4,181 research outputs found
Mechanical properties of a degradable phosphate glass fibre reinforced polymer composite for internal fracture fixation
NOTICE: this is the author’s version of a work that was accepted for publication in Materials Science and Engineering. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Materials Science and Engineering, [VOL 30, ISSUE 7, (2010)] DOI: 10.1016/j.msec.2010.04.017
Gas- and dust evolution in protoplanetary disks
Context. Current models of the size- and radial evolution of dust in
protoplanetary disks generally oversimplify either the radial evolution of the
disk (by focussing at one single radius or by using steady state disk models)
or they assume particle growth to proceed monodispersely or without
fragmentation. Further studies of protoplanetary disks - such as observations,
disk chemistry and structure calculations or planet population synthesis models
- depend on the distribution of dust as a function of grain size and radial
position in the disk.
Aims. We attempt to improve upon current models to be able to investigate how
the initial conditions, the build-up phase, and the evolution of the
protoplanetary disk influence growth and transport of dust.
Methods. We introduce a new version of the model of Brauer et al. (2008) in
which we now include the time-dependent viscous evolution of the gas disk, and
in which more advanced input physics and numerical integration methods are
implemented.
Results. We show that grain properties, the gas pressure gradient, and the
amount of turbulence are much more influencing the evolution of dust than the
initial conditions or the build-up phase of the protoplanetary disk. We
quantify which conditions or environments are favorable for growth beyond the
meter size barrier. High gas surface densities or zonal flows may help to
overcome the problem of radial drift, however already a small amount of
turbulence poses a much stronger obstacle for grain growth.Comment: accepted to A&
Dust retention in protoplanetary disks
Context: Protoplanetary disks are observed to remain dust-rich for up to
several million years. Theoretical modeling, on the other hand, raises several
questions. Firstly, dust coagulation occurs so rapidly, that if the small dust
grains are not replenished by collisional fragmentation of dust aggregates,
most disks should be observed to be dust poor, which is not the case. Secondly,
if dust aggregates grow to sizes of the order of centimeters to meters, they
drift so fast inwards, that they are quickly lost.
Aims: We attempt to verify if collisional fragmentation of dust aggregates is
effective enough to keep disks 'dusty' by replenishing the population of small
grains and by preventing excessive radial drift.
Methods: With a new and sophisticated implicitly integrated coagulation and
fragmentation modeling code, we solve the combined problem of coagulation,
fragmentation, turbulent mixing and radial drift and at the same time solve for
the 1-D viscous gas disk evolution.
Results: We find that for a critical collision velocity of 1 m/s, as
suggested by laboratory experiments, the fragmentation is so effective, that at
all times the dust is in the form of relatively small particles. This means
that radial drift is small and that large amounts of small dust particles
remain present for a few million years, as observed. For a critical velocity of
10 m/s, we find that particles grow about two orders of magnitude larger, which
leads again to significant dust loss since larger particles are more strongly
affected by radial drift.Comment: Letter accepted 3 July 2009, included comments of language edito
Trapped surfaces in spherical expanding open universes
Consider spherically symmetric initial data for a cosmology which, in the
large, approximates an open Friedmann-Lema{\^\i}tre
universe. Further assume that the data is chosen so that the trace of the
extrinsic curvature is a constant and that the matter field is at rest at this
instant of time. One expects that no trapped surfaces appear in the data if no
significant clump of excess matter is to be found. This letter confirms this
belief by displaying a necessary condition for the existence of trapped
surfaces.This necessary condition, simply stated, says that a relatively large
amount of excess matter must be concentrated in a small volume for trapped
surfaces to appear.Comment: 8 pages, Late
Bounds for the greatest characteristic root of an irreducible nonnegative matrix
AbstractA new lower bound for the Perron root for irreducible, non-negative matrices is obtained which is, in particular, a better bound than the Frobenius bound [w = max(akk)] if all the main diagonal elements are zero
Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits
Context. Grains in circumstellar disks are believed to grow by mutual
collisions and subsequent sticking due to surface forces. Results of many
fields of research involving circumstellar disks, such as radiative transfer
calculations, disk chemistry, magneto-hydrodynamic simulations largely depend
on the unknown grain size distribution.
Aims. As detailed calculations of grain growth and fragmentation are both
numerically challenging and computationally expensive, we aim to find simple
recipes and analytical solutions for the grain size distribution in
circumstellar disks for a scenario in which grain growth is limited by
fragmentation and radial drift can be neglected.
Methods. We generalize previous analytical work on self-similar steady-state
grain distributions. Numerical simulations are carried out to identify under
which conditions the grain size distributions can be understood in terms of a
combination of power-law distributions. A physically motivated fitting formula
for grain size distributions is derived using our analytical predictions and
numerical simulations.
Results. We find good agreement between analytical results and numerical
solutions of the Smoluchowski equation for simple shapes of the kernel
function. The results for more complicated and realistic cases can be fitted
with a physically motivated "black box" recipe presented in this paper. Our
results show that the shape of the dust distribution is mostly dominated by the
gas surface density (not the dust-to-gas ratio), the turbulence strength and
the temperature and does not obey an MRN type distribution.Comment: 16 pages, 9 figures, accepted for publication in A&
Selbstbestimmung und Selbstverständnis: Themenschwerpunkte im Umgang mit der Patientenverfügung
Exact properties of Frobenius numbers and fraction of the symmetric semigroups in the weak limit for n=3
We generalize and prove a hypothesis by V. Arnold on the parity of Frobenius
number. For the case of symmetric semigroups with three generators of Frobenius
numbers we found an exact formula, which in a sense is the sum of two
Sylvester's formulaes. We prove that the fraction of the symmetric semigroups
is vanishing in the weak limit
Functional organization of the language network in three- and six-year-old children
The organization of the language network undergoes continuous changes during development as children learn to understand sentences. In the present study, functional magnetic resonance imaging and behavioral measures were utilized to investigate functional activation and functional connectivity (FC) in three-year-old (3yo) and six-year-old (6yo) children during sentence comprehension. Transitive German sentences varying the word order (subject-initial and object-initial) with case marking were presented auditorily. We selected children who were capable of processing the subject-initial sentences above chance level accuracy from each age group to ensure that we were tapping real comprehension. Both age groups showed a main effect of word order in the left posterior superior temporal gyrus (pSTG), with greater activation for object-initial compared to subject-initial sentences. However, age differences were observed in the FC between left pSTG and the left inferior frontal gyrus (IFG). The 6yo group showed stronger FC between the left pSTG and Brodmann area (BA) 44 of the left IFG compared to the 3yo group. For the 3yo group, in turn, the FC between left pSTG and left BA 45 was stronger than with left BA 44. Our study demonstrates that while task-related activation was comparable, the small behavioral differences between age groups were reflected in the underlying functional organization revealing the ongoing development of the neural language network
- …