6,731 research outputs found

### The Tree-Particle-Mesh N-body Gravity Solver

The Tree-Particle-Mesh (TPM) N-body algorithm couples the tree algorithm for
directly computing forces on particles in an hierarchical grouping scheme with
the extremely efficient mesh based PM structured approach. The combined TPM
algorithm takes advantage of the fact that gravitational forces are linear
functions of the density field. Thus one can use domain decomposition to break
down the density field into many separate high density regions containing a
significant fraction of the mass but residing in a very small fraction of the
total volume. In each of these high density regions the gravitational potential
is computed via the tree algorithm supplemented by tidal forces from the
external density distribution. For the bulk of the volume, forces are computed
via the PM algorithm; timesteps in this PM component are large compared to
individually determined timesteps in the tree regions. Since each tree region
can be treated independently, the algorithm lends itself to very efficient
parallelization using message passing. We have tested the new TPM algorithm (a
refinement of that originated by Xu 1995) by comparison with results from
Ferrell & Bertschinger's P^3M code and find that, except in small clusters, the
TPM results are at least as accurate as those obtained with the
well-established P^3M algorithm, while taking significantly less computing
time. Production runs of 10^9 particles indicate that the new code has great
scientific potential when used with distributed computing resources.Comment: 24 pages including 9 figures, uses aaspp4.sty; revised to match
published versio

### Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: III. X-ray spectral modelling

Following the Swift X-ray observations of the 2006 outburst of the recurrent
nova RS Ophiuchi, we developed hydrodynamical models of mass ejection from
which the forward shock velocities were used to estimate the ejecta mass and
velocity. In order to further constrain our model parameters, here we present
synthetic X-ray spectra from our hydrodynamical calculations which we compare
to the Swift data. An extensive set of simulations was carried out to find a
model which best fits the spectra up to 100 days after outburst. We find a good
fit at high energies but require additional absorption to match the low energy
emission. We estimate the ejecta mass to be in the range (2-5) x 10^{-7} solar
masses and the ejection velocity to be greater than 6000 km/s (and probably
closer to 10,000 km/s). We also find that estimates of shock velocity derived
from gas temperatures via standard model fits to the X-ray spectra are much
lower than the true shock velocities.Comment: 13 pages, 5 figures, Accepted for publication in Ap

### The Amplitude of Mass Fluctuations

We determine the linear amplitude of mass fluctuations in the universe,
sigma_8, from the abundance of massive clusters at redshifts z=0.5 to 0.8. The
evolution of massive clusters depends exponentially on the amplitude of mass
fluctuations and thus provides a powerful measure of this important
cosmological parameter. The relatively high abundance of massive clusters
observed at z>0.5, and the relatively slow evolution of their abundance with
time, suggest a high amplitude of mass fluctuations: sigma_8=0.9 +-10% for
Omega_m=0.4, increasing slightly to sigma_8=0.95 for Omega_m=0.25 and
sigma_8=1.0 for Omega_m=0.1 (flat CDM models). We use the cluster abundance
observed at z=0.5 to 0.8 to derive a normalization relation from the
high-redshift clusters, which is only weakly dependent on Omega_m:
sigma_8*Omega_m^0.14 = 0.78 +-0.08. When combined with recent constraints from
the present-day cluster mass function (sigma_8*Omega_m^0.6=0.33 +-0.03) we find
sigma_8=0.98 +-0.1 and Omega_m=0.17 +-0.05. Low sigma_8 values (<0.7) are
unlikely; they produce an order of magnitude fewer massive clusters than
observed.Comment: 12 pages including 3 figures; updated to match published versio

### Peridynamic Galerkin method: an attractive alternative to finite elements

This work presents a meshfree particle scheme designed for arbitrary deformations that possess the accuracy and properties of the Finite-Element-Method. The accuracy is maintained even with arbitrary particle distributions. Mesh-based methods mostly fail if requirements on the location of evaluation points are not satisfied. Hence, with this new scheme not only the range of loadings can be increased but also the pre-processing step can be facilitated compared to the FEM. The key to this new meshfree method lies in the fulfillment of essential requirements for spatial discretization schemes. The new approach is based on the correspondence theory of Peridynamics. Some modifications of this framework allows for a consistent and stable formulation. By applying the peridynamic differentiation concept, it is also shown that the equations of the correspondence theory can be derived from the weak form. Likewise, it is demonstrated that special moving least square shape functions possess the Kronecker-δ property. Thus, Dirichlet boundary conditions can be directly applied. The positive performance of this new meshfree method, especially in comparison to the Finite-Element-Method, is shown in the calculation of several test cases. In order to guarantee a fair comparison enhanced finite element formulations are also used. The test cases include the patch test, an eigenmode analysis as well as the investigation of loadings in the context of large deformations. © 2022, The Author(s)

### Peridynamic Galerkin method: an attractive alternative to finite elements

This work presents a meshfree particle scheme designed for arbitrary deformations that possess the accuracy and properties of the Finite-Element-Method. The accuracy is maintained even with arbitrary particle distributions. Mesh-based methods mostly fail if requirements on the location of evaluation points are not satisfied. Hence, with this new scheme not only the range of loadings can be increased but also the pre-processing step can be facilitated compared to the FEM. The key to this new meshfree method lies in the fulfillment of essential requirements for spatial discretization schemes. The new approach is based on the correspondence theory of Peridynamics. Some modifications of this framework allows for a consistent and stable formulation. By applying the peridynamic differentiation concept, it is also shown that the equations of the correspondence theory can be derived from the weak form. Likewise, it is demonstrated that special moving least square shape functions possess the Kronecker-δ property. Thus, Dirichlet boundary conditions can be directly applied. The positive performance of this new meshfree method, especially in comparison to the Finite-Element-Method, is shown in the calculation of several test cases. In order to guarantee a fair comparison enhanced finite element formulations are also used. The test cases include the patch test, an eigenmode analysis as well as the investigation of loadings in the context of large deformations. © 2022, The Author(s)

### Accurate Realizations of the Ionized Gas in Galaxy Clusters: Calibrating Feedback

Using the full, three-dimensional potential of galaxy cluster halos (drawn
from an N-body simulation of the current, most favored cosmology), the
distribution of the X-ray emitting gas is found by assuming a polytropic
equation of state and hydrostatic equilibrium, with constraints from
conservation of energy and pressure balance at the cluster boundary. The
resulting properties of the gas for these simulated redshift zero clusters (the
temperature distribution, mass-temperature and luminosity-temperature
relations, and the gas fraction) are compared with observations in the X-ray of
nearby clusters. The observed properties are reproduced only under the
assumption that substantial energy injection from non-gravitational sources has
occurred. Our model does not specify the source, but star formation and AGN may
be capable of providing this energy, which amounts to 3 to 5 x10^{-5} of the
rest mass in stars (assuming ten percent of the gas initially in the cluster
forms stars). With the method described here it is possible to generate
realistic X-ray and Sunyaev-Zel'dovich cluster maps and catalogs from N-body
simulations, with the distributions of internal halo properties (and their
trends with mass, location, and time) taken into account.Comment: Matches ApJ published version; 30 pages, 7 figure

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