18,576 research outputs found
Engineering knowledge requirements for sand and dust on Mars
The successful landing of human beings on Mars and the establishment of a permanent outpost there will require an understanding of the Martian environment by the engineers. A key feature of the Martian environment is the nearly ubiquitous presence of sand and dust. The process which the engineering community will undertake to determine the sensitivities of their designs to the current level of knowledge about Mars sand and dust is emphasized. The interaction of the engineering community with the space exploration initiative (SEI) mission planners and management is described
Twisted Split Fermions
The observed flavor structure of the standard model arises naturally in
"split fermion" models which localize fermions at different places in an extra
dimension. It has, until now, been assumed that the bulk masses for such
fermions can be chosen to be flavor diagonal simultaneously at every point in
the extra dimension, with all the flavor violation coming from the Yukawa
couplings to the Higgs. We consider the more natural possibility in which the
bulk masses cannot be simultaneously diagonalized, that is, that they are
twisted in flavor space. We show that, in general, this does not disturb the
natural generation of hierarchies in the flavor parameters. Moreover, it is
conceivable that all the flavor mixing and CP-violation in the standard model
may come only from twisting, with the five-dimensional Yukawa couplings taken
to be universal.Comment: 15 pages, 1 figur
On the Disambiguation of Weighted Automata
We present a disambiguation algorithm for weighted automata. The algorithm
admits two main stages: a pre-disambiguation stage followed by a transition
removal stage. We give a detailed description of the algorithm and the proof of
its correctness. The algorithm is not applicable to all weighted automata but
we prove sufficient conditions for its applicability in the case of the
tropical semiring by introducing the *weak twins property*. In particular, the
algorithm can be used with all acyclic weighted automata, relevant to
applications. While disambiguation can sometimes be achieved using
determinization, our disambiguation algorithm in some cases can return a result
that is exponentially smaller than any equivalent deterministic automaton. We
also present some empirical evidence of the space benefits of disambiguation
over determinization in speech recognition and machine translation
applications
On Toroidal Horizons in Binary Black Hole Inspirals
We examine the structure of the event horizon for numerical simulations of
two black holes that begin in a quasicircular orbit, inspiral, and finally
merge. We find that the spatial cross section of the merged event horizon has
spherical topology (to the limit of our resolution), despite the expectation
that generic binary black hole mergers in the absence of symmetries should
result in an event horizon that briefly has a toroidal cross section. Using
insight gained from our numerical simulations, we investigate how the choice of
time slicing affects both the spatial cross section of the event horizon and
the locus of points at which generators of the event horizon cross. To ensure
the robustness of our conclusions, our results are checked at multiple
numerical resolutions. 3D visualization data for these resolutions are
available for public access online. We find that the structure of the horizon
generators in our simulations is consistent with expectations, and the lack of
toroidal horizons in our simulations is due to our choice of time slicing.Comment: Submitted to Phys. Rev.
New Limits on Radio Emission from X-ray Dim Isolated Neutron Stars
We have carried out a search for radio emission at 820 MHz from six X-ray dim
isolated neutron stars with the Robert C. Byrd Green Bank Radio Telescope. No
transient or pulsed emission was found using fast folding, fast Fourier
transform, and single-pulse searches. The corresponding flux limits are about
0.01 mJy for pulsed emission, depending on the integration time for the
particular source and assuming a duty cycle of 2%, and 20 mJy for single
dispersed pulses. These are the most sensitive limits to date on radio emission
from X-ray dim isolated neutron stars. There is no evidence for isolated radio
pulses, as seen in a class of neutron stars known as rotating radio transients.
Our results imply that either the radio luminosities of these objects are lower
than those of any known radio pulsars, or they could simply be long-period
nearby radio pulsars with high magnetic fields beaming away from the Earth. To
test the latter possibility, we would need around 40 similar sources to provide
a 1 sigma probability of at least one of them beaming toward us. We also give a
detailed description of our implementation of the Fast Folding Algorithm.Comment: 16 pages, 8 figures, 3 tables, accepted to Ap
Dispersion interactions from a local polarizability model
A local approximation for dynamic polarizability leads to a nonlocal
functional for the long-range dispersion interaction energy via an
imaginary-frequency integral. We analyze several local polarizability
approximations and argue that the form underlying the construction of our
recent van der Waals functional [O. A. Vydrov and T. Van Voorhis, Phys. Rev.
