2,547 research outputs found
Exact vortex nucleation and cooperative vortex tunneling in dilute BECs
With the imminent advent of mesoscopic rotating BECs in the lowest Landau
level (LLL) regime, we explore LLL vortex nucleation. An exact many-body
analysis is presented in a weakly elliptical trap for up to 400 particles.
Striking non-mean field features are exposed at filling factors >>1 . Eg near
the critical rotation frequency pairs of energy levels approach each other with
exponential accuracy. A physical interpretation is provided by requantising a
mean field theory, where 1/N plays the role of Planck's constant, revealing two
vortices cooperatively tunneling between classically degenerate energy minima.
The tunnel splitting variation is described in terms of frequency, particle
number and ellipticity.Comment: 4 pages,4 figure
Deep ROSAT Surveys & the contribution of AGNs to the soft X-ray background
The ROSAT Deep Surveys in the Lockman Hole have revealed that AGNs are the main contributors (~75%) to the soft X-ray background in the 1–2 keV band. Using new optical/infrared and radio observations we have obtained a nearly complete identification (93%) of the 91 X-ray sources down to a limiting flux of 1.2·10^(–15) erg cm^(–2) s^(–1) in the 0.5–2.0 keV band. We present the optical colors and the emission line properties of our AGNs in comparison with other X-ray selected AGN samples. Furthermore we discuss the fraction of red AGNs found in the ROSAT Deep Surveys. From the ROSAT Deep Surveys we see no evidence for a new class of X-ray bright galaxies, which significantly contributes to the soft X-ray background
The velocity peaks in the cold dark matter spectrum on Earth
The cold dark matter spectrum on earth is expected to have peaks in velocity
space. We obtain estimates for the sizes and locations of these peaks. To this
end we have generalized the secondary infall model of galactic halo formation
to include angular momentum of the dark matter particles. This new model is
still spherically symmetric and it has self-similar solutions. Our results are
relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio
Large-amplitude isothermal fluctuations and high-density dark-matter clumps
Large-amplitude isothermal fluctuations in the dark matter energy density,
parameterized by \Phi\equiv\delta\rhodm/\rhodm, are studied within the
framework of a spherical collapse model. For \Phi \ga 1, a fluctuation
collapses in the radiation-dominated epoch and produces a dense dark-matter
object. The final density of the virialized object is found to be \rho_F
\approx 140\, \Phi^3 (\Phi+1) \rhoeq, where \rhoeq is the matter density at
equal matter and radiation energy density. This expression is valid for the
entire range of possible values of , both for and . Some astrophysical consequences of high-density dark-matter clumps are
discussed.Comment: 15 pages plus 3 figures (included at the end as a uuencoded
postscript file), LaTeX, FNAL--PUB--94/055--
Oblique ion collection in the drift-approximation: how magnetized Mach-probes really work
The anisotropic fluid equations governing a frictionless obliquely-flowing
plasma around an essentially arbitrarily shaped three-dimensional ion-absorbing
object in a strong magnetic field are solved analytically in the quasi-neutral
drift-approximation, neglecting parallel temperature gradients. The effects of
transverse displacements traversing the magnetic presheath are also quantified.
It is shown that the parallel collection flux density dependence upon external
Mach-number is where is the angle (in the plane of field and
drift velocity) of the object-surface to the magnetic-field and
is the external parallel flow. The perpendicular drift,
\M_\perp, appearing here consists of the external \E\wedge\B drift plus a
weighted sum of the ion and electron electron diamagnetic drifts that depends
upon the total angle of the surface to the magnetic field. It is that somewhat
counter-intuitive combination that an oblique (transverse) Mach probe
experiment measures.Comment: Revised version following refereeing for Physics of Plasma
Cosmological models from quintessence
A generalized quintessence model is presented which corresponds to a richer
vacuum structure that, besides a time-dependent, slowly varying scalar field,
contains a varying cosmological term. From first principles we determine a
number of scalar-field potentials that satisfy the constraints imposed by the
field equations and conservations laws, both in the conventional and
generalized quintessence models. Besides inverse-power law solutions, these
potentials are given in terms of hyperbolic functions or the twelve Jacobian
elliptic functions, and are all related to the luminosity distance by means of
an integral equation. Integration of this equation for the different solutions
leads to a large family of cosmological models characterized by luminosity
distance-redshift relations. Out of such models, only four appear to be able to
predict a required accelerating universe conforming to observations on
supernova Ia, at large or moderate redshifts.Comment: 9 pages, RevTex, to appear in Phys. Rev.
The Evolutionary Status of Clusters of Galaxies at z ~ 1
Combined HST, X-ray, and ground-based optical studies show that clusters of
galaxies are largely "in place" by , an epoch when the Universe was
less than half its present age. High resolution images show that elliptical,
S0, and spiral galaxies are present in clusters at redshifts up to . Analysis of the CMDs suggest that the cluster ellipticals formed their
stars several Gyr earlier, near redshift 3. The morphology--density relation is
well established at , with star-forming spirals and irregulars residing
mostly in the outer parts of the clusters and E/S0s concentrated in dense
clumps. The intracluster medium has already reached the metallicity of
present-day clusters. The distributions of the hot gas and early-type galaxies
are similar in clusters, indicating both have largely virialized in
the deepest potentials wells. In spite of the many similarities between
and present-day clusters, there are significant differences. The
morphologies revealed by the hot gas, and particularly the early-type galaxies,
are elongated rather than spherical. We appear to be observing the clusters at
an epoch when the sub-clusters and groups are still assembling into a single
regular cluster. Support for this picture comes from CL0152 where the gas
appears to be lagging behind the luminous and dark mass in two merging
sub-components. Moreover, the luminosity difference between the first and
second brightest cluster galaxies at is smaller than in 93% of
present-day Abell clusters, which suggests that considerable luminosity
evolution through merging has occurred since that epoch. Evolution is also seen
in the bolometric X-ray luminosity function.Comment: 18 pages, 12 figures, to appear in Penetrating Bars through Masks of
Cosmic Dust: the Hubble Tuing Fork Strikes a New Note, eds. D.L. Block, K.C.
Freeman, I. Puerari & R. Groess. Figures degraded to meet astroph size limit;
a version with higher resolution figures may be downloaded from:
http://acs.pha.jhu.edu/~jpb/z1clusters/ford_clusters.pd
Formation of convective cells in the scrape-off layer of the CASTOR tokamak
Understanding of the scrape-off layer (SOL) physics in tokamaks requires
diagnostics with sufficient temporal and spatial resolution. This contribution
describes results of experiments performed in the SOL of the CASTOR tokamak
(R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the
whole poloidal cross section. The individual probes measure either the ion
saturation current of the floating potential with the spatial resolution up to
3 mm. Experiments are performed in a particular magnetic configuration,
characterized by a long parallel connection length in the SOL, L_par ~q2piR. We
report on measurements in discharges, where the edge electric field is modified
by inserting a biased electrode into the edge plasma. In particular, a complex
picture is observed, if the biased electrode is located inside the SOL. The
poloidal distribution of the floating potential appears to be strongly
non-uniform at biasing. The peaks of potential are observed at particular
poloidal angles. This is interpreted as formation of a biased flux tube, which
emanates from the electrode along the magnetic field lines and snakes q times
around the torus. The resulting electric field in the SOL is 2-dimensional,
having the radial as well as the poloidal component. It is demonstrated that
the poloidal electric field E_pol convects the edge plasma radially due to the
E_pol x B_T drift either inward or outward depending on its sign. The
convective particle flux is by two orders of magnitude larger than the
fluctuation-induced one and consequently dominates.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
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