34,667 research outputs found
Neutron Star Kicks from Asymmetric Collapse
Many neutron stars are observed to be moving with spatial velocities, in
excess of 500km/s. A number of mechanisms have been proposed to give neutron
stars these high velocities. One of the leading classes of models proposed
invokes asymmetries in the core of a massive star just prior to collapse. These
asymmetries grow during the collapse, causing the resultant supernova to also
be asymmetric. As the ejecta is launched, it pushes off (or ``kicks'') the
newly formed neutron star. This paper presents the first 3-dimensional
supernova simulations of this process. The ejecta is not the only matter that
kicks the newly-formed neutron star. Neutrinos also carry away momentum and the
asymmetric collapse leads also to asymmetries in the neutrinos. However, the
neutrino asymmetries tend to damp out the neutron star motions and even the
most extreme asymmetric collapses presented here do not produce final neutron
star velocities above 200km/s.Comment: 7 pages, 4 figures, see http://qso.lanl.gov/~clf/papers/kick.ps.gz
for full figure
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Sedimentary rocks in Bequerel crater: origin as polar layered deposits during high obliquity
Abstract not available
The size distribution of magnetic bright points derived from Hinode/SOT observations
Context. Magnetic Bright Points (MBPs) are small-scale magnetic features in
the solar photosphere. They may be a possible source of coronal heating by
rapid footpoint motions that cause magnetohydrodynamical waves. The number and
size distribution are of vital importance in estimating the small
scale-magnetic-field energy. Aims. The size distribution of MBPs is derived for
G-band images acquired by the Hinode/SOT instrument. Methods. For
identification purposes, a new automated segmentation and identification
algorithm was developed. Results. For a sampling of 0.108 arcsec/pixel, we
derived a mean diameter of (218 +- 48) km for the MBPs. For the full resolved
data set with a sampling of 0.054 arcsec/pixel, the size distribution shifted
to a mean diameter of (166 +- 31) km. The determined diameters are consistent
with earlier published values. The shift is most probably due to the different
spatial sampling. Conclusions. We conclude that the smallest magnetic elements
in the solar photosphere cannot yet be resolved by G-band observations. The
influence of discretisation effects (sampling) has also not yet been
investigated sufficiently.Comment: Astronomy and Astrophysics, Volume 498, Issue 1, 2009, pp.289-29
Combining brain-computer interfaces and assistive technologies: state-of-the-art and challenges
In recent years, new research has brought the field of EEG-based Brain-Computer Interfacing (BCI) out of its infancy and into a phase of relative maturity through many demonstrated prototypes such as brain-controlled wheelchairs, keyboards, and computer games. With this proof-of-concept phase in the past, the time is now ripe to focus on the development of practical BCI technologies that can be brought out of the lab and into real-world applications. In particular, we focus on the prospect of improving the lives of countless disabled individuals through a combination of BCI technology with existing assistive technologies (AT). In pursuit of more practical BCIs for use outside of the lab, in this paper, we identify four application areas where disabled individuals could greatly benefit from advancements in BCI technology, namely,“Communication and Control”, “Motor Substitution”, “Entertainment”, and “Motor Recovery”. We review the current state of the art and possible future developments, while discussing the main research issues in these four areas. In particular, we expect the most progress in the development of technologies such as hybrid BCI architectures, user-machine adaptation algorithms, the exploitation of users’ mental states for BCI reliability and confidence measures, the incorporation of principles in human-computer interaction (HCI) to improve BCI usability, and the development of novel BCI technology including better EEG devices
Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field
The effects of the initial temperature in the out of equilibrium quantum
field dynamics in the presence of an homogeneous external field are
investigated. We consider an initial thermal state of temperature T for a
constant external field J. A subsequent sign flip of the external field, J to
-J, gives rise to an out of equilibrium nonperturbative quantum field dynamics.
