596 research outputs found
Hydrodynamic simulations of shell convection in stellar cores
Shell convection driven by nuclear burning in a stellar core is a common
hydrodynamic event in the evolution of many types of stars. We encounter and
simulate this convection (i) in the helium core of a low-mass red giant during
core helium flash leading to a dredge-down of protons across an entropy
barrier, (ii) in a carbon-oxygen core of an intermediate-mass star during core
carbon flash, and (iii) in the oxygen and carbon burning shell above the
silicon-sulfur rich core of a massive star prior to supernova explosion. Our
results, which were obtained with the hydrodynamics code HERAKLES, suggest that
both entropy gradients and entropy barriers are less important for stellar
structure than commonly assumed. Our simulations further reveal a new dynamic
mixing process operating below the base of shell convection zones.Comment: 8 pages, 3 figures .. submitted to a proceedings of conference about
"Red Giants as Probes of the Structure and Evolution of the Milky Way" which
has taken place between 15-17 November 2010 in Rom
Thermodynamic analysis of black hole solutions in gravitating nonlinear electrodynamics
We perform a general study of the thermodynamic properties of static
electrically charged black hole solutions of nonlinear electrodynamics
minimally coupled to gravitation in three space dimensions. The Lagrangian
densities governing the dynamics of these models in flat space are defined as
arbitrary functions of the gauge field invariants, constrained by some
requirements for physical admissibility. The exhaustive classification of these
theories in flat space, in terms of the behaviour of the Lagrangian densities
in vacuum and on the boundary of their domain of definition, defines twelve
families of admissible models. When these models are coupled to gravity, the
flat space classification leads to a complete characterization of the
associated sets of gravitating electrostatic spherically symmetric solutions by
their central and asymptotic behaviours. We focus on nine of these families,
which support asymptotically Schwarzschild-like black hole configurations, for
which the thermodynamic analysis is possible and pertinent. In this way, the
thermodynamic laws are extended to the sets of black hole solutions of these
families, for which the generic behaviours of the relevant state variables are
classified and thoroughly analyzed in terms of the aforementioned boundary
properties of the Lagrangians. Moreover, we find universal scaling laws (which
hold and are the same for all the black hole solutions of models belonging to
any of the nine families) running the thermodynamic variables with the electric
charge and the horizon radius. These scale transformations form a one-parameter
multiplicative group, leading to universal "renormalization group"-like
first-order differential equations. The beams of characteristics of these
equations generate the full set of black hole states associated to any of these
gravitating nonlinear electrodynamics...Comment: 51 single column pages, 19 postscript figures, 2 tables, GRG tex
style; minor corrections added; final version appearing in General Relativity
and Gravitatio
An area law for entanglement from exponential decay of correlations
Area laws for entanglement in quantum many-body systems give useful
information about their low-temperature behaviour and are tightly connected to
the possibility of good numerical simulations. An intuition from quantum
many-body physics suggests that an area law should hold whenever there is
exponential decay of correlations in the system, a property found, for
instance, in non-critical phases of matter. However, the existence of quantum
data-hiding state--that is, states having very small correlations, yet a volume
scaling of entanglement--was believed to be a serious obstruction to such an
implication. Here we prove that notwithstanding the phenomenon of data hiding,
one-dimensional quantum many-body states satisfying exponential decay of
correlations always fulfil an area law. To obtain this result we combine
several recent advances in quantum information theory, thus showing the
usefulness of the field for addressing problems in other areas of physics.Comment: 8 pages, 3 figures. Short version of arXiv:1206.2947 Nature Physics
(2013
Effective Holographic Theories for low-temperature condensed matter systems
The IR dynamics of effective holographic theories capturing the interplay
between charge density and the leading relevant scalar operator at strong
coupling are analyzed. Such theories are parameterized by two real exponents
that control the IR dynamics. By studying the thermodynamics,
spectra and conductivities of several classes of charged dilatonic black hole
solutions that include the charge density back reaction fully, the landscape of
such theories in view of condensed matter applications is characterized.
Several regions of the plane can be excluded as the extremal
solutions have unacceptable singularities. The classical solutions have
generically zero entropy at zero temperature, except when where
the entropy at extremality is finite. The general scaling of DC resistivity
with temperature at low temperature, and AC conductivity at low frequency and
temperature across the whole plane, is found. There is a
codimension-one region where the DC resistivity is linear in the temperature.
