10,465 research outputs found
Numerical study of weakly unstable electron plasma oscillations
Vlasov equation for solving initial value problem for unstable electron plasm
Power dissipation in nanoscale conductors: classical, semi-classical and quantum dynamics
Modelling Joule heating is a difficult problem because of the need to introduce correct correlations between the motions of the ions and the electrons. In this paper we analyse three different models of current induced heating (a purely classical model, a fully quantum model and a hybrid model in which the electrons are treated quantum mechanically and the atoms are treated classically). We find that all three models allow for both heating and cooling processes in the presence of a current, and furthermore the purely classical and purely quantum models show remarkable agreement in the limit of high biases. However, the hybrid model in the Ehrenfest approximation tends to suppress heating. Analysis of the equations of motion reveals that this is a consequence of two things: the electrons are being treated as a continuous fluid and the atoms cannot undergo quantum fluctuations. A means for correcting this is suggested
Proper Motion of the Faint Star near KIC 8462852 (Boyajian's Star) - Not a Binary System
A faint star located 2 arcsec from KIC 8462852 was discovered in Keck 10 m
adaptive optics imaging in the near-infrared (NIR) in 2014 by Boyajian et
al. (2016). The closeness of the star to KIC 8462852 suggested the two could
constitute a binary, which might have implications for the cause of the
brightness dips seen by {\it Kepler} (Boyajian et al. (2016) and in
ground-based optical studies Boyajian et al. (2018). Here, NIR imaging in 2017
using the Mimir instrument resolved the pair and enabled measuring their
separation. The faint star had moved milliarcsec (mas) relative to
KIC 8462852 since 2014. The relative proper motion of the faint star is mas yr, for a tangential velocity of km s if
it is at the same 390 pc distance as KIC 8462852. Circular velocity at the 750
AU current projected separation is km s, hence the star pair
cannot be bound.Comment: 10 pages, 2 figure
Kinetic modelling and molecular dynamics simulation of ultracold neutral plasmas including ionic correlations
A kinetic approach for the evolution of ultracold neutral plasmas including
interionic correlations and the treatment of ionization/excitation and
recombination/deexcitation by rate equations is described in detail. To assess
the reliability of the approximations inherent in the kinetic model, we have
developed a hybrid molecular dynamics method. Comparison of the results reveals
that the kinetic model describes the atomic and ionic observables of the
ultracold plasma surprisingly well, confirming our earlier findings concerning
the role of ion-ion correlations [Phys. Rev. A {\bf 68}, 010703]. In addition,
the molecular dynamics approach allows one to study the relaxation of the ionic
plasma component towards thermodynamical equilibrium
Statistical Methodologies to Integrate Experimental and Computational Research
Development of advanced algorithms for simulating engine flow paths requires the integration of fundamental experiments with the validation of enhanced mathematical models. In this paper, we provide an overview of statistical methods to strategically and efficiently conduct experiments and computational model refinement. Moreover, the integration of experimental and computational research efforts is emphasized. With a statistical engineering perspective, scientific and engineering expertise is combined with statistical sciences to gain deeper insights into experimental phenomenon and code development performance; supporting the overall research objectives. The particular statistical methods discussed are design of experiments, response surface methodology, and uncertainty analysis and planning. Their application is illustrated with a coaxial free jet experiment and a turbulence model refinement investigation. Our goal is to provide an overview, focusing on concepts rather than practice, to demonstrate the benefits of using statistical methods in research and development, thereby encouraging their broader and more systematic application
Preliminary evaluation of equine respiratory disease in a region of New Zealand
Inflammatory airway disease (IAD) is one of the most widespread conditions affecting horses
world-wide. Over-crowding, stabling conditions and exposure to irritants (such as bedding
dust) have been thought to contribute to the development of IAD. Since the majority of sport
horses in New Zealand are kept at pasture for prolonged periods, differing from intensive
stabling conditions seen in other countries, variations in the type and frequency of respiratory
pathologies might exist. In order to investigate this possibility, and determine the most
common respiratory conditions affecting horses requiring endoscopic examination, this
preliminary epidemiologic study was conducted. A retrospective analysis of 145 respiratory
endoscopies was conducted including examinations made at 2 veterinary clinics from June
2010 to October 2012. Respiratory endoscopies results were initially classified as normal or
abnormal. Abnormal results were further classified as: IAD, anatomical abnormalities,
intermittent airway obstruction, trauma, miscellaneous abnormalities. Presence of tracheal
secretions of any kind and quantity warranted classification as (IAD). Overall, 76.5% of the
animals requiring a respiratory endoscopy demonstrated abnormalities which justified the
examination: the majority of horses (42.7%) displayed some degree of IAD. Other common
findings included intermittent dorsal displacement of the soft palate (6.9%); bleeding in the
upper respiratory tract (3.45%), and laryngeal hemiplegia (15.2%). Inflammatory airway
disease (IAD) is one of the most widespread conditions affecting horses world-wide and it is
also prevalent in the population studied, despite different housing and managing practices
observed in New Zealand in comparison to other countries. Further studies are needed to
investigate subclinical abnormalities and the causes of the IAD in horses kept at pasture
A Bayesian approach to the estimation of maps between riemannian manifolds
Let \Theta be a smooth compact oriented manifold without boundary, embedded
in a euclidean space and let \gamma be a smooth map \Theta into a riemannian
manifold \Lambda. An unknown state \theta \in \Theta is observed via
X=\theta+\epsilon \xi where \epsilon>0 is a small parameter and \xi is a white
Gaussian noise. For a given smooth prior on \Theta and smooth estimator g of
the map \gamma we derive a second-order asymptotic expansion for the related
Bayesian risk. The calculation involves the geometry of the underlying spaces
\Theta and \Lambda, in particular, the integration-by-parts formula. Using this
result, a second-order minimax estimator of \gamma is found based on the modern
theory of harmonic maps and hypo-elliptic differential operators.Comment: 20 pages, no figures published version includes correction to eq.s
31, 41, 4
Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data
With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti
variables (pulsating hydrogen-atmosphere white dwarf stars) are severely
undersampled by long-cadence (29.42 minutes per exposure) K2 observations.
Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling
precision asteroseismology. We report the discovery of two new ZZ Cetis in
long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights
of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we
recover accurate pulsation frequencies for K2 signals that reflected 4-5 times
off the Nyquist with the full precision of over 70 days of monitoring (~0.01
muHz). In turn, the K2 observations enable us to select the correct peaks from
the alias structure of the ground-based signals caused by gaps in the
observations. We identify at least seven independent pulsation modes in the
light curves of each of these stars. For EPIC 220274129, we detect three
complete sets of rotationally split ell=1 (dipole mode) triplets, which we use
to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We
also detect two sub-Nyquist K2 signals that are likely combination (difference)
frequencies. We attribute our inability to match some of the K2 signals to the
ground-based data to changes in pulsation amplitudes between epochs of
observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC
220274129 near the middle of the ZZ Ceti instability strip, with Teff =
11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03
Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap
The oxygen isotope effect in the ab-plane reflectance of underdoped YBa_2Cu_3O_{7-delta}
We have measured the effect of oxygen isotope substitution on the ab-plane
reflectance of underdoped YBCO. The frequency shift of the transverse optic
phonons due to the substitution of O-16 by O-18 yields an isotope effect of the
expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1.
The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of
alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the
superconducting state. These observations suggest that the shoulder is of
electronic origin and not due to a phonon mode as has been suggested recently.Comment: 4 pages 2 figure
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