3,362 research outputs found
Numerical determination of the material properties of porous dust cakes
The formation of planetesimals requires the growth of dust particles through
collisions. Micron-sized particles must grow by many orders of magnitude in
mass. In order to understand and model the processes during this growth, the
mechanical properties, and the interaction cross sections of aggregates with
surrounding gas must be well understood. Recent advances in experimental
(laboratory) studies now provide the background for pushing numerical aggregate
models onto a new level. We present the calibration of a previously tested
model of aggregate dynamics. We use plastic deformation of surface asperities
as the physical model to bring critical velocities for sticking into accordance
with experimental results. The modified code is then used to compute
compression strength and the velocity of sound in the aggregate at different
densities. We compare these predictions with experimental results and conclude
that the new code is capable of studying the properties of small aggregates.Comment: Accepted for publication in A&
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Mapping evidence gaps in anti-corruption: assessing the state of the operationally relevant evidence on donors' actions and approaches to reducing corruption
This paper charts the current evidence on effectiveness of different anti-corruption reforms, and identifies significant evidence gaps. Despite a substantial amount of literature on corruption, this review found very few studies focusing on anti-corruption reforms, and even fewer that credibly assess issues of effectiveness and impact.
The evidence was strong for only two types of interventions: public financial management (PFM) reforms and supreme audit institutions (SAIs). For PFM, the evidence in general showed positive results, whereas the effectiveness was mixed for SAIs. No strong evidence indicates that any of the interventions pursued have been ineffective, but there is fair evidence that anti-corruption authorities, civil service reforms and the use of corruption conditionality in aid allocation decisions in general have not been effective.
The paper advocates more operationally-relevant research and rigorous evaluations to build up the missing evidence base, particularly in conflict-afflicted states, in regards to the private sector, and on the interactions and interdependencies between different anti-corruption interventions
Compression Behaviour of Porous Dust Agglomerates
The early planetesimal growth proceeds through a sequence of sticking
collisions of dust agglomerates. Very uncertain is still the relative velocity
regime in which growth rather than destruction can take place. The outcome of a
collision depends on the bulk properties of the porous dust agglomerates.
Continuum models of dust agglomerates require a set of material parameters that
are often difficult to obtain from laboratory experiments. Here, we aim at
determining those parameters from ab-initio molecular dynamics simulations. Our
goal is to improveon the existing model that describe the interaction of
individual monomers. We use a molecular dynamics approach featuring a detailed
micro-physical model of the interaction of spherical grains. The model includes
normal forces, rolling, twisting and sliding between the dust grains. We
present a new treatment of wall-particle interaction that allows us to perform
customized simulations that directly correspond to laboratory experiments. We
find that the existing interaction model by Dominik & Tielens leads to a too
soft compressive strength behavior for uni and omni-directional compression.
Upon making the rolling and sliding coefficients stiffer we find excellent
agreement in both cases. Additionally, we find that the compressive strength
curve depends on the velocity with which the sample is compressed. The modified
interaction strengths between two individual dust grains will lead to a
different behaviour of the whole dust agglomerate. This will influences the
sticking probabilities and hence the growth of planetesimals. The new parameter
set might possibly lead to an enhanced sticking as more energy can be stored in
the system before breakup.Comment: 11 pages, 14 figures, accepted for publication in A&
Zonal Flows and Long-Lived Axisymmetric Pressure Bumps in Magnetorotational Turbulence
We study the behavior of magnetorotational turbulence in shearing box
simulations with a radial and azimuthal extent up to ten scale heights. Maxwell
and Reynolds stresses are found to increase by more than a factor two when
increasing the box size beyond two scale heights in the radial direction.
Further increase of the box size has little or no effect on the statistical
properties of the turbulence. An inverse cascade excites magnetic field
structures at the largest scales of the box. The corresponding 10% variation in
the Maxwell stress launches a zonal flow of alternating sub- and
super-Keplerian velocity. This in turn generates a banded density structure in
geostrophic balance between pressure and Coriolis forces. We present a
simplified model for the appearance of zonal flows, in which stochastic forcing
by the magnetic tension on short time-scales creates zonal flow structures with
life-times of several tens of orbits. We experiment with various improved
shearing box algorithms to reduce the numerical diffusivity introduced by the
supersonic shear flow. While a standard finite difference advection scheme
shows signs of a suppression of turbulent activity near the edges of the box,
this problem is eliminated by a new method where the Keplerian shear advection
is advanced in time by interpolation in Fourier space.Comment: Accepted for publication in Ap
The dust disk of HR4049
We present the Spectral Energy Distribution of HR 4049 based on literature
data and new continuum measurements at 850 micron. The SED shows variable
absorption in the UV, and a large IR excess, both caused by circumstellar dust.
