4,480 research outputs found
Protostellar Disk Evolution Over Million-Year Timescales with a Prescription for Magnetized Turbulence
Magnetorotational instability (MRI) is the most promising mechanism behind
accretion in low-mass protostellar disks. Here we present the first analysis of
the global structure and evolution of non-ideal MRI-driven T-Tauri disks on
million-year timescales. We accomplish this in a 1+1D simulation by calculating
magnetic diffusivities and utilizing turbulence activity criteria to determine
thermal structure and accretion rate without resorting to a 3-D
magnetohydrodynamical (MHD) simulation. Our major findings are as follows.
First, even for modest surface densities of just a few times the minimum-mass
solar nebula, the dead zone encompasses the giant planet-forming region,
preserving any compositional gradients. Second, the surface density of the
active layer is nearly constant in time at roughly 10 g/cm2, which we use to
derive a simple prescription for viscous heating in MRI-active disks for those
who wish to avoid detailed MHD computations. Furthermore, unlike a standard
disk with constant-alpha viscosity, the disk midplane does not cool off over
time, though the surface cools as the star evolves along the Hayashi track. The
ice line is firmly in the terrestrial planet-forming region throughout disk
evolution and can move either inward or outward with time, depending on whether
pileups form near the star. Finally, steady-state mass transport is a poor
description of flow through an MRI-active disk. We caution that MRI activity is
sensitive to many parameters, including stellar X-ray flux, grain size,
gas/small grain mass ratio and magnetic field strength, and we have not
performed an exhaustive parameter study here.Comment: Accepted for publication in Astrophysical Journal. 19 pages,
including 8 figure
Phonon Band Structure and Thermal Transport Correlation in a Layered Diatomic Crystal
To elucidate the relationship between a crystal's structure, its thermal
conductivity, and its phonon dispersion characteristics, an analysis is
conducted on layered diatomic Lennard-Jones crystals with various mass ratios.
Lattice dynamics theory and molecular dynamics simulations are used to predict
the phonon dispersion curves and the thermal conductivity. The layered
structure generates directionally dependent thermal conductivities lower than
those predicted by density trends alone. The dispersion characteristics are
quantified using a set of novel band diagram metrics, which are used to assess
the contributions of acoustic phonons and optical phonons to the thermal
conductivity. The thermal conductivity increases as the extent of the acoustic
modes increases, and decreases as the extent of the stop bands increases. The
sensitivity of the thermal conductivity to the band diagram metrics is highest
at low temperatures, where there is less anharmonic scattering, indicating that
dispersion plays a more prominent role in thermal transport in that regime. We
propose that the dispersion metrics (i) provide an indirect measure of the
relative contributions of dispersion and anharmonic scattering to the thermal
transport, and (ii) uncouple the standard thermal conductivity
structure-property relation to that of structure-dispersion and
dispersion-property relations, providing opportunities for better understanding
of the underlying physical mechanisms and a potential tool for material design.Comment: 30 pages, 10 figure
Calibration of the LIGO displacement actuators via laser frequency modulation
We present a frequency modulation technique for calibration of the
displacement actuators of the LIGO 4-km-long interferometric gravitational-wave
detectors. With the interferometer locked in a single-arm configuration, we
modulate the frequency of the laser light, creating an effective length
variation that we calibrate by measuring the amplitude of the frequency
modulation. By simultaneously driving the voice coil actuators that control the
length of the arm cavity, we calibrate the voice coil actuation coefficient
with an estimated 1-sigma uncertainty of less than one percent. This technique
enables a force-free, single-step actuator calibration using a displacement
fiducial that is fundamentally different from those employed in other
calibration methods.Comment: 10 pages, 5 figures, submitted to Classical and Quantum Gravit
Optimizing dual energy cone beam CT protocols for preclinical imaging and radiation research
Objective: The aim of this work was to investigate whether quantitative dual-energy CT (DECT) imaging is feasible for small animal irradiators with an integrated cone-beam CT (CBCT) system.
Methods: The optimal imaging protocols were determined by analyzing different energy combinations and dose levels. The influence of beam hardening effects and the performance of a beam hardening correction (BHC) were investigated. In addition, two systems from different manufacturers were compared in terms of errors in the extracted effective atomic numbers (Z(eff)) and relative electron densities (rho(e)) for phantom inserts with known elemental compositions and relative electron densities.
Results: The optimal energy combination was determined to be 50 and 90kVp. For this combination, Z(eff) and r rho(e) can be extracted with a mean error of 0.11 and 0.010, respectively, at a dose level of 60cGy.
Conclusion: Quantitative DECT imaging is feasible for small animal irradiators with an integrated CBCT system. To obtain the best results, optimizing the imaging protocols is required. Well-separated X-ray spectra and a sufficient dose level should be used to minimize the error and noise for Z(eff) and rho(e). When no BHC is applied in the image reconstruction, the size of the calibration phantom should match the size of the imaged object to limit the influence of beam hardening effects. No significant differences in Z(eff) and rho(e) errors are observed between the two systems from different manufacturers.
Advances in knowledge: This is the first study that investigates quantitative DECT imaging for small animal irradiators with an integrated CBCT system
Aligning ICCP Certification With The IS2002 Model Curriculum: A New International Standard
This is a progress report on actions to align vendor neutral certification for computing and Information Systems professionals with academic standards as codified by the IS2002 Model Curriculum
Statistical Properties of Many Particle Eigenfunctions
Wavefunction correlations and density matrices for few or many particles are
derived from the properties of semiclassical energy Green functions. Universal
features of fixed energy (microcanonical) random wavefunction correlation
functions appear which reflect the emergence of the canonical ensemble as the
number of particles approaches infinity. This arises through a little known
asymptotic limit of Bessel functions. Constraints due to symmetries,
boundaries, and collisions between particles can be included.Comment: 13 pages, 4 figure
Accurate calibration of test mass displacement in the LIGO interferometers
We describe three fundamentally different methods we have applied to
calibrate the test mass displacement actuators to search for systematic errors
in the calibration of the LIGO gravitational-wave detectors. The actuation
frequencies tested range from 90 Hz to 1 kHz and the actuation amplitudes range
from 1e-6 m to 1e-18 m. For each of the four test mass actuators measured, the
weighted mean coefficient over all frequencies for each technique deviates from
the average actuation coefficient for all three techniques by less than 4%.
This result indicates that systematic errors in the calibration of the
responses of the LIGO detectors to differential length variations are within
the stated uncertainties.Comment: 10 pages, 6 figures, submitted on 31 October 2009 to Classical and
Quantum Gravity for the proceedings of 8th Edoardo Amaldi Conference on
Gravitational Wave
Developing reverse logistics programs: A resource based view
Previous research proposes a six-process model for reverse logistics (RL) program design and execution. This manuscript advances RL related knowledge by incorporating the previous model into a broader theoretical framework, namely, the Resource Based View (RBV) of the firm. The current research employs exploratory techniques to investigate the applicability of RBV and its main tenants within the RL context. Based on in-depth interviews with 16 executives from seven different companies, the relationships among resources. RL capabilities, and RL competencies are explored
Information Systems Analyst (ISA): A Professional Certification Based On The IS2002 Model Curriculum
The ICCP, ICCP Education Foundation and a coalition of universities have produced an exit examination, and a new professional certification, Information Systems Analyst, based on the IS2002 Model Curriculum. The examination is also the basis for administering IS program evaluation services by a new Center for Computing Education Research
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