99 research outputs found
The core structure of presolar graphite onions
Of the ``presolar particles'' extracted from carbonaceous chondrite
dissolution residues, i.e. of those particles which show isotopic evidence of
solidification in the neighborhood of other stars prior to the origin of our
solar system, one subset has an interesting concentric
graphite-rim/graphene-core structure. We show here that single graphene sheet
defects in the onion cores (e.g. cyclopentane loops) may be observable edge-on
by HREM. This could allow a closer look at models for their formation, and in
particular strengthen the possibility that growth of these assemblages proceeds
atom-by-atom with the aid of such in-plane defects, under conditions of growth
(e.g. radiation fluxes or grain temperature) which discourage the graphite
layering that dominates subsequent formation of the rim.Comment: 4 pages, 7 figures, 11 refs, see also
http://www.umsl.edu/~fraundor/isocore.htm
Shearing behavior of polydisperse media
We study the shearing of polydisperse and bidisperse media with a size ratio
of 10. Simulations are performed with a the two dimensional shear cell using
contact dynamics. With a truncated power law for the polydisperse media we find
that they show a stronger dilatancy and greater resistance to shearing than
bidisperse mixtures. Motivated by the practical problem of reducing the energy
needed to shear granular media, we introduce "point-like particles"
representing charged particles in the distribution. Even though changing the
kinematic behavior very little, they reduce the force necessary to maintain a
fixed shearing velocity.Comment: 17 pages, 15 figure
The reversible polydisperse Parking Lot Model
We use a new version of the reversible Parking Lot Model to study the
compaction of vibrated polydisperse media. The particle sizes are distributed
according to a truncated power law. We introduce a self-consistent desorption
mechanism with a hierarchical initialization of the system. In this way, we
approach densities close to unity. The final density depends on the
polydispersity of the system as well as on the initialization and will reach a
maximum value for a certain exponent in the power law.Comment: 7 pages, Latex, 12 figure
An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials
A fully parallel version of the contact dynamics (CD) method is presented in
this paper. For large enough systems, 100% efficiency has been demonstrated for
up to 256 processors using a hierarchical domain decomposition with dynamic
load balancing. The iterative scheme to calculate the contact forces is left
domain-wise sequential, with data exchange after each iteration step, which
ensures its stability. The number of additional iterations required for
convergence by the partially parallel updates at the domain boundaries becomes
negligible with increasing number of particles, which allows for an effective
parallelization. Compared to the sequential implementation, we found no
influence of the parallelization on simulation results.Comment: 19 pages, 15 figures, published in Journal of Computational Physics
(2011
BVRI Light Curves for 29 Type Ia Supernovae
BVRI light curves are presented for 27 Type Ia supernovae discovered during
the course of the Calan/Tololo Survey and for two other SNe Ia observed during
the same period. Estimates of the maximum light magnitudes in the B, V, and I
bands and the initial decline rate parameter m15(B) are also given.Comment: 17 pages, figures and tables are not included (contact first author
if needed), to appear in the Astronomical Journa
Supernova 2014J at M82 – II. Direct analysis of a middle-class Type Ia supernova
We analyze a time series of optical spectra of SN 2014J from almost two weeks prior to maximum to nearly four months after maximum. We perform our analysis using the SYNOW code, which is well suited to track the distribution of the ions with velocity in the ejecta. We show that almost all of the spectral features during the entire epoch can be identified with permitted transitions of the common ions found in normal SNe Ia in agreement with previous studies. We show that 2014J is a relatively normal SN Ia. At early times the spectral features are dominated by Si II, S II, Mg II, and Ca II. These ions persist to maximum light with the appearance of Na I and Mg I. At later times iron-group elements also appear, as expected in the stratified abundance model of the formation of normal type Ia SNe. We do not find significant spectroscopic evidence for oxygen, until 100 days after maximum light. The +100 day identification of oxygen is tentative, and would imply significant mixing of unburned or only slight processed elements down to a velocity of 6,000 km~s−1. Our results are in relatively good agreement with other analyses in the IR. We briefly compare SN 2011fe to SN 2014J and conclude that the differences could be due to different central densities at ignition or differences in the C/O ratio of the progenitors
Schnelle Algorithmen zur Simulation extrem polydisperser Medien
In this work different algorithms to simulate extremely polydisperse media, which follow a truncated power law, were developed. The basic difficulty is the nonlinear dependence of the computing time on the polydispersity. It is possible to decouple the computing time from the polydispersity through a self-consistent and macroscopical discription of the so called local packings. These are the many small particles inbetween the large ones.In dieser Arbeit wurden verschiedene Algorithmen zur Simulation extrem polydisperser Medien, die einem abgeschnittenen Potenzgesetz folgen, entwickelt. Das grundlegende Problem besteht darin, dass der Rechenaufwand nichtlinear mit der
Polydispersitaet zusammenhaengt. Eine Abkopplung des Rechenaufwandes gelingt durch
die selbstkonsistente makroskopische Beschreibung der sogenannten lokalen Packungen. Dies sind die vielen kleinen Teilchen die zwischen den einzelnen grossen Teilchen zu liegen kommen
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