29,336 research outputs found
KH15D: a star eclipsed by a large scale dusty vortex?
We propose that the large photometric variations of KH15D are due to an
eclipsing swarm of solid particles trapped in giant gaseous vortex rotating at
\~0.2 AU from the star. The efficiency of the capture-in-vortex mechanism
easily explains the observed large optical depth. The weaker opacity at
mid-eclipse is consistent with a size segregation of the particles toward the
center of the vortex. This dusty structure must extend over ~1/3 of an orbit to
account for the long eclipse duration. The estimated size of the trapped
particles is found to range from 1 to 10cm, consistent with the gray extinction
of the star. The observations of KH15D support the idea that giant vortices can
grow in circumstellar disks and play a central role in planet formation.Comment: Accepted in ApJ Letters - 4 pages - 2 figure
Measurement of Spin Transfer Observables in Antiproton-Proton -> Antilambda-Lambda at 1.637 GeV/c
Spin transfer observables for the strangeness-production reaction
Antiproton-Proton -> Antilambda-Lambda have been measured by the PS185
collaboration using a transversely-polarized frozen-spin target with an
antiproton beam momentum of 1.637 GeV/c at the Low Energy Antiproton Ring at
CERN. This measurement investigates observables for which current models of the
reaction near threshold make significantly differing predictions. Those models
are in good agreement with existing measurements performed with unpolarized
particles in the initial state. Theoretical attention has focused on the fact
that these models produce conflicting predictions for the spin-transfer
observables D_{nn} and K_{nn}, which are measurable only with polarized target
or beam. Results presented here for D_{nn} and K_{nn} are found to be in
disagreement with predictions from existing models. These results also
underscore the importance of singlet-state production at backward angles, while
current models predict complete or near-complete triplet-state dominance.Comment: 5 pages, 3 figure
The Relativistic N-body Problem in a Separable Two-Body Basis
We use Dirac's constraint dynamics to obtain a Hamiltonian formulation of the
relativistic N-body problem in a separable two-body basis in which the
particles interact pair-wise through scalar and vector interactions. The
resultant N-body Hamiltonian is relativistically covariant. It can be easily
separated in terms of the center-of-mass and the relative motion of any
two-body subsystem. It can also be separated into an unperturbed Hamiltonian
with a residual interaction. In a system of two-body composite particles, the
solutions of the unperturbed Hamiltonian are relativistic two-body internal
states, each of which can be obtained by solving a relativistic
Schr\"odinger-like equation. The resultant two-body wave functions can be used
as basis states to evaluate reaction matrix elements in the general N-body
problem. We prove a relativistic version of the post-prior equivalence which
guarantees a unique evaluation of the reaction matrix element, independent of
the ways of separating the Hamiltonian into unperturbed and residual
interactions. Since an arbitrary reaction matrix element involves composite
particles in motion, we show explicitly how such matrix elements can be
evaluated in terms of the wave functions of the composite particles and the
relevant Lorentz transformations.Comment: 42 pages, 2 figures, in LaTe
Infrared Surface Brightness Distances to Cepheids: a comparison of Bayesian and linear-bisector calculations
We have compared the results of Bayesian statistical calculations and
linear-bisector calculations for obtaining Cepheid distances and radii by the
infrared surface brightness method. We analyzed a set of 38 Cepheids using a
Bayesian Markov Chain Monte Carlo method that had been recently studied with a
linear-bisector method. The distances obtained by the two techniques agree to
1.5 \pm 0.6% with the Bayesian distances being larger. The radii agree to 1.1%
\pm 0.7% with the Bayesian determinations again being larger. We interpret this
result as demonstrating that the two methods yield the same distances and
radii. This implies that the short distance to the LMC found in recent
linear-bisector studies of Cepheids is not caused by deficiencies in the
mathematical treatment. However, the computed uncertainties in distance and
radius for our dataset are larger in the Bayesian calculation by factors of
1.4-6.7. We give reasons to favor the Bayesian computations of the
uncertainties. The larger uncertainties can have a significant impact upon
interpretation of Cepheid distances and radii obtained from the infrared
surface brightness method.Comment: 27 pages with 9 figure
Stochastic models for atomic clocks
For the atomic clocks used in the National Bureau of Standards Time Scales, an adequate model is the superposition of white FM, random walk FM, and linear frequency drift for times longer than about one minute. The model was tested on several clocks using maximum likelihood techniques for parameter estimation and the residuals were acceptably random. Conventional diagnostics indicate that additional model elements contribute no significant improvement to the model even at the expense of the added model complexity
Blockspin Cluster Algorithms for Quantum Spin Systems
Cluster algorithms are developed for simulating quantum spin systems like the
one- and two-dimensional Heisenberg ferro- and anti-ferromagnets. The
corresponding two- and three-dimensional classical spin models with four-spin
couplings are maped to blockspin models with two-blockspin interactions.
Clusters of blockspins are updated collectively. The efficiency of the method
is investigated in detail for one-dimensional spin chains. Then in most cases
the new algorithms solve the problems of slowing down from which standard
algorithms are suffering.Comment: 11 page
- …