For massive carriers, it is shown that when the scalar operator is not the
dilaton, the DC resistivity scales as the heat capacity (and entropy) for
planar (3d) systems. Regions are identified where the theory at finite density
is a Mott-like insulator at T=0. We also find that at low enough temperatures
the entropy due to the charge carriers is generically larger than at zero
charge density.Comment: (v3): Added discussion on the UV completion of the solutions, and on
extremal spectra in the charged case. Expanded discusion on insulating
extremal solutions. Many other refinements and corrections. 126 pages. 48
figure
Circadian-Related Sleep Disorders and Sleep Medication Use in the New Zealand Blind Population: An Observational Prevalence Survey
STUDY OBJECTIVES: To determine the prevalence of self-reported circadian-related sleep disorders, sleep medication and melatonin use in the New Zealand blind population. DESIGN: A telephone survey incorporating 62 questions on sleep habits and medication together with validated questionnaires on sleep quality, chronotype and seasonality. PARTICIPANTS: PARTICIPANTS WERE GROUPED INTO: (i) 157 with reduced conscious perception of light (RLP); (ii) 156 visually impaired with no reduction in light perception (LP) matched for age, sex and socioeconomic status, and (iii) 156 matched fully-sighted controls (FS). SLEEP HABITS AND DISTURBANCES: The incidence of sleep disorders, daytime somnolence, insomnia and sleep timing problems was significantly higher in RLP and LP compared to the FS controls (p<0.001). The RLP group had the highest incidence (55%) of sleep timing problems, and 26% showed drifting sleep patterns (vs. 4% FS). Odds ratios for unconventional sleep timing were 2.41 (RLP) and 1.63 (LP) compared to FS controls. For drifting sleep patterns, they were 7.3 (RLP) and 6.0 (LP). MEDICATION USE: Zopiclone was the most frequently prescribed sleep medication. Melatonin was used by only 4% in the RLP group and 2% in the LP group. CONCLUSIONS: Extrapolations from the current study suggest that 3,000 blind and visually impaired New Zealanders may suffer from circadian-related sleep problems, and that of these, fewer than 15% have been prescribed melatonin. This may represent a therapeutic gap in the treatment of circadian-related sleep disorders in New Zealand, findings that may generalize to other countries
Trust Perceptions of Online Travel Information by Different Content Creators: Some Social and Legal Implications
Consumers are increasingly turning to the online
environment to provide information to assist them in
making purchase decisions related to travel products. They
often rely on travel recommendations from different
sources, such as sellers, independent experts and, increasingly,
other consumers. A new type of online content, usergenerated
content (UGC), provides a number of legal and
social challenges to providers and users of that content,
especially in relation to areas such as defamation, misrepresentation
and social embarrassment. This paper reports
research that examined the level of trustworthiness of
online travel information from these different sources. The
study used a survey of Australian travel consumers (n=
12,000) and results support the notion that there are
differences in the level of trust for online travel information
from different sources. Respondents ‘tended to agree’ that
they trusted information provided by travel agents, information
from commercial operators and comments made by
travellers on third party websites. However, the highest
level of trust was afforded to information provided on State
government tourism websites. These results suggest that
greater trust is placed in online travel comments when they
are on a specific travel website than when they are on a
more generic social networking website. However, respondents
were ‘not sure’ that they trusted comments made by
travellers on weblogs and on social networking sites. Some
88% of respondents that had not visited UGC websites (or
were unsure if they had) indicated that they thought that
UGC would be useful in the future – suggesting that they
feel that any concerns they may have in relation to legal and
social problems resulting from its use will be resolved
A Cognitive Architecture Based on a Learning Classifier System with Spiking Classifiers
© 2015, Springer Science+Business Media New York. Learning classifier systems (LCS) are population-based reinforcement learners that were originally designed to model various cognitive phenomena. This paper presents an explicitly cognitive LCS by using spiking neural networks as classifiers, providing each classifier with a measure of temporal dynamism. We employ a constructivist model of growth of both neurons and synaptic connections, which permits a genetic algorithm to automatically evolve sufficiently-complex neural structures. The spiking classifiers are coupled with a temporally-sensitive reinforcement learning algorithm, which allows the system to perform temporal state decomposition by appropriately rewarding “macro-actions”, created by chaining together multiple atomic actions. The combination of temporal reinforcement learning and neural information processing is shown to outperform benchmark neural classifier systems, and successfully solve a robotic navigation task
Essential Surgery at the District Hospital: A Retrospective Descriptive Analysis in Three African Countries
In the first of two papers investigating surgical provision in eight district hospitals in Saharan African countries, Margaret Kruk and colleagues find low levels of surgical care provision suggesting unmet need for surgical services
A brief history of learning classifier systems: from CS-1 to XCS and its variants
© 2015, Springer-Verlag Berlin Heidelberg. The direction set by Wilson’s XCS is that modern Learning Classifier Systems can be characterized by their use of rule accuracy as the utility metric for the search algorithm(s) discovering useful rules. Such searching typically takes place within the restricted space of co-active rules for efficiency. This paper gives an overview of the evolution of Learning Classifier Systems up to XCS, and then of some of the subsequent developments of Wilson’s algorithm to different types of learning
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