4,711 research outputs found
Calibration of semi-analytic models of galaxy formation using Particle Swarm Optimization
We present a fast and accurate method to select an optimal set of parameters
in semi-analytic models of galaxy formation and evolution (SAMs). Our approach
compares the results of a model against a set of observables applying a
stochastic technique called Particle Swarm Optimization (PSO), a self-learning
algorithm for localizing regions of maximum likelihood in multidimensional
spaces that outperforms traditional sampling methods in terms of computational
cost. We apply the PSO technique to the SAG semi-analytic model combined with
merger trees extracted from a standard CDM N-body simulation. The
calibration is performed using a combination of observed galaxy properties as
constraints, including the local stellar mass function and the black hole to
bulge mass relation. We test the ability of the PSO algorithm to find the best
set of free parameters of the model by comparing the results with those
obtained using a MCMC exploration. Both methods find the same maximum
likelihood region, however the PSO method requires one order of magnitude less
evaluations. This new approach allows a fast estimation of the best-fitting
parameter set in multidimensional spaces, providing a practical tool to test
the consequences of including other astrophysical processes in SAMs.Comment: 11 pages, 4 figures, 1 table. Accepted for publication in ApJ.
Comments are welcom
Stretchable electronic platform for soft and smart contact lens applications
A stretchable platform with spherical-shaped electronics based on thermo-
plastic polyurethane (TPU) is introduced for soft smart contact lenses. The
low glass transition temperature of TPU, its relatively low hardness, and its
proven biocompatibility (i.e., protection of exterior body wounds) fulfill the
essential requirements for eye wearable devices. These requirements include
optical transparency, conformal fitting, and flexibility comparable with soft
contact lenses (e.g., hydrogel-based). Moreover, the viscoelastic nature of
TPU allows planar structures to be thermoformed into spherical caps with a
well-defined curvature (i.e., eyeâs curvature at the cornea: 9 mm). Numerical
modeling and experimental validation enable fine-tuning of the thermo -
forming parameters and the optimization of strain-release patterns. Such
tight control is proven necessary to achieve oxygen permeable, thin, nonde-
velopable, and wrinkle-free contact lenses with integrated electronics (silicon
die, radio-frequency antenna, and stretchable thin-film interconnections). This
work paves the way toward fully autonomous smart contact lenses potentially
for vision correction or sensing applications, among others
High Contrast Imaging and Wavefront Control with a PIAA Coronagraph: Laboratory System Validation
The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high
performance coronagraph concept able to work at small angular separation with
little loss in throughput. We present results obtained with a laboratory PIAA
system including active wavefront control. The system has a 94.3% throughput
(excluding coating losses) and operates in air with monochromatic light.
Our testbed achieved a 2.27e-7 raw contrast between 1.65 lambda/D (inner
working angle of the coronagraph configuration tested) and 4.4 lambda/D (outer
working angle). Through careful calibration, we were able to separate this
residual light into a dynamic coherent component (turbulence, vibrations) at
4.5e-8 contrast and a static incoherent component (ghosts and/or polarization
missmatch) at 1.6e-7 contrast. Pointing errors are controlled at the 1e-3
lambda/D level using a dedicated low order wavefront sensor.
While not sufficient for direct imaging of Earth-like planets from space, the
2.27e-7 raw contrast achieved already exceeds requirements for a ground-based
Extreme Adaptive Optics system aimed at direct detection of more massive
exoplanets. We show that over a 4hr long period, averaged wavefront errors have
been controlled to the 3.5e-9 contrast level. This result is particularly
encouraging for ground based Extreme-AO systems relying on long term stability
and absence of static wavefront errors to recover planets much fainter than the
fast boiling speckle halo.Comment: 18 pages, 12 figures. Accepted for publication in PASP. The pointing
control scheme for this system is described in a separate paper
(Coronagraphic Low-Order Wave-Front Sensor: Principle and Application to a
Phase-Induced Amplitude Coronagraph, The Astrophysical Journal, Volume 693,
Issue 1, pp. 75-84 (2009)
NASA ExoPAG Study Analysis Group 11: Preparing for the WFIRST Microlensing Survey
NASA's proposed WFIRST-AFTA mission will discover thousands of exoplanets
with separations from the habitable zone out to unbound planets, using the
technique of gravitational microlensing. The Study Analysis Group 11 of the
NASA Exoplanet Program Analysis Group was convened to explore scientific
programs that can be undertaken now, and in the years leading up to WFIRST's
launch, in order to maximize the mission's scientific return and to reduce
technical and scientific risk. This report presents those findings, which
include suggested precursor Hubble Space Telescope observations, a
ground-based, NIR microlensing survey, and other programs to develop and deepen
community scientific expertise prior to the mission.Comment: 35 pages, 5 Figures. A brief overview of the findings is presented in
the Executive Summary (2 pages
- âŠ