6,258 research outputs found
AME - Asteroseismology Made Easy. Estimating stellar properties by use of scaled models
We present a new method to obtain stellar properties for stars exhibiting
solar-like oscillations in an easy, fast, and transparent way. The method,
called Asteroseismology Made Easy (AME), can determine stellar masses,
mean-densities, radii, and surface gravities, as well as estimate ages. In this
writing we present AME as a visual and powerful tool which could be useful; in
particular in the light of the large number of exoplanets being found.
AME consists of a set of figures from which the stellar parameters are
deduced. These figures are made from a grid of stellar evolutionary models that
cover masses ranging from 0.7 Msun to 1.6 Msun in steps of 0.1 Msun and
metallicities in the interval -0.3 dex <= [Fe/H] <= +0.3 dex in increments of
0.1 dex. The stellar evolutionary models are computed using the Modules for
Experiments in Stellar Astrophysics (MESA) code with simple input physics.
We have compared the results from AME with results for three groups of stars;
stars with radii determined from interferometry (and measured parallaxes),
stars with radii determined from measurements of their parallaxes (and
calculated angular diameters), and stars with results based on the modelling of
their individual oscillation frequencies. We find that a comparison of the
radii from interferometry to those from AME yield a weighted mean of the
fractional differences of just 2%. This is also the level of deviation that we
find when we compare the parallax-based radii to the radii determined from AME.
The comparison between independently determined stellar parameters and those
found using AME show that our method can provide reliable stellar masses,
radii, and ages, with median uncertainties in the order of 4%, 2%, and 25%
respectively.Comment: 18 pages, 25 figures. To be published in Astronomy & Astrophysic
Schemata as Building Blocks: Does Size Matter?
We analyze the schema theorem and the building block hypothesis using a
recently derived, exact schemata evolution equation. We derive a new schema
theorem based on the concept of effective fitness showing that schemata of
higher than average effective fitness receive an exponentially increasing
number of trials over time. The building block hypothesis is a natural
consequence in that the equation shows how fit schemata are constructed from
fit sub-schemata. However, we show that generically there is no preference for
short, low-order schemata. In the case where schema reconstruction is favoured
over schema destruction large schemata tend to be favoured. As a corollary of
the evolution equation we prove Geiringer's theorem. We give supporting
numerical evidence for our claims in both non-epsitatic and epistatic
landscapes.Comment: 17 pages, 10 postscript figure
Dynamics and instability of false vacuum bubbles
This paper examines the classical dynamics of false vacuum regions embedded
in surrounding regions of true vacuum, in the thin-wall limit. The dynamics of
all generally relativistically allowed solutions -- most but not all of which
have been previously studied -- are derived, enumerated, and interpreted. We
comment on the relation of these solutions to possible mechanisms whereby
inflating regions may be spawned from non-inflating ones. We then calculate the
dynamics of first order deviations from spherical symmetry, finding that many
solutions are unstable to such aspherical perturbations. The parameter space in
which the perturbations on bound solutions inevitably become nonlinear is
mapped. This instability has consequences for the Farhi-Guth-Guven mechanism
for baby universe production via quantum tunneling.Comment: 16 PRD-style pages including 11 embedded figures; accepted by PRD.
Revised version includes new solution, discussion of 'thermal activation',
added reference, fixed typo
Grassmanian and Bosonic Thirring Models with Jump Defects
In this paper we discuss the Lax formulation of the Grassmanian and Bosonic
Thirring models in the presence of jump defects. For the Grassmanian case, the
defect is described by B\"acklund transformation which is responsible for
preserving the integrability of the model.
We then propose an extension of the B\"acklund transformation for the Bosonic
Thirring model which is verified by some B\"acklund transitions like
Vacuum-One soliton, One soliton - One soliton, One soliton - Two solitons and
Two solitons - Two solitons. The Lax formulation within the space split by the
defect leads to the integrability of Bosonic Thirring model.Comment: Latex 21 page
Thirring Model with Jump Defect
The purpose of our work is to extend the formulation of classical affine Toda
Models in the presence of jump defects to pure fermionic Thirring model. As a
first attempt we construct the Lagrangian of the Grassmanian Thirring model
with jump defect (of Backlund type) and present its conserved modified momentum
and energy expressions giving a first indication of its integrability.Comment: Poster contribution to the 5th International School on Field Theory
and Gravitation, Cuiaba, MT, Brazil, 20-24 Apr 2009. to be published in PoS
ISFTG(2009
Type-II super-Backlund transformation and integrable defects for the N=1 super sinh-Gordon model
A new super-Backlund transformation for the N=1 supersymmetric sinh-Gordon
equation is constructed. Based on this construction we propose a type-II
integrable defect for the supersymmetric sinh-Gordon model consistent with this
new transformation through the Lagrangian formalism. Explicit expressions for
the modified conserved energy, momentum and supercharges are also computed. In
addition, we show for the model that the type-II defect can also been regarded
as a pair of fused defects of a previously introduced type. The explicit
derivation of the associated defect matrices is also presented as a necessary
condition for the integrability of the model.Comment: Latex 31 pages. Version accepted for publicatio
N=1 super sinh-Gordon model with defects revisited
The Lax pair formalism is considered to discuss the integrability of the N=1
supersymmetric sinh-Gordon model with a defect. We derive associated defect
matrix for the model and construct the generating functions of the modified
conserved quantities. The corresponding defect contributions for the modified
energy and momentum of the model are explicitly computed.Comment: Latex 26 page
On the smoothness of nonlinear system identification
We shed new light on the \textit{smoothness} of optimization problems arising
in prediction error parameter estimation of linear and nonlinear systems. We
show that for regions of the parameter space where the model is not
contractive, the Lipschitz constant and -smoothness of the objective
function might blow up exponentially with the simulation length, making it hard
to numerically find minima within those regions or, even, to escape from them.
In addition to providing theoretical understanding of this problem, this paper
also proposes the use of multiple shooting as a viable solution. The proposed
method minimizes the error between a prediction model and the observed values.
Rather than running the prediction model over the entire dataset, multiple
shooting splits the data into smaller subsets and runs the prediction model
over each subset, making the simulation length a design parameter and making it
possible to solve problems that would be infeasible using a standard approach.
The equivalence to the original problem is obtained by including constraints in
the optimization. The new method is illustrated by estimating the parameters of
nonlinear systems with chaotic or unstable behavior, as well as neural
networks. We also present a comparative analysis of the proposed method with
multi-step-ahead prediction error minimization
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