158,376 research outputs found
Calculation of percolation thresholds in high dimensions for fcc, bcc, and diamond lattices
In a recent article, Galam and Mauger proposed an invariant for site and bond
percolation thresholds, based on known values for twenty lattices (Eur. Phys.
J. B 1 (1998) 255-258). Here we give a larger list of values for more than
forty lattices in two to six dimensions. In this list are new results for fcc,
bcc, and diamond lattices in 4, 5, and 6 dimensions.
The list contains examples of lattices with equal site percolation
thresholds, but different bond percolation thresholds. These and other examples
show that there are deviations from the proposed invariant of up to 12% in two
dimensions, increasing to 69% in higher dimensions.Comment: 12 pages, 3 figures (EPS), LaTe
Participatory Scenario Generation: Communicating Usability Issues in Product Design through User Involvement in Scenario Generation\ud
Scenarios have proven to be a valuable tool in evaluating and communicating usability issues in consumer product design. Scenarios are explicit descriptions of hypothetical use situations. Realistic scenarios can serve as a valuable frame of reference to evaluate design solutions with regard to usability. To be able to achieve this required level of realism, involving users in scenario generation is essential. In this presentation we discuss how and where users can be involved in a scenario based product design process by means of examples of design projects that were executed by master students Industrial Design Engineering of the University of Twente. \ud
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We distinguish direct and indirect scenario generation. In direct scenario generation the user is actively involved in a participatory scenario generation session: the scenarios are created together with users. Indirect scenario generation is an approach in which scenarios are created by designers based on common analysis techniques like observations and interviews. These scenarios are then offered to users for confirmation. Both types of user involvement in scenario generation can be aimed at either current use scenarios which describe the current situation or future use scenarios which include a new product design. \ud
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The examples show that all strategies can be applied successfully to create realistic scenarios. Which strategy to choose depends among others upon risks and privacy issues, occurrence of infrequent events and availability of users. Furthermore, the variety of approaches shows that there is still a lot to explore with regard to benefits and limitations of the many techniques that can be applied in generating scenarios for consumer product design. We hope to contribute to this field by means of the research in our group and the work of students in the SBPD course\u
An N-body/SPH Study of Isolated Galaxy Mass Density Profiles
We investigate the evolution of mass density profiles in secular disk galaxy
models, paying special attention to the development of a two-component profile
from a single initial exponential disk free of cosmological evolution (i.e., no
accretion or interactions). As the source of density profile variations, we
examine the parameter space of the spin parameter, halo concentration, virial
mass, disk mass and bulge mass, for a total of 162 simulations in the context
of a plausible model of star formation and feedback (GADGET-2). The evolution
of the galaxy mass density profile, including the development of a
two-component profile with an inner and outer segment, is controlled by the
ratio of the disk mass fraction, , to the halo spin parameter,
. The location of the break between the two components and speed at
which it develops is directly proportional to ; the amplitude of
the transition between the inner and outer regions is however controlled by the
ratio of halo concentration to virial velocity. The location of the divide
between the inner and outer profile does not change with time. (Abridged)Comment: 27 pages, 31 figures. Accepted for publication at MNRAS. A
high-resolution version of the paper with figures can be found here
http://www.mpia-hd.mpg.de/~foyle/papers/MN-07-1491-MJ.R1.pd
Cosmic Evolution of Stellar Disk Truncations: From z~1 to the Local Universe
We have conducted the largest systematic search so far for stellar disk
truncations in disk-like galaxies at intermediate redshift (z<1.1), using the
Great Observatories Origins Deep Survey South (GOODS-S) data from the Hubble
Space Telescope - ACS. Focusing on Type II galaxies (i.e. downbending profiles)
we explore whether the position of the break in the rest-frame B-band radial
surface brightness profile (a direct estimator of the extent of the disk where
most of the massive star formation is taking place), evolves with time. The
number of galaxies under analysis (238 of a total of 505) is an order of
magnitude larger than in previous studies. For the first time, we probe the
evolution of the break radius for a given stellar mass (a parameter well suited
to address evolutionary studies). Our results suggest that, for a given stellar
mass, the radial position of the break has increased with cosmic time by a
factor 1.3+/-0.1 between z~1 and z~0. This is in agreement with a moderate
inside-out growth of the disk galaxies in the last ~8 Gyr. In the same period
of time, the surface brightness level in the rest-frame B-band at which the
break takes place has increased by 3.3+/-0.2 mag/arcsec^2 (a decrease in
brightness by a factor of 20.9+/-4.2). We have explored the distribution of the
scale lengths of the disks in the region inside the break, and how this
parameter relates to the break radius. We also present results of the
statistical analysis of profiles of artificial galaxies, to assess the
reliability of our results.Comment: 22 pages, 14 figures, accepted for publication in ApJ. Figures 1, 3
and 6 have somehow downgraded resolution to match uploading requirement
The stars and gas in outer parts of galaxy disks: Extended or truncated -- flat or warped?