Lett. 103, 063004 (2009)] is particularly well physically justified. Using this
improved formula, we compute dynamic dipole polarizabilities and van der Waals
C_6 coefficients for a set of atoms and molecules. Good agreement with the
benchmark values is obtained in most cases
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Influence of Sediment Redox Conditions on Contaminant Stabilization by Apatite and FE(0)
Efficacy of stabilizing Ce, Co, and Pb by adding apatite and zero-valent Fe (Fe(0)) to contaminated wetland sediments was quantified under oxidizing and reducing conditions. The redox status and the general water chemistry of the oxidized and reduced treatments differed greatly, yet the influences of the amendments on contaminant stabilization were quite similar; both amendments significantly (p less than or equal to 0.05) reduced aqueous contaminant concentrations. Based on resin sorption studies and thermodynamic calculations, Ce existed primarily as cationic and to a smaller extent, anionic species, and Co existed almost as cationic, neutral, and organically complexed species. Based on a series of varying selective extractions, almost 50 wt-percentage of the Co and Pb were already strongly bound to the sediment, thereby limiting the potential (and need) of affecting additional immobilization through the use of amendments
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Short-Term Environmental Impacts of Illite Clays When Used As An In SITU Method for Remediating 137Cs-Contaminated Wetland
Over 1,200 hectares (3,000 acres) of wetlands such as L-Lake; Par Pond; Ponds A, 2, 4, 5, and B; canals; and several creeks (e.g. Steel Creek, Lower Three Runs, and Pen Branch) on the Savannah River Site (SRS) are contaminated with 560 Ci of 137Cs. These environmentally sensitive wetlands pose a significant remediation challenge to the Department of Energy (DOE). A new technology is needed to avoid environmentally destructive remediation. Current muck and truck technologies destroy the sensitive ecosystems, and may increase dose to workers. Because of relatively low amount of clay and weak 137Cs retention capabilities of kaolinite dominant sediments on the SRS, 137Cs has a greater bioavailability here than at any other DOE site. We have previously shown that naturally occurring illite minerals, with a high complexing capability for 137Cs, can sequester 137Cs and reduce its bioavailability when applied to 137Cs contaminated wetlands. Previous research showed that an in situ remediation method using illite minerals reduced 137Cs concentrations in the water 25- to 30-fold, in aquatic plants 3- to 5-fold, and in fish 2- to 3-fold. During this funding period (Fy03) we re-sampled study sites in Pond A and R-Canal that had been treated in 2001 with illite clays. The data revealed that 137Cs concentrations in water are still lower than untreated control sites, some 112 weeks later. This encouraging result indicates that the in situ remediation technique has a longer-term effectiveness than was previously reported. This positive finding occurred despite the study site experiencing a severe drought, as well as flooded conditions during the two-year period. Studies on the in situ application of illite minerals to 137Cs-contaminated wetlands continue to produce positive results. Additional measurements are needed, however, to determine the long-term effectiveness of the technique, and the environmental impacts on parameters not measured in this study. When coupled to earlier work, the results reported herein support the use of illite applications to 137Cs-contaminated wetlands on the SRS as an attractive alternative to more environmentally destructive remediation methods
Optical observations of NEA 162173 (1999 JU3) during the 2011-2012 apparition
Near-Earth asteroid 162173 (1999 JU3) is a potential target of two asteroid
sample return missions, not only because of its accessibility but also because
of the first C-type asteroid for exploration missions. The lightcurve-related
physical properties of this object were investigated during the 2011-2012
apparition. We aim to confirm the physical parameters useful for JAXA's
Hayabusa 2 mission, such as rotational period, absolute magnitude, and phase
function. Our data complement previous studies that did not cover low phase
angles. With optical imagers and 1-2 m class telescopes, we acquired the
photometric data at different phase angles. We independently derived the
rotational lightcurve and the phase curve of the asteroid. We have analyzed the
lightcurve of 162173 (1999 JU3), and derived a synodic rotational period of
7.625 +/- 0.003 h, the axis ratio a/b = 1.12. The absolute magnitude H_R =
18.69 +/- 0.07 mag and the phase slope of G = -0.09 +/- 0.03 were also obtained
based on the observations made during the 2011-2012 apparition.Comment: 4 pages, 3 figure
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