5,335 research outputs found
Population size bias in descendant-weighted diffusion quantum Monte Carlo simulations
We consider the influence of population size on the accuracy of diffusion quantum Monte Carlo simulations that employ descendant weighting or forward walking techniques to compute expectation values of observables that do not commute with the Hamiltonian. We show that for a simple model system, the d-dimensional isotropic harmonic oscillator, the population size must increase rapidly with d in order to ensure that the simulations produce accurate results. When the population size is too small, expectation values computed using descendant-weighted diffusion quantum Monte Carlo simulations exhibit significant systematic biases
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
VLA-ANGST: A high-resolution HI Survey of Nearby Dwarf Galaxies
We present the "Very Large Array survey of Advanced Camera for Surveys Nearby
Galaxy Survey Treasury galaxies (VLA-ANGST)." VLA-ANGST is a National Radio
Astronomy Observatory Large Program consisting of high spectral (0.6-2.6 km/s)
and spatial (~6") resolution observations of neutral, atomic hydrogen (HI)
emission toward 35 nearby dwarf galaxies from the ANGST survey. ANGST is a
systematic HST survey to establish a legacy of uniform multi-color photometry
of resolved stars for a volume-limited sample of nearby galaxies (D\lesssim4
Mpc). VLA-ANGST provides VLA HI observations of the sub-sample of ANGST
galaxies with recent star formation that are observable from the northern
hemisphere and that were not observed in the "The HI Nearby Galaxy Survey"
(THINGS). The overarching scientific goal of VLA-ANGST is to investigate
fundamental characteristics of the neutral interstellar medium (ISM) of dwarf
galaxies. Here we describe the VLA observations, the data reduction, and the
final VLA-ANGST data products. We present an atlas of the integrated HI maps,
the intensity-weighted velocity fields, the second moment maps as a measure for
the velocity dispersion of the HI, individual channel maps, and integrated HI
spectra for each VLA-ANGST galaxy. We closely follow the observational setup
and data reduction of THINGS to achieve comparable sensitivity and angular
resolution. A major difference, however, is the high velocity resolution of the
VLA-ANGST observations (0.65 and 1.3km/s for the majority of the galaxies). The
VLA-ANGST data products are made publicly available at:
https://science.nrao.edu/science/surveys/vla-angst. With available star
formation histories from resolved stellar populations and lower resolution
ancillary observations from the FIR to the UV, VLA-ANGST will enable detailed
studies of the relationship between the ISM and star formation in dwarf
galaxies on a ~100 pc scale.Comment: 64 figures, grouped into 32. 115 pages, accepted for publication in
the Astronomical Journa
Decoherence and Programmable Quantum Computation
An examination of the concept of using classical degrees of freedom to drive
the evolution of quantum computers is given. Specifically, when externally
generated, coherent states of the electromagnetic field are used to drive
transitions within the qubit system, a decoherence results due to the back
reaction from the qubits onto the quantum field. We derive an expression for
the decoherence rate for two cases, that of the single-qubit Walsh-Hadamard
transform, and for an implementation of the controlled-NOT gate. We examine the
impact of this decoherence mechanism on Grover's search algorithm, and on the
proposals for use of error-correcting codes in quantum computation.Comment: submitted to Phys. Rev. A 35 double-spaced pages, 2 figures, in LaTe
Investigation of heat transfer augmentation through use of internally finned and roughened tubes : final summary report
This report summarizes a three-year program concerned with obtaining basic design information for tubes having a random roughness on the inside wall (RID) and tubing having continuous internal fins (Forge Fin). Test apparatus and procedures were developed to obtain accurate heat-transfer and friction data for a wide variety of tube geometries using water as the test fluid. For the random roughness the heat-transfer coefficient was above the smooth tube value, for comparable flow conditions, by over 60 percent at a Reynolds number of 30,000. Larger percentage improvements can be expected for higher Reynolds numbers and for fluids having higher Prandtl numbers. Improvements in performance, based on equal pumping power for augmented and smooth tubes, of about 50 percent were observed. The heat-transfer characteristics for tape-generated swirl flow through rough tubes were investigated in order to determine the interaction of swirl flow and roughness effects. For the particular range of parameters covered, for equal flow rates, the maximum improvement in heat transfer with swirl flow in smooth tubes was 70 percent, whereas with swirl flow in rough tubes, the improvement was as much as 100 percent. The heat-transfer coefficient for rough tube swirl flow was accurately correlated by a modification of an additive expression previously suggested for prediction of smooth tube swirl flow data.