1,358 research outputs found
The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings
The macroscopic mechanical properties of colloidal particle gels strongly
depend on the local arrangement of the powder particles. Experiments have shown
that more heterogeneous microstructures exhibit up to one order of magnitude
higher elastic properties than their more homogeneous counterparts at equal
volume fraction. In this paper, packings of spherical particles are used as
model structures to computationally investigate the elastic properties of
coagulated particle gels as a function of their degree of heterogeneity. The
discrete element model comprises a linear elastic contact law, particle bonding
and damping. The simulation parameters were calibrated using a homogeneous and
a heterogeneous microstructure originating from earlier Brownian dynamics
simulations. A systematic study of the elastic properties as a function of the
degree of heterogeneity was performed using two sets of microstructures
obtained from Brownian dynamics simulation and from the void expansion method.
Both sets cover a broad and to a large extent overlapping range of degrees of
heterogeneity. The simulations have shown that the elastic properties as a
function of the degree of heterogeneity are independent of the structure
generation algorithm and that the relation between the shear modulus and the
degree of heterogeneity can be well described by a power law. This suggests the
presence of a critical degree of heterogeneity and, therefore, a phase
transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February
2012
A New Evolutionary Channel for Type Ia Supernovae
We show that long-period dwarf novae offer a promising route for making Type
Ia supernovae. For typical dwarf nova duty cycles d ~ 0.1 - 0.01, mass is
accreted by the white dwarf mainly during dwarf nova outbursts at rates
allowing steady nuclear burning of most of the accreted matter. Mass gains up
to ~ 0.4 Msun are possible in this way. Although these are too small to allow a
0.7 Msun WD to reach the Chandrasekhar mass, they are sufficient if the WD grew
to ~ 1 Msun in a previous episode of thermal-timescale mass transfer, i.e. for
those long-period dwarf novae which descend from supersoft binaries. A further
advantage of this picture is that the supernova always occurs in a binary of
small secondary/primary mass ratio, with the secondary having very little
remaining hydrogen. Both features greatly reduce the possibility of hydrogen
contamination of the supernova ejecta.Comment: 5 pages, 5 figures, MNRAS, accepte
Generation of folk song melodies using Bayes transforms
The paper introduces the `Bayes transform', a mathematical procedure for putting data into a hierarchical representation. Applicable to any type of data, the procedure yields interesting results when applied to sequences. In this case, the representation obtained implicitly models the repetition hierarchy of the source. There are then natural applications to music. Derivation of Bayes transforms can be the means of determining the repetition hierarchy of note sequences (melodies) in an empirical and domain-general way. The paper investigates application of this approach to Folk Song, examining the results that can be obtained by treating such transforms as generative models
Keck Spectroscopy of Faint 3<z<8 Lyman Break Galaxies:- Evidence for a Declining Fraction of Emission Line Sources In the Redshift Range 6<z<8
Using deep Keck spectroscopy of Lyman break galaxies selected from infrared
imaging data taken with WFC3/IR onboard the Hubble Space Telescope, we present
new evidence for a reversal in the redshift-dependent fraction of star forming
galaxies with detectable Lyman alpha emission in the redshift range 6.3 < z <
8.8. Our earlier surveys with the DEIMOS spectrograph demonstrated a
significant increase with redshift in the fraction of line emitting galaxies
over the interval 4 < z < 6, particularly for intrinsically faint systems which
dominate the luminosity density. Using the longer wavelength sensitivities of
LRIS and NIRSPEC, we have targeted 19 Lyman break galaxies selected using
recent WFC3/IR data whose photometric redshifts are in the range 6.3 < z < 8.8
and which span a wide range of intrinsic luminosities. Our spectroscopic
exposures typically reach a 5-sigma sensitivity of < 50 A for the rest-frame
equivalent width (EW) of Lyman alpha emission. Despite the high fraction of
emitters seen only a few hundred million years later, we find only 2 convincing
and 1 possible line emitter in our more distant sample. Combining with
published data on a further 7 sources obtained using FORS2 on the ESO VLT, and
assuming continuity in the trends found at lower redshift, we discuss the
significance of this apparent reversal in the redshift-dependent Lyman alpha
fraction in the context of our range in continuum luminosity. Assuming all the
targeted sources are at their photometric redshift and our assumptions about
the Lyman alpha EW distribution are correct, we would expect to find so few
emitters in less than 1% of the realizations drawn from our lower redshift
samples. Our new results provide further support for the suggestion that, at
the redshifts now being probed spectroscopically, we are entering the era where
the intergalactic medium is partially neutral.Comment: 8 pages, 5 figures, Accepted to ApJ 10/1/1
Generation of Porous Particle Structures using the Void Expansion Method
The newly developed "void expansion method" allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions using the discrete element method. Particles are randomly
placed under addition of much smaller "void-particles". Then, the void-particle
radius is increased repeatedly, thereby rearranging the structural particles
until formation of a dense particle packing.
The structural particles' mean coordination number was used to characterize
the evolving microstructures. At some void radius, a transition from an
initially low to a higher mean coordination number is found, which was used to
characterize the influence of the various simulation parameters. For structural
and void-particle stiffnesses of the same order of magnitude, the transition is
found at constant total volume fraction slightly below the random close packing
limit. For decreasing void-particle stiffness the transition is shifted towards
a smaller void-particle radius and becomes smoother.Comment: 9 pages, 8 figure
New gas mixtures suitable for rare event detection using a Micromegas-TPC detector
The aim of the presented work was to develop further techniques based on a
Micromegas-TPC, in order to reach a high gas gain with good energy resolution,
and to search for gas mixtures suitable for rare event detection. This paper
focuses on xenon, which is convenient for the search of neutrinoless double
beta decay in 136 Xe. Conversely, a small admixture of xenon to CF 4 can reduce
attachment in the latter. This gas mixture would be suitable for dark matter
searches and the study of solar and reactor neutrinos. Various configurations
of the Micromegas plane were investigated and are described.Comment: 14 pages, 8 figures, article, revised version with improved figures,
text modifications, accepted for publication by JINS
Scaling in a continuous time model for biological aging
In this paper we consider a generalization to the asexual version of the
Penna model for biological aging, where we take a continuous time limit. The
genotype associated to each individual is an interval of real numbers over
which Dirac --functions are defined, representing genetically
programmed diseases to be switched on at defined ages of the individual life.
We discuss two different continuous limits for the evolution equation and two
different mutation protocols, to be implemented during reproduction. Exact
stationary solutions are obtained and scaling properties are discussed.Comment: 10 pages, 6 figure
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