8,946 research outputs found
What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?
We investigate the conditions of ideal magnetohydrodynamic (MHD) turbulence
responsible for the relative orientation between density structures,
characterized by their gradient, , and the magnetic field,
, in molecular clouds (MCs). For that purpose, we construct an
expression for the time evolution of the angle, , between
and based on the transport equations of MHD
turbulence. Using this expression, we find that the configuration where
and are mostly parallel, , and where
and are mostly perpendicular, ,
constitute attractors, that is, the system tends to evolve towards either of
these configurations and they are more represented than others. This fact would
explain the predominant alignment or anti-alignment between column density,
, structures and the projected magnetic field orientation,
, reported in observations. Additionally, we find that
departures from the configurations are related to convergent
flows, quantified by the divergence of the velocity field,
, in the presence of a relatively strong magnetic
field. This would explain the observed change in relative orientation between
-structures and towards MCs, from mostly parallel at low
to mostly perpendicular at the highest , as the result of the
gravitational collapse and/or convergence of flows. Finally, we show that the
density threshold that marks the observed change in relative orientation
towards MCs, from and being mostly parallel at low
to mostly perpendicular at the highest , is related to the magnetic field
strength and constitutes a crucial piece of information for determining the
role of the magnetic field in the dynamics of MCs.Comment: 10 pages, 8 figures. Submitted to A&
The MICE luminosity monitor
The MICE experiment will provide the first measurement of ionisation cooling, a technique suitable for reducing the transverse emittance of a tertiary muon beam in a future neutrino factory accelerator facility. MICE is presently in the final stages of commissioning its beam line. The MICE luminosity monitor has proved an invaluable tool throughout this process, providing independent measurements of particle rate from the MICE target, normalisation for beam line detectors and verification of simulation codes
The magnetic environment of the Orion-Eridanus superbubble as revealed by Planck
Using the 353-GHz polarization observations by the Planck satellite we
characterize the magnetic field in the Orion-Eridanus superbubble, a nearby
expanding structure that spans more than 1600 square degrees in the sky. We
identify a region of both low dispersion of polarization orientations and high
polarization fraction associated with the outer wall of the superbubble
identified in the most recent models of the large-scale shape of the region. We
use the Davis-Chandrasekhar-Fermi method to derive plane-of-the-sky magnetic
field strengths of tens of microGauss toward the southern edge of the bubble.
The comparison of these values with existing Zeeman splitting observations of
HI in emission suggests that the large-scale magnetic field in the region was
primarily shaped by the expanding superbubble.Comment: 7 pages, 8 figures. Accepted for publication as a Letter in A&A,
section 1. Letters to the Editor (08/12/2017
Sustainable assessment revisited
Sustainable assessment has been proposed as an idea that focused on the contribution of assessment to learning beyond the timescale of a given course. It was identified as an assessment that meets the needs of the present in terms of the demands of formative and summative assessment, but which also prepares students to meet their own future learning needs. This paper reviews the value of such a notion for assessment; how it has been taken up over the past 15 years in higher education and why it might still be needed. It identifies how it has been a successful intervention in assessment discourse. It explores what more is needed to locate assessment as an intervention to focus on learning for the longer term. It shows how sustainable assessment can help bridge the gap between assessment and learning, and link to ideas such as self-regulation, students’ making judgements about their own work and course-wide assessment
Reactive Force Field for Proton Diffusion in BaZrO3 using an empirical valence bond approach
A new reactive force field to describe proton diffusion within the
solid-oxide fuel cell material BaZrO3 has been derived. Using a quantum
mechanical potential energy surface, the parameters of an interatomic potential
model to describe hydroxyl groups within both pure and yttrium-doped BaZrO3
have been determined. Reactivity is then incorporated through the use of the
empirical valence bond model. Molecular dynamics simulations (EVB-MD) have been
performed to explore the diffusion of hydrogen using a stochastic thermostat
and barostat whose equations are extended to the isostress-isothermal ensemble.
In the low concentration limit, the presence of yttrium is found not to
significantly influence the diffusivity of hydrogen, despite the proton having
a longer residence time at oxygen adjacent to the dopant. This lack of
influence is due to the fact that trapping occurs infrequently, even when the
proton diffuses through octahedra adjacent to the dopant. The activation energy
for diffusion is found to be 0.42 eV, in good agreement with experimental
values, though the prefactor is slightly underestimated.Comment: Corrected titl
Recovering hidden Bloch character: Unfolding Electrons, Phonons, and Slabs
For a quantum state, or classical harmonic normal mode, of a system of
spatial periodicity "R", Bloch character is encoded in a wavevector "K". One
can ask whether this state has partial Bloch character "k" corresponding to a
finer scale of periodicity "r". Answering this is called "unfolding." A theorem
is proven that yields a mathematically clear prescription for unfolding, by
examining translational properties of the state, requiring no "reference
states" or basis functions with the finer periodicity (r,k). A question then
arises, how should one assign partial Bloch character to a state of a finite
system? A slab, finite in one direction, is used as the example. Perpendicular
components k_z of the wavevector are not explicitly defined, but may be hidden
in the state (and eigenvector |i>.) A prescription for extracting k_z is
offered and tested. An idealized silicon (111) surface is used as the example.
Slab-unfolding reveals surface-localized states and resonances which were not
evident from dispersion curves alone.Comment: 11 pages, 7 figure
A new route towards uniformly functionalized single-layer graphene
It is shown, by DFT calculations, that the uniform functionalization of upper
layer of graphite by hydrogen or fluorine does not change essentially its
bonding energy with the underlying layers, whereas the functionalization by
phenyl groups decreases the bonding energy by a factor of approximately ten.
This means that the functionalized monolayer in the latter case can be easily
separated by mild sonication. According to our computational results, such
layers can be cleaned up to pure graphene, as well as functionalized further up
to 25% coverage, without essential difficulties. The energy gap within the
interval from 0.5 to 3 eV can be obtained by such one-side funtionalization
using different chemical species.Comment: 15 pages, 3 figures, to appear in J. Phys. D: Applied Physic
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