39,953 research outputs found
THE SPECTROSCOPY OF CRYSTAL DEFECTS - A COMPENDIUM OF DEFECT NOMENCLATURE
The authors bring together tables of current defect nomenclature and a summary of the rules actually practised (rather than idealised schemes) in choosing such labels for signals obtained with a range of spectroscopies. As well as providing a source of reference for the user lost in a maze of labels, the compilation also indicates parallels between similar defect species in very different systems (e.g. ice and quartz), even though the relationships may be far from obvious from the labels. The systems considered are all non-metals, namely ionic crystals (including oxides), silica, semiconductors (e.g. III-V and tetrahedrally coordinated II-VI), valence crystals (e.g. diamond, c-Si, a-Si) and other special hosts like ice and conducting polymers
Inhomogeneous Reionization Models in Cosmological Hydrodynamical Simulations
In this work we present a new hybrid method to simulate the thermal effects
of the reionization in cosmological hydrodynamical simulations. The method
improves upon the standard approach used in simulations of the intergalactic
medium (IGM) and galaxy formation without a significant increase of the
computational cost allowing for efficient exploration of the parameter space.
The method uses a small set of phenomenological input parameters and combines a
semi-numerical reionization model to solve for the topology of reionization and
an approximate model of how reionization heats the IGM, with the massively
parallel \texttt{Nyx} hydrodynamics code, specifically designed to solve for
the structure of diffuse IGM gas. We have produced several large-scale high
resolution cosmological hydrodynamical simulations (, Mpc/h) with different instantaneous and inhomogeneous HI reionization
models that use this new methodology. We study the IGM thermal properties of
these models and find that large scale temperature fluctuations extend well
beyond the end of reionization. Analyzing the 1D flux power spectrum of these
models, we find up to differences in the large scale properties
(low modes, s/km) of the post-reionization power spectrum due
to the thermal fluctuations. We show that these differences could allow one to
distinguish between different reionization scenarios already with existing
Ly forest measurements. Finally, we explore the differences in the
small-scale cutoff of the power spectrum and we find that, for the same heat
input, models show very good agreement provided that the reionization redshift
of the instantaneous reionization model happens at the midpoint of the
inhomogeneous model.Comment: 24 pages, 16 figures. Accepted by MNRAS. Minor changes to match
published versio
The existence and detection of optically dark galaxies by 21cm surveys
One explanation for the disparity between Cold Dark Matter (CDM) predictions
of galaxy numbers and observations could be that there are numerous dark
galaxies in the Universe. These galaxies may still contain baryons, but no
stars, and may be detectable in the 21cm line of atomic hydrogen. The results
of surveys for such objects, and simulations that do/do not predict their
existence, are controversial. In this paper we use an analytical model of
galaxy formation, consistent with CDM, to firstly show that dark galaxies are
certainly a prediction of the model. Secondly, we show that objects like
VIRGOHI21, a dark galaxy candidate recently discovered by us, while rare are
predicted by the model. Thirdly, we show that previous 'blind' HI surveys have
placed few constraints on the existence of dark galaxies. This is because they
have either lacked the sensitivity and/or velocity resolution or have not had
the required detailed optical follow up. We look forward to new 21cm blind
surveys (ALFALFA and AGES) using the Arecibo multi-beam instrument which should
find large numbers of dark galaxies if they exist
Rotating gravity currents: small-scale and large-scale laboratory experiments and a geostrophic model
Laboratory experiments simulating gravity-driven coastal surface currents produced by estuarine fresh-water discharges into the ocean are discussed. The currents are generated inside a rotating tank filled with salt water by the continuous release of buoyant fresh water from a small source at the fluid surface. The height, the width and the length of the currents are studied as a function of the background rotation rate, the volumetric discharge rate and the density difference at the source. Two complementary experimental data sets are discussed and compared with each other. One set of experiments was carried out in a tank of diameter 1 m on a small-scale rotating turntable. The second set of experiments was conducted at the large-scale Coriolis Facility (LEGI, Grenoble) which has a tank of diameter 13 m. A simple geostrophic model predicting the current height, width and propagation velocity is developed. The experiments and the model are compared with each other in terms of a set of non-dimensional parameters identified in the theoretical analysis of the problem. These parameters enable the corresponding data of the large-scale and the small-scale experiments to be collapsed onto a single line. Good agreement between the model and the experiments is found
Propagation of the First Flames in Type Ia Supernovae
We consider the competition of the different physical processes that can
affect the evolution of a flame bubble in a Type Ia supernovae -- burning,
turbulence and buoyancy. Even in the vigorously turbulent conditions of a
convecting white dwarf, thermonuclear burning that begins at a point near the
center (within 100 km) of the star is dominated by the spherical laminar
expansion of the flame, until the burning region reaches kilometers in size.
