2,310 research outputs found
Decimetre dust aggregates in protoplanetary discs
The growth of planetesimals is an essential step in planet formation.
Decimetre-size dust agglomerates mark a transition point in this growth
process. In laboratory experiments we simulated the formation, evolution, and
properties of decimetre-scale dusty bodies in protoplanetary discs. Small
sub-mm size dust aggregates consisting of micron-size SiO particles
randomly interacted with dust targets of varying initial conditions in a
continuous sequence of independent collisions. Impact velocities were 7.7 m/s
on average and in the range expected for collisions with decimetre bodies in
protoplanetary discs. The targets all evolved by forming dust \emph{crusts}
with up to several cm thickness and a unique filling factor of 31% 3%. A
part of the projectiles sticks directly. In addition, some projectile fragments
slowly return to the target by gravity. All initially porous parts of the
surface, i.e. built from the slowly returning fragments, are compacted and
firmly attached to the underlying dust layers by the subsequent impacts. Growth
is possible at impact angles from 0 (central collision) to
70. No growth occurs at steeper dust surfaces. We measured the
velocity, angle, and size distribution of collision fragments. The average
restitution coefficient is 3.8% or 0.29 m/s ejection velocity. Ejecta sizes are
comparable to the projectile sizes. The high filling factor is close to the
most compact configuration of dust aggregates by local compression (%). This implies that the history of the surface formation and target growth
is completely erased. In view of this, the filling factor of 31% seems to be a
universal value in the collision experiments of all self-consistently evolving
targets at the given impact velocities. We suggest that decimetre and probably
larger bodies can simply be characterised by one single filling factor.Comment: 10 pages, 9 figure
Elliptic Flow and Semi-hard Scattering at SPS
Results on elliptic flow and two-particle correlations in the semi-hard
regime are presented.Comment: 4 pages, 4 figures, 3 of which contain 2 eps file
Growth of Dust as the Initial Step Toward Planet Formation
We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.Comment: Protostars and Planets V (PPV) review. 18 pages, 5 figure
Regularized Ordinal Regression and the ordinalNet R Package
Regularization techniques such as the lasso (Tibshirani 1996) and elastic net
(Zou and Hastie 2005) can be used to improve regression model coefficient
estimation and prediction accuracy, as well as to perform variable selection.
Ordinal regression models are widely used in applications where the use of
regularization could be beneficial; however, these models are not included in
many popular software packages for regularized regression. We propose a
coordinate descent algorithm to fit a broad class of ordinal regression models
with an elastic net penalty. Furthermore, we demonstrate that each model in
this class generalizes to a more flexible form, for instance to accommodate
unordered categorical data. We introduce an elastic net penalty class that
applies to both model forms. Additionally, this penalty can be used to shrink a
non-ordinal model toward its ordinal counterpart. Finally, we introduce the R
package ordinalNet, which implements the algorithm for this model class
Structural Glass Fin Façade St. Davids Hall, Cardiff
The remodelling of the entrance of St. David’s Hall in Cardiff features an array oftranslucent and partially pigmented structural glass fins with artistic lighting. Thefins are composed of 2 x 10 mm toughened glass incorporating a red screen frit andacid etching. The fins are 300 mm deep and up to 5 m long, supported by a stainlesssteel shoe bracket at the bottom and a fork bracket in the upper zone of the fin.Only by using a stiff Ionoplast interlayer the fins could be designed to suit the loadbearing capacity of the existing structure. The robustness and safety of the designwas demonstrated by a series of impact tests
Elliptic flow contribution to two-particle correlations at different orientations to the reaction plane
Collective anisotropic particle flow, a general phenomenon present in
relativistic heavy-ion collisions, can be separated from direct
particle-particle correlations of different physics origin by virtue of its
specific azimuthal pattern. We provide expressions for flow-induced
two-particle azimuthal correlations, if one of the particles is detected under
fixed directions with respect to the reaction plane. We consider an ideal case
when the reaction plane angle is exactly known, as well as present the general
expressions in case of finite event-plane resolution. We foresee applications
for the study of generic two-particle correlations at large transverse momentum
originating from jet fragmentation.Comment: 5 pages, 3 figures, to be published as Rapid Communications in
Phys.Rev.C Re-submit paper to with small improvements in text for better
understanding, some minor changes in notation, and correcting one formula
where a summation was forgotten. One new reference, one reference to
conference report removed since full paper was already reference
Interfaces within graphene nanoribbons
We study the conductance through two types of graphene nanostructures: nanoribbon junctions in which the width changes from wide to narrow, and curved nanoribbons. In the wide-narrow structures, substantial reflection occurs from the wide-narrow interface, in contrast to the behavior of the much studied electron gas waveguides. In the curved nanoribbons, the conductance is very sensitive to details such as whether regions of a semiconducting armchair nanoribbon are included in the curved structure -- such regions strongly suppress the conductance. Surprisingly, this suppression is not due to the band gap of the semiconducting nanoribbon, but is linked to the valley degree of freedom. Though we study these effects in the simplest contexts, they can be expected to occur for more complicated structures, and we show results for rings as well. We conclude that experience from electron gas waveguides does not carry over to graphene nanostructures. The interior interfaces causing extra scattering result from the extra effective degrees of freedom of the graphene structure, namely the valley and sublattice pseudospins
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