2,968 research outputs found
The second US Naval Observatory CCD Astrograph Catalog (UCAC2)
The second USNO CCD Astrograph Catalog, UCAC2 was released in July 2003.
Positions and proper motions for 48,330,571 sources (mostly stars) are
available on 3 CDs, supplemented with 2MASS photometry for 99.5% of the
sources. The catalog covers the sky area from -90 to +40 degrees declination,
going up to +52 in some areas; this completely supersedes the UCAC1 released in
2001. Current epoch positions are obtained from observations with the USNO
8-inch Twin Astrograph equipped with a 4k CCD camera. The precision of the
positions are 15 to 70 mas, depending on magnitude, with estimated systematic
errors of 10 mas or below. Proper motions are derived by utilizing over 140
ground-and space-based catalogs, including Hipparcos/Tycho, the AC2000.2, as
well as yet unpublished re-measures of the AGK2 plates and scans from the NPM
and SPM plates. Proper motion errors are about 1 to 3 mas/yr for stars to 12th
magnitude, and about 4 to 7 mas/yr for fainter stars to 16th magnitude. The
observational data, astrometric reductions, results, and important information
for the users of this catalog are presented.Comment: accepted by AJ, AAS LaTeX, 14 figures, 10 table
Computational study of the thermal conductivity in defective carbon nanostructures
We use non-equilibrium molecular dynamics simulations to study the adverse
role of defects including isotopic impurities on the thermal conductivity of
carbon nanotubes, graphene and graphene nanoribbons. We find that even in
structurally perfect nanotubes and graphene, isotopic impurities reduce thermal
conductivity by up to one half by decreasing the phonon mean free path. An even
larger thermal conductivity reduction, with the same physical origin, occurs in
presence of structural defects including vacancies and edges in narrow graphene
nanoribbons. Our calculations reconcile results of former studies, which
differed by up to an order of magnitude, by identifying limitations of various
computational approaches
A PRODUCER-LEVEL CROSS-HEDGE FOR ROUGH RICE USING WHEAT FUTURES
This study explores the potential of routine preharvest cross-hedging of rough rice using wheat futures contract prices. A numerical simulation approach combined with risk efficiency analysis evaluates a wide rage of cross-hedging alternatives. Results establish that farm-level cross-hedging can be considered a viable marketing alternative.Marketing,
Numerical comparison between a Gyrofluid and Gyrokinetic model investigating collisionless magnetic reconnection
The first detailed comparison between gyrokinetic and gyrofluid simulations
of collisionless magnetic reconnection has been carried out. Both the linear
and nonlinear evolution of the collisionless tearing mode have been analyzed.
In the linear regime, we have found a good agreement between the two approaches
over the whole spectrum of linearly unstable wave numbers, both in the drift
kinetic limit and for finite ion temperature. Nonlinearly, focusing on the
small- regime, with indicating the standard tearing
stability parameter, we have compared relevant observables such as the
evolution and saturation of the island width, as well as the island oscillation
frequency in the saturated phase.The results are basically the same, with small
discrepancies only in the value of the saturated island width for moderately
high values of . Therefore, in the regimes investigated here, the
gyrofluid approach can describe the collisionless reconnection process as well
as the more complete gyrokinetic model.Comment: Accepted for publication on Physics of Plasma
A comparison of head and manual control for a position-control pursuit tracking task
Head control was compared with manual control in a pursuit tracking task involving proportional controlled-element dynamics. An integrated control/display system was used to explore tracking effectiveness in horizontal and vertical axes tracked singly and concurrently. Compared with manual tracking, head tracking resulted in a 50 percent greater rms error score, lower pilot gain, greater high-frequency phase lag and greater low-frequency remnant. These differences were statistically significant, but differences between horizontal- and vertical-axis tracking and between 1- and 2-axis tracking were generally small and not highly significant. Manual tracking results were matched with the optimal control model using pilot-related parameters typical of those found in previous manual control studies. Head tracking performance was predicted with good accuracy using the manual tracking model plus a model for head/neck response dynamics obtained from the literature
Maximum likelihood estimation in possibly misspecified dynamic models with time-inhomogeneous Markov Regimes
This paper considers maximum likelihood (ML) estimation in a large class of models with hidden Markov regimes. We investigate consistency and local asymptotic normality of the ML estimator under general conditions which allow for autoregressive dynamics in the observable process, time-inhomogeneous Markov regime sequences, and possible model misspecification. A Monte Carlo study examines the finite-sample properties of the ML estimator. An empirical application is also discussed
Atomistic potential for graphene and other sp carbon systems
We introduce a torsional force field for sp carbon to augment an in-plane
atomistic potential of a previous work (Kalosakas et al, J. Appl. Phys. {\bf
113}, 134307 (2013)) so that it is applicable to out-of-plane deformations of
graphene and related carbon materials. The introduced force field is fit to
reproduce DFT calculation data of appropriately chosen structures. The aim is
to create a force field that is as simple as possible so it can be efficient
for large scale atomistic simulations of various sp carbon structures
without significant loss of accuracy. We show that the complete proposed
potential reproduces characteristic properties of fullerenes and carbon
nanotubes. In addition, it reproduces very accurately the out-of-plane ZA and
ZO modes of graphene's phonon dispersion as well as all phonons with
frequencies up to 1000~cm.Comment: 9 pages, 6 figure
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