15,130 research outputs found
A pattern-recognition theory of search in expert problem solving
Understanding how look-ahead search and pattern recognition interact is one of the important research questions in the study of expert problem-solving. This paper examines the implications of the template theory (Gobet & Simon, 1996a), a recent theory of expert memory, on the theory of problem solving in chess. Templates are "chunks" (Chase & Simon, 1973) that have evolved into more complex data structures and that possess slots allowing values to be encoded rapidly. Templates may facilitate search in three ways: (a) by allowing information to be stored into LTM rapidly; (b) by allowing a search in the template space in addition to a search in the move space; and (c) by compensating loss in the "mind's eye" due to interference and decay. A computer model implementing the main ideas of the theory is presented, and simulations of its search behaviour are discussed. The template theory accounts for the slight skill difference in average depth of search found in chess players, as well as for other empirical data
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
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Relativistic Nucleus-Nucleus Collisions: Zone of Reactions and Space-Time Structure of a Fireball
A zone of reactions is determined and then exploited as a tool in studying
the space-time structure of an interacting system formed in a collision of
relativistic nuclei. The time dependence of the reaction rates integrated over
spatial coordinates is also considered. Evaluations are made with the help of
the microscopic transport model UrQMD. The relation of the boundaries of
different zones of reactions and the hypersurfaces of sharp chemical and
kinetic freeze-outs is discussed.Comment: 6 pages, 5 figure
Scattering Theory of Charge-Current Induced Magnetization Dynamics
In ferromagnets, charge currents can excite magnons via the spin-orbit
coupling. We develop a novel and general scattering theory of charge current
induced macrospin magnetization torques in normal metalferromagnetnormal
metal layers. We apply the formalism to a dirty GaAs(Ga,Mn)AsGaAs system.
By computing the charge current induced magnetization torques and solving the
Landau-Lifshitz-Gilbert equation, we find magnetization switching for current
densities as low as ~A/cm. Our results are in agreement
with a recent experimental observation of charge-current induced magnetization
switching in (Ga,Mn)As.Comment: Final version accepted by EP
Preparation of a Resorbable Osteoinductive Tricalcium Phosphate Ceramic
Over the past decade we have demonstrated numerous times that calcium phosphates can be rendered with osteoinductive properties by introducing specific surface microstructures1. Since most of these calcium phosphates contained hydroxyapatite, they are either slowly or not resorbable2. Resorbability is an often sought after characteristic of calcium phosphates so that they can be gradually replaced by newly formed bone. The objective of this study was to prepare a resorbable surface microstructured tricalcium phosphate (TCP) ceramic and evaluate its osteoinductive property and resorption rate after intramuscular implantation in dogs. This material was then compared to the established and slowly resorbable osteoinductive biphasic calcium phosphate ceramic (BCP)
Kinematics of the ultracompact helium accretor AM canum venaticorum
We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated
Onsager approach to 1D solidification problem and its relation to phase field description
We give a general phenomenological description of the steady state 1D front
propagation problem in two cases: the solidification of a pure material and the
isothermal solidification of two component dilute alloys.
The solidification of a pure material is controlled by the heat transport in
the bulk and the interface kinetics.
The isothermal solidification of two component alloys is controlled by the
diffusion in the bulk and the interface kinetics.
We find that the condition of positive-definiteness of the symmetric Onsager
matrix of interface kinetic coefficients still allows an arbitrary sign of the
slope of the velocity-concentration line near the solidus in the alloy problem
or of the velocity-temperature line in the case of solidification of a pure
material. This result offers a very simple and elegant way to describe the
interesting phenomenon of a possible non-single-value behavior of velocity
versus concentration which has previously been discussed by different
approaches. We also discuss the relation of this Onsager approach to the thin
interface limit of the phase field description.Comment: 5 pages, 1 figure, submitted to Physical Review
On the orbital periods of the AM CVn stars HP Librae and V803 Centauri
We analyse high-time-resolution spectroscopy of the AM CVn stars HP Librae
and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very
Large Telescope (VLT) of the European Southern Observatory, Chile.