I review observations of truncations of stellar disks and models for their
origin, compare observations of truncations in moderately inclined galaxies to
those in edge-on systems and discuss the relation between truncations and
HI-warps and their systematics and origin. Truncations are a common feature in
edge-on stellar disks, but the relation of truncations in face-on to those in
edge-on galaxies needs further clarification. The origin of truncations is most
likely related to a maximum in the specific angular momentum in the material
that formed the stellar disks, but this model does probably require some
redistribution of angular momentum. HI-warps start just beyond the truncation
radius and disks and warps appear distinct components. This suggests that inner
disks form initially and settle as rigid, very flat structures, while HI-warps
result from later infall of gas with a different orientation of the angular
momentum.Comment: Invited Review at the Vatican Symposium: Formation and Evolution of
Galaxy Disks, October 2007, proceeding editors Jose G. Funes, SJ and Enrico
M. Corsin
Lessons from the Milky Way: the Kapteyn Universe
Jacobus Cornelius Kapteyn (1851-1922) presented a model for the distribution
of stars in space together with a dynamical interpretation in terms of an
equilibrium between the gravitational field of the stars and their random
motion and rotation. In the vertical direction Kapteyn's results are
substantially correct. Usually the Kapteyn Universe is described as being
flawed due to neglect of interstellar absorption. Kapteyn was led to adopt this
on the basis of widely accepted evidence by Shapley on an absence of reddening
of stars in globular clusters. But another, equally important misconception was
Kapteyn's interpretation of the two Star Streams as manifestations of two
groups of stars rotating around a center in opposite directions. This was
supported by the observation of very different mixes in stellar types in the
two streams. Had Kapteyn adopted the absorption as he himself had determined it
he would not have been able to arrive at a consistent picture.Comment: To appear in LESSONS FROM THE LOCAL GROUP: A Conference in Honour of
David Block and Bruce Elmegreen, eds. Freeman, K.C., Elmegreen, B.G., Block,
D.L.and Woolway, M., Springer: New Yor
Parameterized Complexity of Graph Constraint Logic
Graph constraint logic is a framework introduced by Hearn and Demaine, which
provides several problems that are often a convenient starting point for
reductions. We study the parameterized complexity of Constraint Graph
Satisfiability and both bounded and unbounded versions of Nondeterministic
Constraint Logic (NCL) with respect to solution length, treewidth and maximum
degree of the underlying constraint graph as parameters. As a main result we
show that restricted NCL remains PSPACE-complete on graphs of bounded
bandwidth, strengthening Hearn and Demaine's framework. This allows us to
improve upon existing results obtained by reduction from NCL. We show that
reconfiguration versions of several classical graph problems (including
independent set, feedback vertex set and dominating set) are PSPACE-complete on
planar graphs of bounded bandwidth and that Rush Hour, generalized to boards, is PSPACE-complete even when is at most a constant
The effect of a brief teacher training on student achievement
On the basis of previously determined correlations between teacher behavior and student achievement, a training program for mathematics teachers was developed. Previous research had established that this training did change the behavior of teachers. This article reports the effects of the training program on teacher behavior, student achievement, and student attitudes towards mathematics. From this study it appeared that the timing of the training was important for its effect on student achievement. There was no effect on student attitude
Electromagnons and instabilities in magnetoelectric materials with non-collinear spin orders
We show that strong electromagnon peaks can be found in absorption spectra of
non-collinear magnets exhibiting a linear magnetoelectric effect. The
frequencies of these peaks coincide with the frequencies of antiferromagnetic
resonances and the ratio of the spectral weights of the electromagnon and
antiferromagnetic resonance is related to the ratio of the static
magnetoelectric constant and magnetic susceptibility. Using a Kagome lattice
antiferromagnet as an example, we show that frustration of spin ordering gives
rise to magnetoelastic instabilities at strong spin-lattice coupling, which
transform a non-collinear magnetoelectric spin state into a collinear
multiferroic state with a spontaneous electric polarization and magnetization.
The Kagome lattice antiferromagnet also shows a ferroelectric
incommensurate-spiral phase, where polarization is induced by the exchange
striction mechanism.Comment: 9 pages, 4 figure
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