3,830 research outputs found
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
Nuclear Magnetic Resonance Quantum Computing Using Liquid Crystal Solvents
Liquid crystals offer several advantages as solvents for molecules used for
nuclear magnetic resonance quantum computing (NMRQC). The dipolar coupling
between nuclear spins manifest in the NMR spectra of molecules oriented by a
liquid crystal permits a significant increase in clock frequency, while short
spin-lattice relaxation times permit fast recycling of algorithms, and save
time in calibration and signal-enhancement experiments. Furthermore, the use of
liquid crystal solvents offers scalability in the form of an expanded library
of spin-bearing molecules suitable for NMRQC. These ideas are demonstrated with
the successful execution of a 2-qubit Grover search using a molecule
(CHCl) oriented in a liquid crystal and a clock speed eight
times greater than in an isotropic solvent. Perhaps more importantly, five
times as many logic operations can be executed within the coherence time using
the liquid crystal solvent.Comment: Minor changes. Published in Appl. Phys. Lett. v.75, no.22, 29 Nov
1999, p.3563-356
Universal Baxterization for -graded Hopf algebras
We present a method for Baxterizing solutions of the constant Yang-Baxter
equation associated with -graded Hopf algebras. To demonstrate the
approach, we provide examples for the Taft algebras and the quantum group
.Comment: 8 page
TetraDENSITY. A database of population density estimates in terrestrial vertebrates
Motivation: Population density is a key demographic parameter influencing many ecological processes, and macroecology has described both intra- and interspecific patterns of variation. Population density data are expensive to collect and contain many forms of noise and potential bias; these factors have impeded investigation of macroecological patterns, and many hypotheses remain largely unexplored. Population density also represents fundamental information for conservation, because it underlies population dynamics and, ultimately, extinction risk. Here we present TetraDENSITY, an extensive dataset with > 18,000 records of density estimates for terrestrial vertebrates, in order to facilitate new research on this topic. Main types of variable contained: The dataset includes taxonomic information on species, population density estimate, year of data collection, season, coordinates of the locality, locality name, habitat, sampling method and sampling area. Spatial location and grain: Global. Spatial accuracy varies across studies; conservatively, it can be considered at 1°, but for many data it is much finer. Time period and grain: From 1926 to 2017. Temporal accuracy is yearly in most cases, but studies with higher temporal resolution (season, month) are also present. Major taxa and level of measurement: Amphibians in terrestrial phase, reptiles, birds and mammals. Estimates derive from multiple methods, reflecting the study taxon, location and techniques available at the time of density estimation
Cavity approach for real variables on diluted graphs and application to synchronization in small-world lattices
We study XY spin systems on small world lattices for a variety of graph
structures, e.g. Poisson and scale-free, superimposed upon a one dimensional
chain. In order to solve this model we extend the cavity method in the one
pure-state approximation to deal with real-valued dynamical variables. We find
that small-world architectures significantly enlarge the region in parameter
space where synchronization occurs. We contrast the results of population
dynamics performed on a truncated set of cavity fields with Monte Carlo
simulations and find excellent agreement. Further, we investigate the
appearance of replica symmetry breaking in the spin-glass phase by numerically
analyzing the proliferation of pure states in the message passing equations.Comment: 10 pages, 3 figure
Finite element analysis of polyethylene wear in total hip replacement: A literature review
Evaluation and prediction of wear play a key role in product design and material selection of total hip replacements, because wear debris is one of the main causes of loosening and failure. Multifactorial clinical or laboratory studies are high cost and require unfeasible timeframes for implant development. Simulation using finite element methods is an efficient and inexpensive alternative to predict wear and pre-screen various parameters. This article presents a comprehensive literature review of the state-of-the-art finite element modelling techniques that have been applied to evaluate wear in polyethylene hip replacement components. A number of knowledge gaps are identified including the need to develop appropriate wear coefficients and the analysis of daily living activities
Stellar Bar Evolution in Cuspy and Flat-Cored Triaxial CDM Halos
We analyze the evolution of stellar bars in galactic disks in mildly triaxial
flat-core and cuspy CDM halos. We use tailored simulations of rigid and live
halos which include the feedback from disk/bar onto the halo in order to test
the work by El-Zant & Shlosman (2002). The latter used the Liapunov exponents
to analyze the fate of bars in analytical asymmetric halos. We find: (1) The
bar growth is similar in all rigid axisymmetric and triaxial halos. (2) Bars in
live models vertically buckle and form a pseudobulge with a boxy/peanut shape.
