42,372 research outputs found
On The Orbital Evolution of Jupiter Mass Protoplanet Embedded in A Self-Gravity Disk
We performed a series of hydro-dynamic simulations to investigate the orbital
migration of a Jovian planet embedded in a proto-stellar disk. In order to take
into account of the effect of the disk's self gravity, we developed and adopted
an \textbf{Antares} code which is based on a 2-D Godunov scheme to obtain the
exact Reimann solution for isothermal or polytropic gas, with non-reflecting
boundary conditions. Our simulations indicate that in the study of the runaway
(type III) migration, it is important to carry out a fully self consistent
treatment of the gravitational interaction between the disk and the embedded
planet. Through a series of convergence tests, we show that adequate numerical
resolution, especially within the planet's Roche lobe, critically determines
the outcome of the simulations. We consider a variety of initial conditions and
show that isolated, non eccentric protoplanet planets do not undergo type III
migration. We attribute the difference between our and previous simulations to
the contribution of a self consistent representation of the disk's self
gravity. Nevertheless, type III migration cannot be completely suppressed and
its onset requires finite amplitude perturbations such as that induced by
planet-planet interaction. We determine the radial extent of type III migration
as a function of the disk's self gravity.Comment: 19 pages, 13 figure
Distributed Stochastic Optimization over Time-Varying Noisy Network
This paper is concerned with distributed stochastic multi-agent optimization
problem over a class of time-varying network with slowly decreasing
communication noise effects. This paper considers the problem in composite
optimization setting which is more general in noisy network optimization. It is
noteworthy that existing methods for noisy network optimization are Euclidean
projection based. We present two related different classes of non-Euclidean
methods and investigate their convergence behavior. One is distributed
stochastic composite mirror descent type method (DSCMD-N) which provides a more
general algorithm framework than former works in this literature. As a
counterpart, we also consider a composite dual averaging type method (DSCDA-N)
for noisy network optimization. Some main error bounds for DSCMD-N and DSCDA-N
are obtained. The trade-off among stepsizes, noise decreasing rates,
convergence rates of algorithm is analyzed in detail. To the best of our
knowledge, this is the first work to analyze and derive convergence rates of
optimization algorithm in noisy network optimization. We show that an optimal
rate of in nonsmooth convex optimization can be obtained for
proposed methods under appropriate communication noise condition. Moveover,
convergence rates in different orders are comprehensively derived in both
expectation convergence and high probability convergence sense.Comment: 27 page
Single-Transverse Spin Asymmetry in Dijet Correlations at Hadron Colliders
We present a phenomenological study of the single-transverse spin asymmetry
in azimuthal correlations of two jets produced nearly "back-to-back" in pp
collisions at RHIC. We properly take into account the initial- and final-state
interactions of partons that can generate this asymmetry in QCD
hard-scattering. Using distribution functions fitted to the existing
single-spin data, we make predictions for various weighted single-spin
asymmetries in dijet correlations that are now readily testable at RHIC.Comment: 14 pages, 2 figure
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
A Novel FastICA Method for the Reference-based Contrast Functions
This paper deals with the efficient optimization problem of Cumulant-based contrast criteria in the Blind Source Separation (BSS) framework, in which sources are retrieved by maximizing the Kurtosis contrast function. Combined with the recently proposed reference-based contrast schemes, a new fast fixed-point (FastICA) algorithm is proposed for the case of linear and instantaneous mixture. Due to its quadratic dependence on the number of searched parameters, the main advantage of this new method consists in the significant decrement of computational speed, which is particularly striking with large number of samples. The method is essentially similar to the classical algorithm based on the Kurtosis contrast function, but differs in the fact that the reference-based idea is utilized. The validity of this new method was demonstrated by simulations
Time-optimal synthesis of unitary transformations in coupled fast and slow qubit system
In this paper, we study time-optimal control problems related to system of
two coupled qubits where the time scales involved in performing unitary
transformations on each qubit are significantly different. In particular, we
address the case where unitary transformations produced by evolutions of the
coupling take much longer time as compared to the time required to produce
unitary transformations on the first qubit but much shorter time as compared to
the time to produce unitary transformations on the second qubit. We present a
canonical decomposition of SU(4) in terms of the subgroup SU(2)xSU(2)xU(1),
which is natural in understanding the time-optimal control problem of such a
coupled qubit system with significantly different time scales. A typical
setting involves dynamics of a coupled electron-nuclear spin system in pulsed
electron paramagnetic resonance experiments at high fields. Using the proposed
canonical decomposition, we give time-optimal control algorithms to synthesize
various unitary transformations of interest in coherent spectroscopy and
quantum information processing.Comment: 8 pages, 3 figure
Spectrum of low-lying configurations with negative parity
Spectrum of low-lying five-quark configurations with strangeness quantum
number and negative parity is studied in three kinds of constituent
quark models, namely the one gluon exchange, Goldstone Boson exchange, and
instanton-induced hyperfine interaction models, respectively. Our numerical
results show that the lowest energy states in all the three employed models are
lying at 1800 MeV, about 200 MeV lower than predictions of various
quenched three-quark models. In addition, it is very interesting that the state
with the lowest energy in one gluon exchange model is with spin 3/2, but 1/2 in
the other two models.Comment: Version published in Phys. Rev.
An Efficient Algorithm by Kurtosis Maximization in Reference-Based Framework
This paper deals with the optimization of kurtosis for complex-valued signals in the independent component analysis (ICA) framework, where source signals are linearly and instantaneously mixed. Inspired by the recently proposed reference-based contrast schemes, a similar contrast function is put forward, based on which a new fast fixed-point (FastICA) algorithm is proposed. The new optimization method is similar in spirit to the former classical kurtosis-based FastICA algorithm but differs in the fact that it is much more efficient than the latter in terms of computational speed, which is significantly striking with large number of samples. The performance of this new algorithm is confirmed through computer simulations
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