11,067 research outputs found
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Unofficial trade when states are weak: the case of cross-border commerce in the Horn of Africa
This paper addresses informal cross-border trade in the Horn of Africa, with an emphasis on the Somalia borderlands. It will be shown that despite the collapse of a government in 1991, Somalia’s unofficial exports of cattle to Kenya have grown considerably during the past 13 years.Qoraalkani wuxuu ka hadlayaa ganacsiga ka gudbo xuduudaha Geeska Afrika. Gaar ahaan markii uu dhacay taliskii militeriga waxaa aad u sii kordhay xoolixii soomaaliyeed ee loo suuq geynaayay Kenya.Questo documento è dedicato al commercio transnazionale informale nel Corno D'Africa, con particolare attenzione ai confini della Somalia. Si dimostra che nonostante la caduta del governo nel 1991, le esportazioni di bestiame non ufficiali somale in Kenya sono aumentate considerevolmente durante gli ultimi 13 anni
Non-ergodic Jackson Networks with Infinite Supply - Local Stabilization and Local Equilibrium Analysis
Classical Jackson networks are a well established tool for the analysis of complex systems. In this paper we analyze Jackson networks with the additional features that (i) nodes may have an in nite supply of low priority work and (ii) nodes may be unstable in the sense that the queue length at these nodes grows beyond any bound. We provide the limiting distribution of the queue length distribution at stable nodes, which turns out to be of product-form. A key step in establishing this result is the development of a new algorithm based on adjusted trac equations for detecting instable nodes. Our results complement the results known in the literature for the sub-cases of Jackson networks with either innite supply nodes or unstable nodes by providing an analysis of the signicantly more challenging case of networks with both types of nonstandard nodes present. Building on our product-form results, we provide closed-form solutions for common customer and system oriented performance measures
Three-Quark Potential in SU(3) Lattice QCD
The static three-quark (3Q) potential is measured in the SU(3) lattice QCD
with and at the quenched level. From the 3Q Wilson
loop, the 3Q ground-state potential is extracted using the
smearing technique for the ground-state enhancement. With accuracy better than
a few %, is well described by a sum of a constant, the two-body
Coulomb term and the three-body linear confinement term , where denotes the minimal length of the color flux tube
linking the three quarks. By comparing with the Q- potential, we
find a universal feature of the string tension, , as well as the one-gluon-exchange result for the
Coulomb coefficient, .Comment: 7 pages, 3 figur
Towards a precision computation of f_Bs in quenched QCD
We present a computation of the decay constant f_Bs in quenched QCD. Our
strategy is to combine new precise data from the static approximation with an
interpolation of the decay constant around the charm quark mass region. This
computation is the first step in demonstrating the feasability of a strategy
for f_B in full QCD. The continuum limits in the static theory and at finite
mass are taken separately and will be further improved.Comment: Lattice2003(heavy), 3 pages, 2 figure
Evolution of Fermion Pairing from Three to Two Dimensions
We follow the evolution of fermion pairing in the dimensional crossover from
3D to 2D as a strongly interacting Fermi gas of Li atoms becomes confined
to a stack of two-dimensional layers formed by a one-dimensional optical
lattice. Decreasing the dimensionality leads to the opening of a gap in
radio-frequency spectra, even on the BCS-side of a Feshbach resonance. The
measured binding energy of fermion pairs closely follows the theoretical
two-body binding energy and, in the 2D limit, the zero-temperature mean-field
BEC-BCS theory.Comment: 5 pages, 4 figure
Localization transition of random copolymers at interfaces
We consider adsorption of random copolymer chains onto an interface within
the model of Garel et al. Europhysics Letters 8, 9 (1989). By using the replica
method the adsorption of the copolymer at the interface is mapped onto the
problem of finding the ground state of a quantum mechanical Hamiltonian. To
study this ground state we introduce a novel variational principle for the
Green's function, which generalizes the well-known Rayleigh-Ritz method of
Quantum Mechanics to nonstationary states. Minimization with an appropriate
trial Green's function enables us to find the phase diagram for the
localization-delocalization transition for an ideal random copolymer at the
interface.Comment: 5 page
Electrostatic extraction of cold molecules from a cryogenic reservoir
We present a method which delivers a continuous, high-density beam of slow
and internally cold polar molecules. In our source, warm molecules are first
cooled by collisions with a cryogenic helium buffer gas. Cold molecules are
then extracted by means of an electrostatic quadrupole guide. For ND the
source produces fluxes up to molecules/s with
peak densities up to molecules/cm. For
HCO the population of rovibrational states is monitored by depletion
spectroscopy, resulting in single-state populations up to .Comment: 4 pages, 4 figures, changes to the text, updated figures and
reference
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