The shape of the IR excess from 1 micron all the way down to 850 micron can be
nearly perfectly fitted with a single blackbody function at T ~ 1150 K or
alternatively with a sum of blackbodies in a narrow temperature range. The
energy emitted in this IR continuum radiation is about one-third of the stellar
luminosity. We show that this blackbody radiation must be due to the presence
of a circumbinary disk with a large height. This disk must also be gas-rich, in
agreement with the observations of molecular bands in the ISO-SWS spectrum. We
present two possible scenario's for explaining the shape and the intensity of
the IR excess. The first scenario involves large grains (a >1 mm) that each
radiate like a blackbody. The second scenario argues that the blackbody
radiation is due to a very optically thick circumbinary disk. We investigate if
such a disk would indeed produce blackbody radiation by presenting results from
radiative transfer calculations. We further quantify the properties of such a
disk and its stability in the framework of (hydro)dynamics, grain settling,
radiation pressure and grain drift. The virtues and shortcomings of both models
for the origin of the IR blackbody are discussed by contrasting them with other
observations and assessing them in the framework of (binary) (post-)AGB
evolution.Comment: 16 pages, 12 figures, accepted for publication in A&
Reflected Light from Sand Grains in the Terrestrial Zone of a Protoplanetary Disk
We show that grains have grown to ~mm size (sand sized) or larger in the
terrestrial zone (within ~3 AU) of the protoplanetary disk surrounding the 3
Myr old binary star KH 15D. We also argue that the reflected light in the
system reaches us by back scattering off the far side of the same ring whose
near side causes the obscuration.Comment: 22 pages, 5 figures. To be published in Nature, March 13, 2008.
Contains a Supplemen
Soil CO2 venting as one of the mechanisms for tolerance of Zn deficiency by rice in flooded soils
We sought to explain rice (Oryza sativa) genotype differences in tolerance of zinc (Zn) deficiency in flooded paddy soils and the counter-intuitive observation, made in earlier field experiments, that Zn uptake per plant increases with increasing planting density. We grew tolerant and intolerant genotypes in a Zn-deficient flooded soil at high and low planting densities, and found (a) plant Zn concentrations and growth increased with planting density and more so in the tolerant genotype, whereas the concentrations of other nutrients decreased, indicating a specific effect on Zn uptake; (b) the effects of planting density and genotype on Zn uptake could only be explained if the plants induced changes in the soil to make Zn more soluble; and (c) the genotype and planting density effects were both associated with decreases in dissolved CO2 in the rhizosphere soil solution and resulting increases in pH. We suggest the increases in pH caused solubilisation of soil Zn by dissolution of alkali-soluble, Zn-complexing organic ligands from soil organic matter. We conclude that differences in venting of soil CO2 through root aerenchyma were responsible for the genotype and planting density effects
Lyman alpha emission from the first galaxies: Signatures of accretion and infall in the presence of line trapping
The formation of the first galaxies is accompanied by large accretion flows
and virialization shocks, during which the gas is shock-heated to temperatures
of K, leading to potentially strong fluxes in the Lyman alpha line.
Indeed, a number of Lyman alpha blobs has been detected at high redshift. In
this letter, we explore the origin of such Lyman alpha emission using
cosmological hydrodynamical simulations that include a detailed model of atomic
hydrogen as a multi-level atom and the effects of line trapping with the
adaptive mesh refinement code FLASH. We see that baryons fall into the center
of a halo through cold streams of gas, giving rise to a Lyman alpha luminosity
of at least at , similar to observed Lyman
alpha blobs. We find that a Lyman alpha flux of emerges from the envelope of the halo rather than its center,
where the photons are efficiently trapped. Such emission can be probed in
detail with the upcoming James Webb Space Telescope (JWST) and will constitute
an important probe of gas infall and accretion.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS LETTER
Incidence and survival of remnant disks around main-sequence stars
We present photometric ISO 60 and 170um measurements, complemented by some
IRAS data at 60um, of a sample of 84 nearby main-sequence stars of spectral
class A, F, G and K in order to determine the incidence of dust disks around
such main-sequence stars. Of the stars younger than 400 Myr one in two has a
disk; for the older stars this is true for only one in ten. We conclude that
most stars arrive on the main sequence surrounded by a disk; this disk then
decays in about 400 Myr. Because (i) the dust particles disappear and must be
replenished on a much shorter time scale and (ii) the collision of
planetesimals is a good source of new dust, we suggest that the rapid decay of
the disks is caused by the destruction and escape of planetesimals. We suggest
that the dissipation of the disk is related to the heavy bombardment phase in
our Solar System. Whether all stars arrive on the main sequence surrounded by a
disk cannot be established: some very young stars do not have a disk. And not
all stars destroy their disk in a similar way: some stars as old as the Sun
still have significant disks.Comment: 16 pages, 9 figures, Astron & Astrophys. in pres
Thermodynamically consistent Reference Interaction Site Model theory of the tangent diatomic fluid
Thermodynamic and structural properties of the tangent diatomic fluid are
studied in the framework provided by the Reference Interaction Site Model
(RISM) theory, coupled with a Modified Hypernetted Chain closure. The
enforcement of the internal thermodynamic consistency of the theory is
described in detail. The results we obtain almost quantitatively agree with
available or newly generated simulation data. We envisage the possibility to
extend the consistent RISM formalism to generic, more realistic molecular
fluids.Comment: Typeset with LaTeX, 6 pages, 3 figures (5 subfigures), 28 references,
submitted to Chem. Phys. Let
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