877 research outputs found
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A Portfolio approach to wind and solar deployment in Australia
We develop a new framework that can be used to analyse interactions between solar and wind generation using a Mean-Variance Portfolio Theory (MPT) framework. We use this framework to understand the role of electricity transmission integrating a high shar
Wetting to Non-wetting Transition in Sodium-Coated C_60
Based on ab initi and density-functional theory calculations, an empirical
potential is proposed to model the interaction between a fullerene molecule and
many sodium atoms. This model predicts homogeneous coverage of C_60 below 8 Na
atoms, and a progressive droplet formation above this size. The effects of
ionization, temperature, and external electric field indicate that the various,
and apparently contradictory, experimental results can indeed be put into
agreement.Comment: 4 pages, 4 postscript figure
A single sub-km Kuiper Belt object from a stellar Occultation in archival data
The Kuiper belt is a remnant of the primordial Solar System. Measurements of
its size distribution constrain its accretion and collisional history, and the
importance of material strength of Kuiper belt objects (KBOs). Small, sub-km
sized, KBOs elude direct detection, but the signature of their occultations of
background stars should be detectable. Observations at both optical and X-ray
wavelengths claim to have detected such occultations, but their implied KBO
abundances are inconsistent with each other and far exceed theoretical
expectations. Here, we report an analysis of archival data that reveals an
occultation by a body with a 500 m radius at a distance of 45 AU. The
probability of this event to occur due to random statistical fluctuations
within our data set is about 2%. Our survey yields a surface density of KBOs
with radii larger than 250 m of 2.1^{+4.8}_{-1.7} x 10^7 deg^{-2}, ruling out
inferred surface densities from previous claimed detections by more than 5
sigma. The fact that we detected only one event, firmly shows a deficit of
sub-km sized KBOs compared to a population extrapolated from objects with r>50
km. This implies that sub-km sized KBOs are undergoing collisional erosion,
just like debris disks observed around other stars.Comment: To appear in Nature on December 17, 2009. Under press embargo until
1800 hours London time on 16 December. 19 pages; 7 figure
Cathodoluminescence Applied to the Microcharacterization of Mineral Materials: A Present Status in Experimentation and Interpretation
Experimentation and interpretation of cathodoluminescence (CL) microscopy and spectroscopy applied to the microcharacterization of material minerals are reviewed. The origins of the intrinsic (host lattice) and extrinsic (impurities) luminescence emissions in crystals are briefly discussed. Merits and limitations of the available techniques are illustrated. CL emission changes as a function of the incident electron dose are illustrated for the case of natural quartz and sphalerite (ZnS) crystals. These effects are discussed in terms of the development of bulk charging, production of heat, diffusion of impurities, and creation of lattice defects induced by the incident ionizing particles. Although CL emission is mostly extrinsic in origin there is no general rule for identifying the nature of impurities from the CL emission spectra of minerals. However there is potential for using CL spectroscopy for trace element analysis as presented for the case of minerals containing rare-earth luminescent ions. The CL emission is a signature of the crystal-chemistry properties of minerals and hence contains potential genetic information. Some of the applications of CL emissions in the geosciences are summarized
Science with Simbol-X
Simbol-X is a French-Italian mission, with a participation of German
laboratories, for X-ray astronomy in the wide 0.5-80 keV band. Taking advantage
of emerging technology in mirror manufacturing and spacecraft formation flying,
Simbol-X will push grazing incidence imaging up to ~80 keV, providing an
improvement of roughly three orders of magnitude in sensitivity and angular
resolution compared to all instruments that have operated so far above 10 keV.
This will open a new window in X-ray astronomy, allowing breakthrough studies
on black hole physics and census and particle acceleration mechanisms. We
describe briefly the main scientific goals of the Simbol-X mission, giving a
few examples aimed at highlighting key issues of the Simbol-X design.Comment: Proc. of the workshop "Simbol-X: The hard X-ray universe in focus",
Bologna 14-16 May, 200
INTEGRAL upper limits on gamma-ray emission associated with the gravitational wave event GW150914
Using observations of the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL), we put upper limits on the gamma-ray and hard X-ray prompt emission
associated with the gravitational wave event GW150914, discovered by the
LIGO/Virgo collaboration. The omni-directional view of the INTEGRAL/SPI-ACS has
allowed us to constrain the fraction of energy emitted in the hard X-ray
electromagnetic component for the full high-probability sky region of LIGO
trigger. Our upper limits on the hard X-ray fluence at the time of the event
range from erg cm to
erg cm in the 75 keV - 2 MeV energy range for typical spectral models.
Our results constrain the ratio of the energy promptly released in gamma-rays
in the direction of the observer to the gravitational wave energy
EE. We discuss the implication of gamma-ray
limits on the characteristics of the gravitational wave source, based on the
available predictions for prompt electromagnetic emission.Comment: accepted for publication in ApJ
High Momentum Probes of Nuclear Matter
We discuss how the chemical composition of QCD jets is altered by final state
interactions in surrounding nuclear matter. We describe this process through
conversions of leading jet particles. We find that conversions lead to an
enhancement of kaons at high transverse momentum in Au+Au collisions at RHIC,
while their azimuthal asymmetry v_2 is suppressed.Comment: Contribution to the 4th international workshop High-pT physics at LHC
09, Prague; 6 pages, 6 figure
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