400,368 research outputs found
The persistent cosmic web and its filamentary structure I: Theory and implementation
We present DisPerSE, a novel approach to the coherent multi-scale
identification of all types of astrophysical structures, and in particular the
filaments, in the large scale distribution of matter in the Universe. This
method and corresponding piece of software allows a genuinely scale free and
parameter free identification of the voids, walls, filaments, clusters and
their configuration within the cosmic web, directly from the discrete
distribution of particles in N-body simulations or galaxies in sparse
observational catalogues. To achieve that goal, the method works directly over
the Delaunay tessellation of the discrete sample and uses the DTFE density
computed at each tracer particle; no further sampling, smoothing or processing
of the density field is required.
The idea is based on recent advances in distinct sub-domains of computational
topology, which allows a rigorous application of topological principles to
astrophysical data sets, taking into account uncertainties and Poisson noise.
Practically, the user can define a given persistence level in terms of
robustness with respect to noise (defined as a "number of sigmas") and the
algorithm returns the structures with the corresponding significance as sets of
critical points, lines, surfaces and volumes corresponding to the clusters,
filaments, walls and voids; filaments, connected at cluster nodes, crawling
along the edges of walls bounding the voids. The method is also interesting as
it allows for a robust quantification of the topological properties of a
discrete distribution in terms of Betti numbers or Euler characteristics,
without having to resort to smoothing or having to define a particular scale.
In this paper, we introduce the necessary mathematical background and
describe the method and implementation, while we address the application to 3D
simulated and observed data sets to the companion paper.Comment: A higher resolution version is available at
http://www.iap.fr/users/sousbie together with complementary material.
Submitted to MNRA
Diagnostics of active galaxies: I. Modeling the infrared properties of dusty cores of starburst galaxies
An evolutionary model of star formation in ULIRGs was created using existing
star formation and radiative transfer codes (STARBURST99, RADMC and RADICAL) as
building blocks. The results of the simulations are compared to data from two
IRAS catalogs. From the simulations it is found that the dust surrounding the
starburst region is made up from two components. There is a low optical depth
(tau=0.1, which corresponds to 0.1% of the total dust mass), hot (T~400K)
non-grey component close to the starburst (scale size 10pc) and a large scale,
colder grey component (100pc, 75K) with a much larger column (tau=10). The
simulations also show that starburst galaxies can be powered by massive star
formation. The parameters for this star forming region are difficult to
determine, since the IR continuum luminosity is only sensitive to the total UV
input. Therefore, there is a degeneracy between the total starburst mass and
the initial mass function (IMF) slope. A less massive star formation with a
shallower IMF will produce the same amount of OB stars and therefore the same
amount of irradiating UV flux. Assuming the stars are formed according to a
Salpeter IMF (Psi(M) ~ M^-2.35), the star formation region should produce 10^9
Msun of stars (either in one instantaneous burst, or in a continuous process)
in order to produce enough IR radiation.Comment: 37 pages, 32 figures, accepted by Astronomy and Astrophysics. Version
with Appendices can be obtained from
http://www.astro.rug.nl/~loenen/Loenen_et_al_2006.pd
Self-consistent Modeling of the of HTS Devices: How Accurate do Models Really Need to Be?
Numerical models for computing the effective critical current of devices made
of HTS tapes require the knowledge of the Jc(B,theta) dependence, i.e. of the
way the critical current density Jc depends on the magnetic flux density B and
its orientation theta with respect to the tape. In this paper we present a
numerical model based on the critical state with angular field dependence of Jc
to extract the Jc(B,theta) relation from experimental data. The model takes
into account the self-field created by the tape, which gives an important
contribution when the field applied in the experiments is low. The same model
can also be used to compute the effective critical current of devices composed
of electromagnetically interacting tapes. Three examples are considered here:
two differently current rated Roebel cables composed of REBCO coated conductors
and a power cable prototype composed of Bi-2223 tapes. The critical currents
computed with the numerical model show good agreement with the measured ones.
The simulations reveal also that several parameter sets in the Jc(B,theta) give
an equally good representation of the experimental characterization of the
tapes and that the measured Ic values of cables are subjected to the influence
of experimental conditions, such as Ic degradation due to the manufacturing and
assembling process and non-uniformity of the tape properties. These two aspects
make the determination of a very precise Jc(B,theta) expression probably
unnecessary, as long as that expression is able to reproduce the main features
of the angular dependence. The easiness of use of this model, which can be
straightforwardly implemented in finite-element programs able to solve static
electromagnetic problems, is very attractive both for researchers and devices
manufactures who want to characterize superconducting tapes and calculate the
effective critical current of superconducting devices
A search for ferromagnetism in transition-metal-doped piezoelectric ZnO
We present the results of a computational study of ZnO in the presence of Co
and Mn substitutional impurities. The goal of our work is to identify potential
ferromagnetic ground states within the (Zn,Co)O or (Zn,Mn)O material systems
that are also good candidates for piezoelectricity. We find that, in contrast
to previous results, robust ferromagnetism is not obtained by substitution of
Co or Mn on the Zn site, unless additional carriers (holes) are also
incorporated. We propose a practical scheme for achieving such -type doping
in ZnO
- …