We present evidence that the literature value for V803 Cen's orbital period
is incorrect, based on an observed `S-wave' in the binary's spectrogram. We
measure a spectroscopic period P=1596.4+/-1.2s of the S-wave feature, which is
significantly shorter than the 1611-second periods found in previous
photometric studies. We conclude that the latter period likely represents a
`superhump'. If one assumes that our S-wave period is the orbital period, V803
Cen's mass ratio can be expected to be much less extreme than previously
thought, at q~0.07 rather than q~0.016. This relaxes the constraints on the
masses of the components considerably: the donor star does then not need to be
fully degenerate, and the mass of the accreting white dwarf no longer has to be
very close to the Chandrasekhar limit.
For HP Lib, we similarly measure a spectroscopic period P=1102.8+/-0.2s. This
supports the identification of HP Lib's photometric periods found in the
literature, and the constraints upon the masses derived from them.Comment: Accepted for publication in MNRA
Phase-resolved spectroscopy of the helium dwarf nova 'SN 2003aw' in quiescence
High time resolution spectroscopic observations of the ultracompact helium dwarf nova 'SN 2003aw' in its quiescent state at V similar to 20.5 reveal its orbital period at 2027.8 +/- 0.5 s or 33.80 min. Together with the photometric 'superhump' period of 2041.5 +/- 0.5 s, this implies a mass ratio q approximate to 0.036. We compare both the average and time-resolved spectra of 'SN 2003aw' and Sloan Digital Sky Survey (SDSS) J124058.03-015919.2. Both show a DB white dwarf spectrum plus an optically thin, helium-dominated accretion disc. 'SN 2003aw' distinguishes itself from the SDSS source by its strong calcium H & K emission lines, suggesting higher abundances of heavy metals than the SDSS source. The silicon and iron emission lines observed in the SDSS source are about twice as strong in 'SN 2003aw'. The peculiar 'double bright spot' accretion disc feature seen in the SDSS source is also present in time-resolved spectra of 'SN 2003aw', albeit much weaker
Dissipative hydrodynamics in 2+1 dimension
In 2+1 dimension, we have simulated the hydrodynamic evolution of QGP fluid
with dissipation due to shear viscosity. Comparison of evolution of ideal and
viscous fluid, both initialised under the same conditions e.g. same
equilibration time, energy density and velocity profile, reveal that the
dissipative fluid evolves slowly, cooling at a slower rate. Cooling get still
slower for higher viscosity. The fluid velocities on the otherhand evolve
faster in a dissipative fluid than in an ideal fluid. The transverse expansion
is also enhanced in dissipative evolution. For the same decoupling temperature,
freeze-out surface for a dissipative fluid is more extended than an ideal
fluid. Dissipation produces entropy as a result of which particle production is
increased. Particle production is increased due to (i) extension of the
freeze-out surface and (ii) change of the equilibrium distribution function to
a non-equilibrium one, the last effect being prominent at large transverse
momentum. Compared to ideal fluid, transverse momentum distribution of pion
production is considerably enhanced. Enhancement is more at high than at
low . Pion production also increases with viscosity, larger the viscosity,
more is the pion production. Dissipation also modifies the elliptic flow.
Elliptic flow is reduced in viscous dynamics. Also, contrary to ideal dynamics
where elliptic flow continues to increase with transverse momentum, in viscous
dynamics, elliptic flow tends to saturate at large transverse momentum. The
analysis suggest that initial conditions of the hot, dense matter produced in
Au+Au collisions at RHIC, as extracted from ideal fluid analysis can be changed
significantly if the QGP fluid is viscous.Comment: 11 pages, 10 figures (revised). In the revised version, calculations
are redone with ADS/CFT and perurbative estimate of viscosity. Comments on
the unphysical effects like early reheating of the fluid, in 1st order
dissipative theories are added. The particle spectra calculations are redone
with modified programm
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