45,720 research outputs found
Manufacture of DPFC-DMS polymer in the SKG range
BPFC-DMS block copolymers were synthesized on a pre-pilot scale (i.e., to 5 Kg lots) and subsequently fabricated into clear, colorless films. Details of the synthesis procedures, property determinations, and film casting techniques are presented. Solubility, viscosity and molecular weight characteristics of the resulting product are reported
Assessment worlds colliding? Negotiating between discourses of assessment on an online open course
Using the badged open course, Taking your first steps into Higher Education, this case study examines how assessment on online open courses draws on concepts of assessment used within formal and informal learning. Our experience was that assessment used within open courses, such as massive open online courses, is primarily determined by the requirements of quality assurance processes to award a digital badge or statement of participation as well as what is technologically possible. However, this disregards much recent work in universities that use assessment in support of learning. We suggest that designers of online open courses should pay greater attention to the relationship of assessment and learning to improve participant course completion
Universal Markovian reduction of Brownian particle dynamics
Non-Markovian processes can often be turned Markovian by enlarging the set of
variables. Here we show, by an explicit construction, how this can be done for
the dynamics of a Brownian particle obeying the generalized Langevin equation.
Given an arbitrary bath spectral density , we introduce an orthogonal
transformation of the bath variables into effective modes, leading stepwise to
a semi-infinite chain with nearest-neighbor interactions. The transformation is
uniquely determined by and defines a sequence
of residual spectral densities describing the
interaction of the terminal chain mode, at each step, with the remaining bath.
We derive a simple, one-term recurrence relation for this sequence, and show
that its limit is the quasi-Ohmic expression provided by the Rubin model of
dissipation. Numerical calculations show that, irrespective of the details of
, convergence is fast enough to be useful in practice for an effective
Markovian reduction of quantum dissipative dynamics
Daylight quantum key distribution over 1.6 km
Quantum key distribution (QKD) has been demonstrated over a point-to-point
-km atmospheric optical path in full daylight. This record
transmission distance brings QKD a step closer to surface-to-satellite and
other long-distance applications.Comment: 4 pages, 2 figures, 1 table. Submitted to PRL on 14 January 2000 for
publication consideratio
The X-ray Iron Emission from Tycho's Supernova Remnant
We present the results of broadband fits to the X-ray spectrum of Tycho's
supernova remnant obtained by the Solid-State Imaging Spectrometers on the ASCA
Observatory. We use single-temperature, single-ionization-age, nonequilibrium
ionization models to characterize the ejecta and the blast-shocked interstellar
medium. Based on the Fe K emission at 6.5 keV, previous spectral studies have
suggested that the Fe ejecta in this Type Ia remnant are stratified interior to
the other ejecta. The ASCA data provide important constraints from the Fe L
emission near 1 keV as well as the Fe K emission. We find that the simplest
models, with emission from the ejecta and blast wave each at a single
temperature and ionization age, severely underestimate the Fe K flux. We show
that there is little Fe emission associated with the Si and S ejecta shell. The
blast-shocked interstellar medium has abundances roughly 0.3 times the solar
value, while the ejecta, with the exception of Fe, have relative abundances
that are typical of Type Ia supernovae. The addition of another component of Fe
emission, which we associate with ejecta, at a temperature at least two times
higher and an ionization age 100 times lower than the Si ejecta, does
provide a good fit to the spectrum. This model is consistent with X-ray imaging
results. Although fluorescent emission from dust in the remnant may contribute
to the Fe K flux, we conclude that it is unlikely to dominate.Comment: 23 pages, LaTex; 4 postscript figures, 2 postscript tables. To appear
in ApJ, vol 49
Genetic distance predicts trait differentiation at the subpopulation but not the individual level in eelgrass, Zostera marina.
Ecological studies often assume that genetically similar individuals will be more similar in phenotypic traits, such that genetic diversity can serve as a proxy for trait diversity. Here, we explicitly test the relationship between genetic relatedness and trait distance using 40 eelgrass (Zostera marina) genotypes from five sites within Bodega Harbor, CA. We measured traits related to nutrient uptake, morphology, biomass and growth, photosynthesis, and chemical deterrents for all genotypes. We used these trait measurements to calculate a multivariate pairwise trait distance for all possible genotype combinations. We then estimated pairwise relatedness from 11 microsatellite markers. We found significant trait variation among genotypes for nearly every measured trait; however, there was no evidence of a significant correlation between pairwise genetic relatedness and multivariate trait distance among individuals. However, at the subpopulation level (sites within a harbor), genetic (FST) and trait differentiation were positively correlated. Our work suggests that pairwise relatedness estimated from neutral marker loci is a poor proxy for trait differentiation between individual genotypes. It remains to be seen whether genomewide measures of genetic differentiation or easily measured "master" traits (like body size) might provide good predictions of overall trait differentiation
The Extraordinarily Rapid Expansion of the X-ray Remnant of Kepler's Supernova (SN1604)
Four individual high resolution X-ray images from ROSAT and the Einstein
Observatory have been used to measure the expansion rate of the remnant of
Kepler's supernova (SN 1604). Highly significant measurements of the expansion
have been made for time baselines varying from 5.5 yrs to 17.5 yrs. All
measurements are consistent with a current expansion rate averaged over the
entire remnant of 0.239 (+/-0.015) (+0.017,-0.010) % per yr, which, when
combined with the known age of the remnant, determines the expansion parameter
m, defined as , to be 0.93 (+/-0.06) (+0.07,-0.04). The error
bars on these results include both statistical (first set of errors) and
systematic (second set) uncertainty. According to this result the X-ray remnant
is expanding at a rate that is remarkably close to free expansion and nearly
twice as fast as the mean expansion rate of the radio remnant. The expansion
rates as a function of radius and azimuthal angle are also presented based on
two ROSAT images that were registered to an accuracy better than 0.5
arcseconds. Significant radial and azimuthal variations that appear to arise
from the motion of individual X-ray knots are seen. The high expansion rate of
the X-ray remnant appears to be inconsistent with currently accepted dynamical
models for the evolution of Kepler's SNR.Comment: 14 pages, including 7 postscript figs, LaTeX, emulateapj. Accepted by
Ap
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