Consequently flames that ignite in the inner ~20 km promptly burn through the
center, and flame bubbles anywhere must grow quite large--indeed, resolvable by
large-scale simulations of the global system--for significant motion or
deformation occur. As a result, any hot-spot that successfully ignites into a
flame can burn a significant amount of white dwarf material. This potentially
increases the stochastic nature of the explosion compared to a scenario where a
simmering progenitor can have small early hot-spots float harmlessly away.
Further, the size where the laminar flame speed dominates other relevant
velocities sets a characteristic scale for fragmentation of larger flame
structures, as nothing--by definition--can easily break the burning region into
smaller volumes. This makes possible the development of semi-analytic
descriptions of the earliest phase of the propagation of burning in a Type Ia
supernovae, which we present here. Our analysis is supported by fully resolved
numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap
Stability of negative and positive trions in quantum wires
Binding energies of negative () and positive trions () in quantum
wires are studied for strong quantum confinement of carriers which results in a
numerical exactly solvable model. The relative electron and hole localization
has a strong effect on the stability of trions. For equal hole and electron
confinement, is more stable but a small imbalance of the particle
localization towards a stronger hole localization e.g. due to its larger
effective mass, leads to the interchange of and recombination lines
in the photoluminescent spectrum as was recently observed experimentally. In
case of larger stability, a magnetic field oriented parallel to the wire
axis leads to a stronger increase of the binding energy resulting in a
crossing of the and lines
Heavy meson masses and decay constants from relativistic heavy quarks in full lattice QCD
We determine masses and decay constants of heavy-heavy and heavy-charm
pseudoscalar mesons as a function of heavy quark mass using a fully
relativistic formalism known as Highly Improved Staggered Quarks for the heavy
quark. We are able to cover the region from the charm quark mass to the bottom
quark mass using MILC ensembles with lattice spacing values from 0.15 fm down
to 0.044 fm. We obtain f_{B_c} = 0.427(6) GeV; m_{B_c} = 6.285(10) GeV and
f_{\eta_b} = 0.667(6) GeV. Our value for f_{\eta_b} is within a few percent of
f_{\Upsilon} confirming that spin effects are surprisingly small for heavyonium
decay constants. Our value for f_{B_c} is significantly lower than potential
model values being used to estimate production rates at the LHC. We discuss the
changing physical heavy-quark mass dependence of decay constants from
heavy-heavy through heavy-charm to heavy-strange mesons. A comparison between
the three different systems confirms that the B_c system behaves in some ways
more like a heavy-light system than a heavy-heavy one. Finally we summarise
current results on decay constants of gold-plated mesons.Comment: 16 pages, 12 figure
The P Cygni supergiant [OMN2000] LS1 – implications for the star formation history of W51
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200911980Aims. We investigate the nature of the massive star [OMN2000] LS1 and use these results to constrain the history of star formation within the host complex W51. Methods. We utilised a combination of near-IR spectroscopy and non-LTE model atmosphere analysis to derive the physical properties of [OMN2000] LS1 , and a combination of theoretical evolutionary calculations and Monte Carlo simulations to apply limits on the star formation history of W51. Results. We find the spectrum of [OMN2000] LS1 to be consistent with that of a P Cygni supergiant. With a temperature in the range of 13.2–13.7 kK and log( ) , it is significantly cooler, less luminous, and less massive than proposed by previous authors. The presence of such a star within W51 shows that star formation has been underway for at least 3 Myr, while the formation of massive O stars is still on going. The lack of a population of evolved red supergiants within the complex shows that the rate of formation of young massive clusters at ages 9 Myr was lower than currently observed. We find no evidence of internally triggered, sequential star formation within W51, and favour the suggestion that star formation has proceeded at multiple indepedent sites within the GMC. Along with other examples, such as the G305 and Carina star-forming regions, we suggest that W51 is a Galactic analogue of the ubiquitous star cluster complexes seen in external galaxies such as M51 and NGC2403.Peer reviewe
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