647 research outputs found
Quantum railroads and directed localization at the juncture of quantum Hall systems
The integer quantum Hall effect (QHE) and one-dimensional Anderson
localization (AL) are limiting special cases of a more general phenomenon,
directed localization (DL), predicted to occur in disordered one-dimensional
wave guides called "quantum railroads" (QRR). Here we explain the surprising
results of recent measurements by Kang et al. [Nature 403, 59 (2000)] of
electron transfer between edges of two-dimensional electron systems and
identify experimental evidence of QRR's in the general, but until now entirely
theoretical, DL regime that unifies the QHE and AL. We propose direct
experimental tests of our theory.Comment: 11 pages revtex + 3 jpeg figures, to appear in Phys. Rev.
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
The mass-to-light ratio of rich star clusters
We point out a strong time-evolution of the mass-to-light conversion factor
eta commonly used to estimate masses of unresolved star clusters from observed
cluster spectro-photometric measures. We present a series of gas-dynamical
models coupled with the Cambridge stellar evolution tracks to compute
line-of-sight velocity dispersions and half-light radii weighted by the
luminosity. We explore a range of initial conditions, varying in turn the
cluster mass and/or density, and the stellar population's IMF. We find that
eta, and hence the estimated cluster mass, may increase by factors as large as
3 over time-scales of 50 million years. We apply these results to an hypothetic
cluster mass distribution function (d.f.) and show that the d.f. shape may be
strongly affected at the low-mass end by this effect. Fitting truncated
isothermal (Michie-King) models to the projected light profile leads to
over-estimates of the concentration parameter c of delta c ~ 0.3 compared to
the same functional fit applied to the projected mass density.Comment: 6 pages, 2 figures, to appear in the proceedings of the "Young
massive star clusters", Granada, Spain, September 200
Recommended from our members
The extent that certain dairy farmer attitudes and behaviors are associated with farm business profitability
The way in which farm managers' attitudes, personality, behavior, values, and sociodemographic characteristics influence farm business performance is, at best, only partially understood. The study reported here expands on this understanding by analyzing the attitudes and personal attributes of 80 dairy farmers in Great Britain in relation to the profitability over 3 yr of their farm businesses. Business goals, temperament, purchasing behavior, and having a growth mindset toward the business were found to be associated with profitability. A linear regression model consisting of 5 variables related to the above was presented that predicts 34% of the observed variation in profitability. Each of these variables were questions related to the participants' personal attitudes or beliefs. Other assessed variables, such as specific husbandry behaviors or practices, or management practices and sociodemographic characteristics, did not warrant inclusion in the final model. These results uniquely contribute to understanding how the attitudes, personality, behaviors, and attributes of dairy farmers are associated with, and thus likely to influence, the profitability of their farm businesses
The evolution of galaxy groups and of galaxies therein
Properties of groups of galaxies depend sensitively on the algorithm for
group selection, and even the most recent catalogs of groups built from
redshift-space selection should suffer from projections and infalling galaxies.
The cosmo-dynamical evolution of groups from initial Hubble expansion to
collapse and virialization leads to a fundamental track (FT) in
virial-theorem-M/L vs crossing time. The increased rates of mergers, both
direct and after dynamical friction, in groups relative to clusters, explain
the higher fraction of elliptical galaxies at given local number density in
X-ray selected groups, relative to clusters, even when the hierarchical
evolution of groups is considered. Galaxies falling into groups and clusters
should later travel outwards to typically 2 virial radii, which is somewhat
less than the outermost radius where observed galaxy star formation
efficiencies are enhanced relative to field galaxies of same morphological
type. An ongoing analysis of the internal kinematics of X-ray selected groups
suggests that the radial profiles of line of sight velocity dispersion are
consistent with isotropic NFW distributions for the total mass density, with
higher (lower) concentrations than LambdaCDM predictions in groups of high
(low) mass. The critical mass, at M200 ~ 10^13 M_sun is consistent with
possible breaks in the X-ray luminosity-temperature and Fundamental Plane
relations. The internal kinematics of groups indicate that the M-T relation of
groups should agree with that extrapolated from clusters with no break at the
group scale. The analyses of observed velocity dispersion profiles and of the
FT both suggest that low velocity dispersion groups (compact and loose, X-ray
emitting or undetected) are quite contaminated by chance projections.Comment: Invited review, ESO workshop "Groups of Galaxies in the Nearby
Universe", held in Santiago, Chile, 5-9 December 2005, ed. I. Saviane, V.
Ivanov & J. Borissova, 16 page
N-body simulations of gravitational dynamics
We describe the astrophysical and numerical basis of N-body simulations, both
of collisional stellar systems (dense star clusters and galactic centres) and
collisionless stellar dynamics (galaxies and large-scale structure). We explain
and discuss the state-of-the-art algorithms used for these quite different
regimes, attempt to give a fair critique, and point out possible directions of
future improvement and development. We briefly touch upon the history of N-body
simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu
Spin-Charge Separation in the Model: Magnetic and Transport Anomalies
A real spin-charge separation scheme is found based on a saddle-point state
of the model. In the one-dimensional (1D) case, such a saddle-point
reproduces the correct asymptotic correlations at the strong-coupling
fixed-point of the model. In the two-dimensional (2D) case, the transverse
gauge field confining spinon and holon is shown to be gapped at {\em finite
doping} so that a spin-charge deconfinement is obtained for its first time in
2D. The gap in the gauge fluctuation disappears at half-filling limit, where a
long-range antiferromagnetic order is recovered at zero temperature and spinons
become confined. The most interesting features of spin dynamics and transport
are exhibited at finite doping where exotic {\em residual} couplings between
spin and charge degrees of freedom lead to systematic anomalies with regard to
a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic
fluctuation with a small, doping-dependent energy scale is found, which is
characterized in momentum space by a Gaussian peak at (, ) with
a doping-dependent width (, is the doping
concentration). This commensurate magnetic fluctuation contributes a
non-Korringa behavior for the NMR spin-lattice relaxation rate. There also
exits a characteristic temperature scale below which a pseudogap behavior
appears in the spin dynamics. Furthermore, an incommensurate magnetic
fluctuation is also obtained at a {\em finite} energy regime. In transport, a
strong short-range phase interference leads to an effective holon Lagrangian
which can give rise to a series of interesting phenomena including linear-
resistivity and Hall-angle. We discuss the striking similarities of these
theoretical features with those found in the high- cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request;
minor revisions in the text and references have been made; To be published in
July 1 issue of Phys. Rev. B52, (1995
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Growth of Forage Legumes and Grasses in Acidic Soil Amended with Flue Gas Desulfurization Products
- …