Las Campanas Observatory for local Type Ia Supernovae (SNe Ia) show clear evidence that SNe Ia with the same brightness decline or stretch may have systematic and independent deviations at times < 5 days before and at times > 30 days after maximum light. This suggests the existence of two independent secondary parameters which control the shape of SN Ia light curves in addition to the brightness decline relation. stretch. The differences are consistent in morphology of the time dependence and size with predictions by models within the delayed detonation scenario. The secondary parameters may reflect two independent physical effects caused by variations in the progenitor and accretion rates, and link the LC variations in shape with the intrinsic, absolute brightness. Comment: 32 pages, 9 Figures, ApJ accepted
12/2009;
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ABSTRACT: We demonstrate the application of our 3D radiative transfer framework in the model atmosphere code PHOENIX/3D for a number of spectrum synthesis calculations for very different conditions. The 3DRT framework discussed in the previous papers of this series was added to our general-purpose model atmosphere code PHOENIX/1D and an extended 3D version PHOENIX/3D was created. The \phxT code is parallelized via the MPI library using a hierarchical domain decomposition and displays very good strong scaling. We present the results of several test cases for widely different atmosphere conditions and compare the 3D calculations with equivalent 1D models to assess the internal accuracy of the 3D modeling. In addition, we show the results for a number of parameterized 3D structures. With presently available computational resources it is possible to solve the full 3D radiative transfer (including scattering) problem with the same micro-physics as included in 1D modeling. Comment: fixed PDF generation problem. Also available at tp://ftp.hs.uni-hamburg.de/pub/outgoing/phoenix/preprints/3DRT_paper6.pdf A&A, in press (Ref: AA/2009/13064)
11/2009;
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ABSTRACT: NLTE radiative transfer calculations of differentially expanding supernovae atmospheres are computationally intensive and are almost universally performed in time-independent snapshot mode. The validity of the steady-state approximation in the rate equations has recently been questioned. We calculate the effective recombination time of hydrogen in SN II using our general purpose model atmosphere code PHOENIX. While we find that the recombination time for the conditions of SNe II at early times is increased over the classical value for the case of a simple hydrogen model atom with energy levels corresponding to just the first 2 principle quantum numbers, the classical value of the recombination time is recovered in the case of a multi-level hydrogen atom. We also find that the recombination time at most optical depths is smaller in the case of a multi-level atom than for a simple two-level hydrogen atom. We find that time dependence in the rate equations is important in the early epochs of a supernova's lifetime. The changes due to the time dependent rate equation (at constant input luminosity) are manifested in physical parameters such as the level populations which directly affects the spectra. The H-alpha profile is affected by the time dependent rate equations at early times. At later times time dependence does not significantly modify the level populations and therefore the H-alpha profile is roughly independent of whether the steady-state or time-dependent approach is used. Comment: Abstract shortened for arXiv. 39 pages, 16 figures, MNRAS in press
10/2009;
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ABSTRACT: A solution of the radiative-transfer problem in arbitrary velocity fields introduced in a previous paper, has limitations in its applicability. For large-scale applications, the methods described also require large memory sets that are commonly not available to state-of-the-art computing hardware. In this work, we modify the algorithm to allow the computation of large-scale problems. We reduce the memory footprint via a domain decomposition. By introducing iterative Gauss-Seidel type solvers, we improve the speed of the overall computation. Because of the domain decomposition, the new algorithm requires the use of parallel-computing systems. The algorithm that we present permits large-scale solutions of radiative-transfer problems that include arbitrary wavelength couplings. In addition, we discover a quasi-analytic formal solution of the radiative transfer that significantly improves the overall computation speed. More importantly, this method ensures that our algorithm can be applied to multi-dimensional Lagrangian radiative-transfer calculations. In multi-dimensional atmospheres, velocity fields are in general chaotic ensuring that the inclusion of arbitrary wavelength couplings are mandatory.
