23,841 research outputs found
Water impact loads
Computer program to generate time history of load factor and pressure on conical body of revolution during impact with water is discussed. Program calculates depth of penetration, velocity, force, load factor, maximum pressure at water line, and average pressure. Program is written in FORTRAN 4 Level H for IBM 360/85/195 Release 20.1 computer
Diffusion coefficients for multi-step persistent random walks on lattices
We calculate the diffusion coefficients of persistent random walks on
lattices, where the direction of a walker at a given step depends on the memory
of a certain number of previous steps. In particular, we describe a simple
method which enables us to obtain explicit expressions for the diffusion
coefficients of walks with two-step memory on different classes of one-, two-
and higher-dimensional lattices.Comment: 27 pages, 2 figure
Investigating the cores of fossil systems with Chandra
We investigate the cores of fossil galaxy groups and clusters (`fossil
systems') using archival Chandra data for a sample of 17 fossil systems. We
determined the cool-core fraction for fossils via three observable diagnostics,
the central cooling time, cuspiness, and concentration parameter. We quantified
the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy
(BCG), and the X-ray peak/emission weighted centre separations. We studied the
X-ray emission coincident with the BCG to detect the presence of potential
thermal coronae. A deprojection analysis was performed for z < 0.05 fossils to
obtain cooling time and entropy profiles, and to resolve subtle temperature
structures. We investigated the Lx-T relation for fossils from the 400d
catalogue to see if the scaling relation deviates from that of other groups.
Most fossils are identified as cool-core objects via at least two cool-core
diagnostics. All fossils have their dominant elliptical galaxy within 50 kpc of
the X-ray peak, and most also have the emission weighted centre within that
distance. We do not see clear indications of a X-ray corona associated with the
BCG unlike that has been observed for some other objects. Fossils do not have
universal temperature profiles, with some low-temperature objects lacking
features that are expected for ostensibly relaxed objects with a cool-core. The
entropy profiles of the z < 0.05 fossil systems can be well-described by a
power law model, albeit with indices smaller than 1. The 400d fossils Lx-T
relation shows indications of an elevated normalisation with respect to other
groups, which seems to persist even after factoring in selection effects.Comment: Accepted for publication in Astronomy and Astrophysic
The ground-state of General Relativity, Topological Theories and Dark Matter
We suggest a limit of Einstein equations incorporating the state
as a solution. The large scale behavior of this theory has
interesting properties. For a spherical source, the velocity profile for
circular motions is of the form observed in galaxies (approximately flat). For
FRW cosmologies, the Friedman equation contains an additional contribution in
the matter sector.Comment: More clarifications on the interpretation of the limits. Shorter
version. 4 pages, two column, no figure
Is dark matter an extra-dimensional effect?
We investigate the possibility that the observed behavior of test particles
outside galaxies, which is usually explained by assuming the presence of dark
matter, is the result of the dynamical evolution of particles in higher
dimensional space-times. Hence, dark matter may be a direct consequence of the
presence of an extra force, generated by the presence of extra-dimensions,
which modifies the dynamic law of motion, but does not change the intrinsic
properties of the particles, like, for example, the mass (inertia). We discuss
in some detail several possible particular forms for the extra force, and the
acceleration law of the particles is derived. Therefore, the constancy of the
galactic rotation curves may be considered as an empirical evidence for the
existence of the extra dimensions.Comment: 11 pages, no figures, accepted for publication in MPLA; references
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MOND rotation curves for spiral galaxies with Cepheid-based distances
Rotation curves for four spiral galaxies with recently determined
Cepheid-based distances are reconsidered in terms of modified Newtonian
dynamics (MOND). For two of the objects, NGC 2403 and NGC 7331, the rotation
curves predicted by MOND are compatible with the observed curves when these
galaxies are taken to be at the Cepheid distance. For NGC 3198, the largest
distance for which reasonable agreement is obtained is 10% smaller than the
Cepheid-based distance; i.e., MOND clearly prefers a smaller distance. This
conclusion is unaltered when new near-infrared photometry of NGC 3198 is taken
as the tracer of the stellar mass distribution. For the large Sc spiral, NGC
2841, MOND requires a distance which is at least 20% larger than the
Cepheid-based distance. However, the discrepancy of the Tully-Fisher and SNIa
distances with the Cepheid determination casts some doubt upon the Cepheid
method in this case.Comment: Accepted for publication in astronomy and astrophysics 9 pages, 9
figure
Modifying gravity with the Aether: an alternative to Dark Matter
There is evidence that Newton and Einstein's theories of gravity cannot
explain the dynamics of a universe made up solely of baryons and radiation. To
be able to understand the properties of galaxies, clusters of galaxies and the
universe on the whole it has become commonplace to invoke the presence of dark
matter. An alternative approach is to modify the gravitational field equations
to accommodate observations. We propose a new class of gravitational theories
in which we add a new degree of freedom, the Aether, in the form of a vector
field that is coupled covariantly, but non-minimally, with the space-time
metric. We explore the Newtonian and non-Newtonian limits, discuss the
conditions for these theories to be consistent and explore their effect on
cosmology.Comment: Updated version: Notation improved - TG
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Newtonian limit of String-Dilaton Gravity
We study the weak-field limit of string-dilaton gravity and derive
corrections to the Newtonian potential which strength directly depends on the
self interaction potential and the nonminimal coupling of the dilaton scalar
field. We discuss also possible astrophysical applications of the results, in
particular the flat rotation curves of spiral galaxies.Comment: 11 pages, LATEX file, to appear in IJMP
BOOMERanG Data Suggest a Purely Baryonic Universe
The amplitudes of peaks in the angular power spectrum of anisotropies in the
microwave background radiation depend on the mass content of the universe. The
second peak should be prominent when cold dark matter is dominant, but is
depressed when baryons dominate. Recent microwave background data are
consistent with a purely baryonic universe with Omega(matter) = Omega(baryon) ~
0.03 and Omega(Lambda) ~ 1.Comment: 10 pages AASTeX with 1 color postscript figure. Accepted for
publication in ApJ Letters. And yes, the prediction was in the literature
before the dat
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