73 research outputs found
Effect of Density Inhomogeneity on YORP: The case of Itokawa
The effect of density inhomogeneity on the YORP effect for a given shape
model is investigated. A density inhomogeneity will cause an offset between the
center of figure and the center of mass and a re-orientation of the principal
axes away from those associated with the shape alone. Both of these effects can
alter the predicted YORP rate of change in angular velocity and obliquity. We
apply these corrections to the Itokawa shape model and find that its YORP
angular velocity rate is sensitive to offsets between its center of mass and
center of figure, with a shift on the order of 10 meters being able to change
the sign of the YORP effect for that asteroid. Given the non-detection of YORP
for Itokawa as of 2008, this can shed light on the density distribution within
that body. The theory supports a shift of the asteroid center of mass towards
Itokawa's neck region, where there is an accumulation of finer gravels.
Detection of the YORP effect for Itokawa should provide some strong constraints
on its density distribution. This theory could also be applied to asteroids
visited by future spacecraft to constrain density inhomogeneities.Comment: 23 pages, 3 figure
Finite element simulation of three-dimensional free-surface flow problems
An adaptive finite element algorithm is described for the stable solution of three-dimensional free-surface-flow problems based primarily on the use of node movement. The algorithm also includes a discrete remeshing procedure which enhances its accuracy and robustness. The spatial discretisation allows an isoparametric piecewise-quadratic approximation of the domain geometry for accurate resolution of the curved free surface.
The technique is illustrated through an implementation for surface-tension-dominated viscous flows modelled in terms of the Stokes equations with suitable boundary conditions on the deforming free surface. Two three-dimensional test problems are used to demonstrate the performance of the method: a liquid bridge problem and the formation of a fluid droplet
Eye-tracking the timeâcourse of novel word learning and lexical competition in adults and children
Lexical competition is a hallmark of proficient, automatic word recognition. Previous research suggests that there is a delay before a new spoken word becomes engaged in this process, with sleep playing an important role. However, data from one method--the visual world paradigm--consistently show competition without a delay. We trained 42 adults and 40 children (aged 7-8) on novel word-object pairings, and employed this paradigm to measure the time-course of lexical competition. Fixations to novel objects upon hearing existing words (e.g., looks to the novel object biscal upon hearing âclick on the biscuitâ) were compared to fixations on untrained objects. Novel word-object pairings learned immediately before testing and those learned the previous day exhibited significant competition effects, with stronger competition for the previous day pairings for children but not adults. Crucially, this competition effect was significantly smaller for novel than existing competitors (e.g., looks to candy upon hearing âclick on the candleâ), suggesting that novel items may not compete for recognition like fully-fledged lexical items, even after 24 hours. Explicit memory (cued recall) was superior for words learned the day before testing, particularly for children; this effect (but not the lexical competition effects) correlated with sleep-spindle density. Together, the results suggest that different aspects of new word learning follow different time courses: visual world competition effects can emerge swiftly, but are qualitatively different from those observed with established words, and are less reliant upon sleep. Furthermore, the findings fit with the view that word learning earlier in development is boosted by sleep to a greater degree
Epidemiological status of felid herpesvirus type-1 and feline calicivirus infections in Brazil
Charged pion form factor between Q^2=0.60 and 2.45 GeV^2. II. Determination of, and results for, the pion form factor
The charged pion form factor, Fpi(Q^2), is an important quantity which can be
used to advance our knowledge of hadronic structure. However, the extraction of
Fpi from data requires a model of the 1H(e,e'pi+)n reaction, and thus is
inherently model dependent. Therefore, a detailed description of the extraction
of the charged pion form factor from electroproduction data obtained recently
at Jefferson Lab is presented, with particular focus given to the dominant
uncertainties in this procedure. Results for Fpi are presented for
Q^2=0.60-2.45 GeV^2. Above Q^2=1.5 GeV^2, the Fpi values are systematically
below the monopole parameterization that describes the low Q^2 data used to
determine the pion charge radius. The pion form factor can be calculated in a
wide variety of theoretical approaches, and the experimental results are
compared to a number of calculations. This comparison is helpful in
understanding the role of soft versus hard contributions to hadronic structure
in the intermediate Q^2 regime.Comment: 18 pages, 11 figure
The G0 Experiment: Apparatus for Parity-Violating Electron Scattering Measurements at Forward and Backward Angles
In the G0 experiment, performed at Jefferson Lab, the parity-violating
elastic scattering of electrons from protons and quasi-elastic scattering from
deuterons is measured in order to determine the neutral weak currents of the
nucleon. Asymmetries as small as 1 part per million in the scattering of a
polarized electron beam are determined using a dedicated apparatus. It consists
of specialized beam-monitoring and control systems, a cryogenic hydrogen (or
deuterium) target, and a superconducting, toroidal magnetic spectrometer
equipped with plastic scintillation and aerogel Cerenkov detectors, as well as
fast readout electronics for the measurement of individual events. The overall
design and performance of this experimental system is discussed.Comment: Submitted to Nuclear Instruments and Method
Search for three-nucleon short-range correlations in light nuclei
We present new data probing short-range correlations (SRCs) in nuclei through the measurement of electron scattering off high-momentum nucleons in nuclei. The inclusive ^{4}He/^{3}He cross section ratio is observed to be both x and Q^{2} independent for 1.52, our data support the hypothesis that a previous claim of three-nucleon correlation dominance was an artifact caused by the limited resolution of the measurement. While 3N-SRCs appear to have an important contribution, our data show that isolating 3N-SRCs is significantly more complicated than for 2N-SRCs.United States. Department of Energy (Contract DE-AC05-06OR23177)United States. Department of Energy (Contract DE-AC02-06CH11357)United States. Department of Energy (Contract DE-FG02-96ER40950
Astronomical Distance Determination in the Space Age: Secondary Distance Indicators
The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)
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