1,865 research outputs found
Simulations of Astrophysical Fluid Instabilities
We present direct numerical simulations of mixing at Rayleigh-Taylor unstable
interfaces performed with the FLASH code, developed at the ASCI/Alliances
Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We
present initial results of single-mode studies in two and three dimensions. Our
results indicate that three-dimensional instabilities grow significantly faster
than two-dimensional instabilities and that grid resolution can have a
significant effect on instability growth rates. We also find that unphysical
diffusive mixing occurs at the fluid interface, particularly in poorly resolved
simulations.Comment: 3 pages, 1 figure. To appear in the proceedings of the 20th Texas
Symposium on Relativistic Astrophysic
Large-Scale Simulations of Clusters of Galaxies
We discuss some of the computational challenges encountered in simulating the
evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR)
techniques can successfully address these challenges but are currently being
used by only a few groups. We describe our publicly available AMR code, FLASH,
which uses an object-oriented framework to manage its AMR library, physics
modules, and automated verification. We outline the development of the FLASH
framework to include collisionless particles, permitting it to be used for
cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International
Workshop on Advanced Computing and Analysis Techniques in Physics Research
(ACAT 2000), Fermilab, Oct. 16-20, 200
Better Nonlinear Models from Noisy Data: Attractors with Maximum Likelihood
A new approach to nonlinear modelling is presented which, by incorporating
the global behaviour of the model, lifts shortcomings of both least squares and
total least squares parameter estimates. Although ubiquitous in practice, a
least squares approach is fundamentally flawed in that it assumes independent,
normally distributed (IND) forecast errors: nonlinear models will not yield IND
errors even if the noise is IND. A new cost function is obtained via the
maximum likelihood principle; superior results are illustrated both for small
data sets and infinitely long data streams.Comment: RevTex, 11 pages, 4 figure
In flight performance and first results of FREGATE
The gamma-ray detector of HETE-2, called FREGATE, has been designed to detect
gamma-ray bursts in the energy range [6-400] keV. Its main task is to alert the
other instruments of the occurrence of a gamma-ray burst (GRB) and to provide
the spectral coverage of the GRB prompt emission in hard X-rays and soft
gamma-rays. FREGATE was switched on on October 16, 2000, one week after the
successful launch of HETE-2, and has been continuously working since then. We
describe here the main characteristics of the instrument, its in-flight
performance and we briefly discuss the first GRB observations.Comment: Invited lecture at the Woods Hole 2001 GRB Conference, 8 pages, 15
figure
Microscopic theory of glassy dynamics and glass transition for molecular crystals
We derive a microscopic equation of motion for the dynamical orientational
correlators of molecular crystals. Our approach is based upon mode coupling
theory. Compared to liquids we find four main differences: (i) the memory
kernel contains Umklapp processes, (ii) besides the static two-molecule
orientational correlators one also needs the static one-molecule orientational
density as an input, where the latter is nontrivial, (iii) the static
orientational current density correlator does contribute an anisotropic,
inertia-independent part to the memory kernel, (iv) if the molecules are
assumed to be fixed on a rigid lattice, the tensorial orientational correlators
and the memory kernel have vanishing l,l'=0 components. The resulting mode
coupling equations are solved for hard ellipsoids of revolution on a rigid
sc-lattice. Using the static orientational correlators from Percus-Yevick
theory we find an ideal glass transition generated due to precursors of
orientational order which depend on X and p, the aspect ratio and packing
fraction of the ellipsoids. The glass formation of oblate ellipsoids is
enhanced compared to that for prolate ones. For oblate ellipsoids with X <~ 0.7
and prolate ellipsoids with X >~ 4, the critical diagonal nonergodicity
parameters in reciprocal space exhibit more or less sharp maxima at the zone
center with very small values elsewhere, while for prolate ellipsoids with 2 <~
X <~ 2.5 we have maxima at the zone edge. The off-diagonal nonergodicity
parameters are not restricted to positive values and show similar behavior. For
0.7 <~ X <~ 2, no glass transition is found. In the glass phase, the
nonergodicity parameters show a pronounced q-dependence.Comment: 17 pages, 12 figures, accepted at Phys. Rev. E. v4 is almost
identical to the final paper version. It includes, compared to former
versions v2/v3, no new physical content, but only some corrected formulas in
the appendices and corrected typos in text. In comparison to version v1, in
v2-v4 some new results have been included and text has been change
The Impact of Galaxy Cluster Mergers on Cosmological Parameter Estimation from Surveys of the Sunyaev-Zel'dovich Effect
Sensitive surveys of the Cosmic Microwave Background will detect thousands of
galaxy clusters via the Sunyaev-Zel'dovich (SZ) effect. Two SZ observables, the
central or maximum and integrated Comptonization parameters y_max and Y, relate
in a simple way to the total cluster mass, which allow the construction of mass
functions (MFs) that can be used to estimate cosmological parameters such as
Omega_M, sigma_8, and the dark energy parameter w. However, clusters form from
the mergers of smaller structures, events that can disrupt the equilibrium of
intracluster gas upon which SZ-M relations rely. From a set of
N-body/hydrodynamical simulations of binary cluster mergers, we calculate the
evolution of Y and y_max over the course of merger events and find that both
parameters are transiently "boosted," primarily during the first core passage.
We then use a semi-analytic technique developed by Randall et al. (2002) to
estimate the effect of merger boosts on the distribution functions YF and yF of
Y and y_max, respectively, via cluster merger histories determined from
extended Press-Schechter (PS) merger trees. We find that boosts do not induce
an overall systematic effect on YFs, and the values of Omega_M, sigma_8, and w
were returned to within 2% of values expected from the nonboosted YFs. The
boosted yFs are significantly biased, however, causing Omega_M to be
underestimated by 15-45%, sigma_8 to be overestimated by 10-25%, and w to be
pushed to more negative values by 25-45%. We confirm that the integrated SZ
effect, Y, is far more robust to mergers than y_max, as previously reported by
Motl et al. (2005) and similarly found for the X-ray equivalent Y_X, and we
conclude that Y is the superior choice for constraining cosmological
parameters.Comment: 16 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
The ECLAIRs micro-satellite mission for gamma-ray burst multi-wavelength observations
Gamma-ray bursts (GRB), at least those with a duration longer than a few
seconds are the most energetic events in the Universe and occur at cosmological
distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide
multi-wavelength observations of GRB, to study their astrophysics and to use
them as cosmological probes. Furthermore in 2009 ECLAIRs is expected to be the
only space borne instrument capable of providing a GRB trigger in near
real-time with sufficient localization accuracy for GRB follow-up observations
with the powerful ground based spectroscopic telescopes available by then. A
"Phase A study" of the ECLAIRs project has recently been launched by the French
Space Agency CNES, aiming at a detailed mission design and selection for flight
in 2006. The ECLAIRs mission is based on a CNES micro-satellite of the
"Myriade" family and dedicated ground-based optical telescopes. The satellite
payload combines a 2 sr field-of-view coded aperture mask gamma-camera using
6400 CdTe pixels for GRB detection and localization with 10 arcmin precision in
the 4 to 50 keV energy band, together with a soft X-ray camera for onboard
position refinement to 1 arcmin. The ground-based optical robotic telescopes
will detect the GRB prompt/early afterglow emission and localize the event to
arcsec accuracy, for spectroscopic follow-up observations.Comment: 7 pages, 1 figure, proceedings of the conference "New Developments in
Photodetection", Beaune (France), June 25005. Submitted to NIM-A (Elsevier
Science
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