09/2009;
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ABSTRACT: Using the methods of general relativity Lindquist derived the radiative transfer equation that is correct to all orders in v/c. Mihalas developed a method of solution for the important case of monotonic velocity fields with spherically symmetry. We have developed the generalized atmosphere code PHOENIX, which in 1-D has used the framework of Mihalas to solve the radiative transfer equation (RTE) in 1-D moving flows. We describe our recent work including 3-D radiation transfer in PHOENIX and particularly including moving flows exactly using a novel affine method. We briefly discuss quantitative spectroscopy in supernovae. Comment: 13 pages, 9 figures, to appear in Recent Directions in Astrophysical Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny, American Institute of Physics (2009)
08/2009;
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ABSTRACT: Observations and theoretical calculations have shown the importance of non-spherically symmetric structures in supernovae. Thus, the interpretation of observed supernova spectra requires the ability to solve the transfer equation in 3-D moving atmospheres. We present an implementation of the solution of the radiative transfer equation in 3-D homologously expanding atmospheres in spherical coordinates. The implementation is exact to all orders in v/c. We use a new affine method that makes use of the fact that photons travel on straight lines. We compare our results in 3-D for spherically symmetric test problems with high velocity fields and find excellent agreement. Our well-tested 1-D results are based on methods where the momentum directions vary along the characteristic (co-moving momentum directions). Thus, we are able to verify both the analytic framework and its numerical implementation. Additionally, we have been able to test the parallelization over characteristics. Using 512^2 momentum angles we ran the code on 16,384 Opteron processors and achieved excellent scaling. It is now possible to calculate synthetic spectra from realistic 3D hydro simulations. This should open an era of progress in hydro modeling, similar to that that occurred in the 1980s when 1-D models were confronted with synthetic spectra.
04/2009;
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ABSTRACT: We extend our framework for 3D radiative transfer calculations with a non-local operator splitting methods along (full) characteristics to spherical and cylindrical coordinate systems. These coordinate systems are better suited to a number of physical problems than Cartesian coordinates. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate $\Lambda$ operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter for the different coordinate systems. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.
04/2009;
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D. A. Howell,
A. Conley,
M. Della Valle,
P. E. Nugent,
S. Perlmutter,
G. H. Marion,
K. Krisciunas,
C. Badenes,
P. Mazzali,
G. Aldering, [......],
R. Quimby,
A. Rest,
A. Riess,
M. Sako,
A. M. Soderberg,
L. Strolger,
R Thomas,
M. Turatto,
S. Van Dyk,
W. M. Wood-Vasey
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ABSTRACT: In the next decade Type Ia supernovae (SNe Ia) will be used to test theories predicting changes in the Dark Energy equation of state with time. Ultimately this requires a dedicated space mission like JDEM. SNe Ia are mature cosmological probes --- their limitations are well characterized, and a path to improvement is clear. Dominant systematic errors include photometric calibration, selection effects, reddening, and population-dependent differences. Building on past lessons, well-controlled new surveys are poised to make strides in these areas: the Palomar Transient Factory, Skymapper, La Silla QUEST, Pan-STARRS, the Dark Energy Survey, LSST, and JDEM. They will obviate historical calibrations and selection biases, and allow comparisons via large subsamples. Some systematics follow from our ignorance of SN Ia progenitors, which there is hope of determining with SN Ia rate studies from 0<z<4. Aside from cosmology, SNe Ia regulate galactic and cluster chemical evolution, inform stellar evolution, and are laboratories for extreme physics. Essential probes of SNe Ia in these contexts include spectroscopy from the UV to the IR, X-ray cluster and SN remnant observations, spectropolarimetry, and advanced theoretical studies. While there are an abundance of discovery facilities planned, there is a deficit of follow-up resources. Living in the systematics era demands deep understanding rather than larger statistics. NOAO ReSTAR initiative to build 2-4m telescopes would provide necessary follow-up capability. Finally, to fully exploit LSST, well-matched wide-field spectroscopic capabilities are desirable.
04/2009;
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ABSTRACT: We present synthetic spectra around maximum for the type II supernova SN 2003Z, which was first detected on January 29.7 2003. Comparison with observed spectra aim at the determination of physical parameters for SN 2003Z. Synthetic spectra are calculated with our stellar atmosphere code PHOENIX. It solves the special relativistic equation of radiative transfer, including large NLTE-calculations and line blanketing by design, in 1-dimensional spherical symmetry. The observed spectra were obtained at the 3.5 meter telescope at Calar Alto. The TWIN instrument was used so that a spectral range from about 3600 to 7500 Angstroem was covered. The spectra were taken on Feb. 4, 5, 9, and 11, 2003. The physical parameters of the models give the luminosities, a range of possible velocity profiles for the SN, an estimate of the colour excess, and the observed metalicity. Comment: 8 figures
02/2009;
Institutions
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1970–2009
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University of Oklahoma
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Homer L. Dodge Department of Physics and Astronomy
Norman,
OK,
USA
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2007
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Lawrence Berkeley National Laboratory
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Computational Research Division (CRD)
Berkeley,
CA,
USA
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1993–1999
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Arizona State University
Tempe,
AZ,
USA