(cont.) The test program for internally finned tubes established that short spiralled fins produce the greatest improvement in heat transfer. On the basis of equal flow conditions, the heat transfer was improved by over 200 percent; while at equal pumping power, the performance was as high as 170 percent. These improvements, which are attributed to increased area and turbulence promotion, appear to equal the improvements displayed by any of the schemes used to augment heat transfer inside tubes. In order to bring the augmentation problem into perspective, a discussion of data for other types of roughness and finning is included.DS
Validation of the UNC OCT Index for the Diagnosis of Early Glaucoma
Purpose:To independently validate the performance of the University of North Carolina Optical Coherence Tomography (UNC OCT) Index in diagnosing and predicting early glaucoma. Methods:Data of 118 normal subjects (118 eyes) and 96 subjects (96 eyes) with early glaucoma defined as visual field mean deviation (MD) greater than -4 decibels (dB), aged 40 to 80 years, and who were enrolled in the Full-Threshold Testing Size III, V, VI comparison study were used in this study. CIRRUS OCT average and quadrants' retinal nerve fiber layer (RNFL); optic disc vertical cup-to-disc ratio (VCDR), cup-to-disc area ratio, and rim area; and average, minimum, and six sectoral ganglion cell-inner plexiform layer (GCIPL) measurements were run through the UNC OCT Index algorithm. Area under the receiver operating characteristic curve (AUC) and sensitivities at 95% and 99% specificity were calculated and compared between single parameters and the UNC OCT Index. Results:Mean age was 60.1 ± 11.0 years for normal subjects and 66.5 ± 8.1 years for glaucoma patients (P < 0.001). MD was 0.29 ± 1.04 dB and -1.30 ± 1.35 dB in normal and glaucomatous eyes (P < 0.001), respectively. The AUC of the UNC OCT Index was 0.96. The best single metrics when compared to the UNC OCT Index were VCDR (0.93, P = 0.054), average RNFL (0.92, P = 0.014), and minimum GCIPL (0.91, P = 0.009). The sensitivities at 95% and 99% specificity were 85.4% and 76.0% (UNC OCT Index), 71.9% and 62.5% (VCDR, all P < 0.001), 64.6% and 53.1% (average RNFL, all P < 0.001), and 66.7% and 58.3% (minimum GCIPL, all P < 0.001), respectively. Conclusions:The findings confirm that the UNC OCT Index may provide improved diagnostic perforce over that of single OCT parameters and may be a good tool for detection of early glaucoma. Translational Relevance:The UNC OCT Index algorithm may be incorporated easily into routine clinical practice and be useful for detecting early glaucoma
Validation of the UNC OCT Index for the Diagnosis of Early Glaucoma
Observations of binary stars containing an accreting black hole or neutron star often show x-ray emission extending to high energies (>10 kilo–electron volts), which is ascribed to an accretion disk corona of energetic particles akin to those seen in the solar corona. Despite their ubiquity, the physical conditions in accretion disk coronae remain poorly constrained. Using simultaneous infrared, optical, x-ray, and radio observations of the Galactic black hole system V404 Cygni, showing a rapid synchrotron cooling event in its 2015 outburst, we present a precise 461 ± 12 gauss magnetic field measurement in the corona. This measurement is substantially lower than previous estimates for such systems, providing constraints on physical models of accretion physics in black hole and neutron star binary systems
Neurodegenerative disorder masquerading as psychosis in a forensic psychiatry setting
A man presenting in his 50s, following conviction for a non-violent crime, to forensic psychiatric services, and then to a neuropsychiatry service with an unusual presentation of psychosis: second person auditory hallucinations, grandiose delusions and somatic delusions. Detailed collateral and family history revealed a background of progressive cognitive deficit and a family history of motor neuron disease. MRI of the brain revealed asymmetrical parieto-occipital volume loss and genetic testing demonstrated a pathogenic expansion of the chromosome 9 open reading frame 72 (C9ORF72) gene consistent with familial frontotemporal dementia caused by a hexanucleotide repeat expansion at C9ORF72, a recently discovered cause of familial frontotemporal dementia/motor neuron disease. This form of frontotemporal dementia should be considered as an important potential differential diagnosis for patients presenting with psychotic symptoms in later life, in whom a detailed family history and thorough cognitive assessment is essential
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