The dynamics is studied here for the symmetry broken lambda(Phi^2)^2 scalar N
component field theory in the large N limit. We find a dynamical effective
potential for the expectation value that helps to understand the dynamics. The
dynamics presents two regimes defined by the presence or absence of a temporal
trapping close to the metastable equilibrium position of the potential. The two
regimes are separated by a critical value of the external field that depends on
the initial temperature. The temporal trapping is shorter for larger initial
temperatures or larger external fields. Parametric resonances and spinodal
instabilities amplify the quantum fluctuations in the field components
transverse to the external field. When there is a temporal trapping this is the
main mechanism that allows the system to escape from the metastable state for
large N. Subsequently backreaction stops the growth of the quantum fluctuations
and the system enters a quasiperiodic regime.Comment: LaTeX, 19 pages, 12 .eps figures, improved version to appear in Phys
Rev
Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations
Small-scale magnetic fields in the solar photosphere can be identified in
high-resolution magnetograms or in the G-band as magnetic bright points (MBPs).
Rapid motions of these fields can cause magneto-hydrodynamical waves and can
also lead to nanoflares by magnetic field braiding and twisting. The MBP
velocity distribution is a crucial parameter for estimating the amplitudes of
those waves and the amount of energy they can contribute to coronal heating.
The velocity and lifetime distributions of MBPs are derived from solar G-band
images of a quiet sun region acquired by the Hinode/SOT instrument with
different temporal and spatial sampling rates. We developed an automatic
segmentation, identification and tracking algorithm to analyse G-Band image
sequences to obtain the lifetime and velocity distributions of MBPs. The
influence of temporal/spatial sampling rates on these distributions is studied
and used to correct the obtained lifetimes and velocity distributions for these
digitalisation effects. After the correction of algorithm effects, we obtained
a mean MBP lifetime of (2.50 +- 0.05) min and mean MBP velocities, depending on
smoothing processes, in the range of (1 - 2) km/s. Corrected for temporal
sampling effects, we obtained for the effective velocity distribution a
Rayleigh function with a coefficient of (1.62 +- 0.05) km/s. The x- and y-
components of the velocity distributions are Gaussians. The lifetime
distribution can be fitted by an exponential function.Comment: Astronomy and Astrophysics (in press
Gravitational Waves from Axisymmetric, Rotational Stellar Core Collapse
We have carried out an extensive set of two-dimensional, axisymmetric,
purely-hydrodynamic calculations of rotational stellar core collapse with a
realistic, finite-temperature nuclear equation of state and realistic massive
star progenitor models. For each of the total number of 72 different
simulations we performed, the gravitational wave signature was extracted via
the quadrupole formula in the slow-motion, weak-field approximation. We
investigate the consequences of variation in the initial ratio of rotational
kinetic energy to gravitational potential energy and in the initial degree of
differential rotation. Furthermore, we include in our model suite progenitors
from recent evolutionary calculations that take into account the effects of
rotation and magnetic torques. For each model, we calculate gravitational
radiation wave forms, characteristic wave strain spectra, energy spectra, final
rotational profiles, and total radiated energy. In addition, we compare our
model signals with the anticipated sensitivities of the 1st- and 2nd-generation
LIGO detectors coming on line. We find that most of our models are detectable
by LIGO from anywhere in the Milky Way.Comment: 13 pages, 22 figures, accepted for publication in ApJ (v600, Jan.
2004). Revised version: Corrected typos and minor mistakes in text and
references. Minor additions to the text according to the referee's
suggestions, conclusions unchange
Carrier scattering, mobilities and electrostatic potential in mono-, bi- and tri-layer graphenes
The carrier density and temperature dependence of the Hall mobility in mono-,
bi- and tri-layer graphene has been systematically studied. We found that as
the carrier density increases, the mobility decreases for mono-layer graphene,
while it increases for bi-layer/tri-layer graphene. This can be explained by
the different density of states in mono-layer and bi-layer/tri-layer graphenes.
In mono-layer, the mobility also decreases with increasing temperature
primarily due to surface polar substrate phonon scattering. In
bi-layer/tri-layer graphene, on the other hand, the mobility increases with
temperature because the field of the substrate surface phonons is effectively
screened by the additional graphene layer(s) and the mobility is dominated by
Coulomb scattering.
We also find that the temperature dependence of the Hall coefficient in
mono-, bi- and tri-layer graphene can be explained by the formation of electron
and hole puddles in graphene. This model also explains the temperature
dependence of the minimum conductance of mono-, bi- and tri-layer graphene. The
electrostatic potential variations across the different graphene samples are
extracted.Comment: 18 pages, 7 figure
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