(3) In live axisymmetric halos, the bar strength varies little during the
secular evolution. The bar pattern speed anticorrelates with the halo core
size. The bar strength is larger for smaller disk-to-halo mass ratios within
disk radii, the bar size correlates with the halo core sizes, and the bar
pattern speeds -- with the halo central mass concentration. Bars embedded in
live triaxial halos have a starkly different fate: they dissolve on ~1.5-5 Gyr
due to the onset of chaos over continuous zones, leaving behind a weak oval
distortion. The onset of chaos is related to the halo triaxiality, the fast
rotating bar and the halo cuspiness. Before the bar dissolves, the region
outside it develops strong spiral structures, especially in the live triaxial
halos. (4) More angular momentum is absorbed by the triaxial halos as compared
to the axisymmetric models and its exchange is mediated by resonances. (5)
Cuspy halos are more susceptible than flat-core halos to having their
prolateness washed out by the bar. We analyze these results in terms of the
stability of trajectories and development of chaos. We set constraints on the
triaxiality of DM halos by comparing our predictions to recent observations of
bars out to z~1.Comment: 17 pages, 14 figures, Astrophysical Journal, in press, Vol. 637.
Updated version (text, references
Framework for classifying logical operators in stabilizer codes
Entanglement, as studied in quantum information science, and non-local
quantum correlations, as studied in condensed matter physics, are fundamentally
akin to each other. However, their relationship is often hard to quantify due
to the lack of a general approach to study both on the same footing. In
particular, while entanglement and non-local correlations are properties of
states, both arise from symmetries of global operators that commute with the
system Hamiltonian. Here, we introduce a framework for completely classifying
the local and non-local properties of all such global operators, given the
Hamiltonian and a bi-partitioning of the system. This framework is limited to
descriptions based on stabilizer quantum codes, but may be generalized. We
illustrate the use of this framework to study entanglement and non-local
correlations by analyzing global symmetries in topological order, distribution
of entanglement and entanglement entropy.Comment: 20 pages, 9 figure
Constraints on the Progenitor of SN 2010jl and Pre-Existing Hot Dust in its Surrounding Medium
A search for the progenitor of SN~2010jl, an unusually luminous core-collapse
supernova of Type~IIn, using pre-explosion {\it Hubble}/WFPC2 and {\it
Spitzer}/IRAC images of the region, yielded upper limits on the UV and
near-infrared (IR) fluxes from any candidate star. These upper limits constrain
the luminosity and effective temperature of the progenitor, the mass of any
preexisting dust in its surrounding circumstellar medium (CSM), and dust
proximity to the star. A {\it lower} limit on the CSM dust mass is required to
hide a luminous progenitor from detection by {\it Hubble}. {\it Upper} limits
on the CSM dust mass and constraints on its proximity to the star are set by
requiring that the absorbed and reradiated IR emission not exceed the IRAC
upper limits. Using the combined extinction-IR emission constraints we present
viable combinations, where and are the CSM dust mass and
its inner radius. These depend on the CSM outer radius, dust composition and
grain size, and the properties of the progenitor. The results constrain the
pre-supernova evolution of the progenitor, and the nature and origin of the
observed post-explosion IR emission from SN~2010jl. In particular, an
~Car-type progenitor will require at least 4~mag of visual extinction to
avoid detection by the {\it Hubble}. This can be achieved with dust masses
~\msun\ (less than the estimated 0.2-0.5~\msun\ around
~Car) which must be located at distances of ~cm from the
star to avoid detection by {\it Spitzer}.Comment: Accepted for publication in the ApJ. 14 pages 10 figures. The
complete figure set for Figure 10 (24 images) is available in the online
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