1,829 research outputs found
Counting the ions surrounding nucleic acids.
Nucleic acids are strongly negatively charged, and thus electrostatic interactions-screened by ions in solution-play an important role in governing their ability to fold and participate in biomolecular interactions. The negative charge creates a region, known as the ion atmosphere, in which cation and anion concentrations are perturbed from their bulk values. Ion counting experiments quantify the ion atmosphere by measuring the preferential ion interaction coefficient: the net total number of excess ions above, or below, the number expected due to the bulk concentration. The results of such studies provide important constraints on theories, which typically predict the full three-dimensional distribution of the screening cloud. This article reviews the state of nucleic acid ion counting measurements and critically analyzes their ability to test both analytical and simulation-based models
Using Covers to Minimize Odor and Gas Emissions from Manure Storages
Odor emissions from animal production sites are typically the leading cause of nuisance complaints. Locating facilities far from neighbors is often a solution to the odor problem, but this is not always an option for producers. Most often, reducing odor emissions from the farm is the only option available
Designing Volumetric Truss Structures
We present the first algorithm for designing volumetric Michell Trusses. Our
method uses a parametrization approach to generate trusses made of structural
elements aligned with the primary direction of an object's stress field. Such
trusses exhibit high strength-to-weight ratios. We demonstrate the structural
robustness of our designs via a posteriori physical simulation. We believe our
algorithm serves as an important complement to existing structural optimization
tools and as a novel standalone design tool itself
Analysis of Carbon/Carbon Fragments From the Columbia Tragedy
The extensive investigation following the Space Shuttle Orbiter Columbia accident of February 1, 2003 determined that hot gases entered the wing through a breach in the protective reinforced carbon/carbon (RCC) leading edge. In the current study, the exposed edges of the recovered RCC from the vicinity of the breach are examined with scanning electron microscopy and Raman spectroscopy. Electron microscopy of the exposed edges revealed regions of pointed carbon fibers, characteristic of exposure to high temperature oxidizing gases. The Raman technique relates the observed 1350 and 1580 to 1600 cm(-1) bands to graphitic dom ains and their corresponding temperatures of formation. Some of the regions showed evidence of exposure temperatures beyond 2700 ?C during the accident
Unitarity and the Hilbert space of quantum gravity
Under the premises that physics is unitary and black hole evaporation is
complete (no remnants, no topology change), there must exist a one-to-one
correspondence between states on future null and timelike infinity and on any
earlier spacelike Cauchy surface (e.g., slices preceding the formation of the
hole). We show that these requirements exclude a large set of semiclassical
spacetime configurations from the Hilbert space of quantum gravity. In
particular, the highest entropy configurations, which account for almost all of
the volume of semiclassical phase space, would not have quantum counterparts,
i.e. would not correspond to allowed states in a quantum theory of gravity.Comment: 7 pages, 3 figures, revtex; minor changes in v2 (version published in
Class. Quant. Grav.
The Aquarius Co-Moving Group is Not a Disrupted Classical Globular Cluster
We present a detailed analysis of high-resolution, high S/N spectra for 5
Aquarius stream stars observed with the MIKE spectrograph on the Magellan Clay
telescope. Our sample represents one third of the 15 known members in the
stream. We find the stream is not mono-metallic: the metallicity ranges from
[Fe/H] = -0.63 to -1.58. No anti-correlation in Na-O abundances is present, and
we find a strong positive Mg-Al relationship, similar to that observed in the
thick disk. We find no evidence that the stream is a result of a disrupted
classical globular cluster, contrary to a previously published claim. High
[(Na, Ni, alpha)/Fe] and low [Ba/Y] abundance ratios in the stream suggests it
is not a tidal tail from a disrupted dwarf galaxy, either. The stream is
chemically indistinguishable from Milky Way field stars with the exception of
one candidate, C222531-145437. From its position, velocity, and detailed
chemical abundances, C222531-145437 is likely a star that was tidally disrupted
from omega-Centauri. We propose the Aquarius stream is Galactic in origin, and
could be the result from a disk-satellite perturbation in the Milky Way thick
disk on the order of a few Gyr ago: derived orbits, UVW velocities, and angular
momenta of the Aquarius members offer qualitative support for our hypothesis.
Assuming C222531-145437 is a tidally disrupted member of omega-Centauri, this
system is the most likely disk perturber. In the absence of compelling chemical
and/or dynamical evidence that the Aquarius stream is the tidal tail of a
disrupted satellite, we advocate the "Aquarius group" as a more appropriate
description. Like the Canis Major over-density, as well as the Hercules and
Monoceros groups, the Aquarius group joins the list of kinematically-identified
substructures that are not actually accreted material: they are simply part of
the rich complexity of the Milky Way structure.Comment: Accepted to MNRAS. Updated to journal versio
Generally covariant model of a scalar field with high frequency dispersion and the cosmological horizon problem
Short distance structure of spacetime may show up in the form of high
freqency dispersion. Although such dispersion is not locally Lorentz invariant,
we show in a scalar field model how it can nevertheless be incorporated into a
generally covariant metric theory of gravity provided the locally preferred
frame is dynamical. We evaluate the resulting energy-momentum tensor and
compute its expectation value for a quantum field in a thermal state. The
equation of state differs at high temperatures from the usual one, but not by
enough to impact the problems of a hot big bang cosmology. We show that a
superluminal dispersion relation can solve the horizon problem via superluminal
equilibration, however it cannot do so while remaining outside the Planck
regime unless the dispersion relation is artificially chosen to have a rather
steep dependence on wavevector.Comment: 4 pages, 1 figure; New section added with discussion of solution to
the cosmological horizon problem using superluminal dispersion, title changed
to reflect new content, various additional minor change
- …