40,162 research outputs found
Gauge Dressing of 2D Field Theories
By using the gauge Ward identities, we study correlation functions of gauged
WZNW models. We show that the gauge dressing of the correlation functions can
be taken into account as a solution of the Knizhnik-Zamolodchikov equation. Our
method is analogous to the analysis of the gravitational dressing of 2D field
theories.Comment: 13 pages, Late
T Tauri variability in the context of the beat-frequency model
We examine the implications of a beat frequency modulated model of T Tauri
accretion. In particular we show that measurements of the variability of
accretion generated lines can be used in conjunction with existing photometry
to obtain a measurement of the underlying photospheric and disc flux. This
provides an independent way of checking spectral energy distribution modelling.
In addition, we show how spectroscopy of T Tauri stars can reveal the
inclination angle between the magnetic axis and the plane of the disc.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Dynamical Modeling of NGC 6809: Selecting the best model using Bayesian Inference
The precise cosmological origin of globular clusters remains uncertain, a
situation hampered by the struggle of observational approaches in conclusively
identifying the presence, or not, of dark matter in these systems. In this
paper, we address this question through an analysis of the particular case of
NGC 6809. While previous studies have performed dynamical modeling of this
globular cluster using a small number of available kinematic data, they did not
perform appropriate statistical inference tests for the choice of best model
description; such statistical inference for model selection is important since,
in general, different models can result in significantly different inferred
quantities. With the latest kinematic data, we use Bayesian inference tests for
model selection and thus obtain the best fitting models, as well as mass and
dynamic mass-to-light ratio estimates. For this, we introduce a new likelihood
function that provides more constrained distributions for the defining
parameters of dynamical models. Initially we consider models with a known
distribution function, and then model the cluster using solutions of the
spherically symmetric Jeans equation; this latter approach depends upon the
mass density profile and anisotropy parameter. In order to find the
best description for the cluster we compare these models by calculating their
Bayesian evidence. We find smaller mass and dynamic mass-to-light ratio values
than previous studies, with the best fitting Michie model for a constant
mass-to-light ratio of and
. We exclude the
significant presence of dark matter throughout the cluster, showing that no
physically motivated distribution of dark matter can be present away from the
cluster core.Comment: 12 pages, 10 figures, accepted for publication in MNRA
Randomized Rounding for the Largest Simplex Problem
The maximum volume -simplex problem asks to compute the -dimensional
simplex of maximum volume inside the convex hull of a given set of points
in . We give a deterministic approximation algorithm for this
problem which achieves an approximation ratio of . The problem
is known to be -hard to approximate within a factor of for
some constant . Our algorithm also gives a factor
approximation for the problem of finding the principal submatrix of
a rank positive semidefinite matrix with the largest determinant. We
achieve our approximation by rounding solutions to a generalization of the
-optimal design problem, or, equivalently, the dual of an appropriate
smallest enclosing ellipsoid problem. Our arguments give a short and simple
proof of a restricted invertibility principle for determinants
Lensed CMB power spectra from all-sky correlation functions
Weak lensing of the CMB changes the unlensed temperature anisotropy and
polarization power spectra. Accounting for the lensing effect will be crucial
to obtain accurate parameter constraints from sensitive CMB observations.
Methods for computing the lensed power spectra using a low-order perturbative
expansion are not good enough for percent-level accuracy. Non-perturbative
flat-sky methods are more accurate, but curvature effects change the spectra at
the 0.3-1% level. We describe a new, accurate and fast, full-sky
correlation-function method for computing the lensing effect on CMB power
spectra to better than 0.1% at l<2500 (within the approximation that the
lensing potential is linear and Gaussian). We also discuss the effect of
non-linear evolution of the gravitational potential on the lensed power
spectra. Our fast numerical code is publicly available.Comment: 16 pages, 4 figures. Changes to match PRD version including new
section on non-linear corrections. CAMB code available at http://camb.info
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