10,724 research outputs found
On empirical models of the upper atmosphere in the polar regions
Modified expression for exospheric temperature in Jacchia static diffusion models of upper atmosphere in polar region
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier
The sticking of micron sized dust particles due to surface forces in
circumstellar disks is the first stage in the production of asteroids and
planets. The key ingredients that drive this process are the relative velocity
between the dust particles in this environment and the complex physics of dust
aggregate collisions. Here we present the results of a collision model, which
is based on laboratory experiments of these aggregates. We investigate the
maximum aggregate size and mass that can be reached by coagulation in
protoplanetary disks. We model the growth of dust aggregates at 1 AU at the
midplane at three different gas densities. We find that the evolution of the
dust does not follow the previously assumed growth-fragmentation cycles.
Catastrophic fragmentation hardly occurs in the three disk models. Furthermore
we see long lived, quasi-steady states in the distribution function of the
aggregates due to bouncing. We explore how the mass and the porosity change
upon varying the turbulence parameter and by varying the critical mass ratio of
dust particles. Particles reach Stokes numbers of roughly 10^-4 during the
simulations. The particle growth is stopped by bouncing rather than
fragmentation in these models. The final Stokes number of the aggregates is
rather insensitive to the variations of the gas density and the strength of
turbulence. The maximum mass of the particles is limited to approximately 1
gram (chondrule-sized particles). Planetesimal formation can proceed via the
turbulent concentration of these aerodynamically size-sorted chondrule-sized
particles.Comment: accepted for publication in A&
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
Security of Quantum Bit-String Generation
We consider the cryptographic task of bit-string generation. This is a
generalisation of coin tossing in which two mistrustful parties wish to
generate a string of random bits such that an honest party can be sure that the
other cannot have biased the string too much. We consider a quantum protocol
for this task, originally introduced in Phys. Rev. A {\bf 69}, 022322 (2004),
that is feasible with present day technology. We introduce security conditions
based on the average bias of the bits and the Shannon entropy of the string.
For each, we prove rigorous security bounds for this protocol in both noiseless
and noisy conditions under the most general attacks allowed by quantum
mechanics. Roughly speaking, in the absence of noise, a cheater can only bias
significantly a vanishing fraction of the bits, whereas in the presence of
noise, a cheater can bias a constant fraction, with this fraction depending
quantitatively on the level of noise. We also discuss classical protocols for
the same task, deriving upper bounds on how well a classical protocol can
perform. This enables the determination of how much noise the quantum protocol
can tolerate while still outperforming classical protocols. We raise several
conjectures concerning both quantum and classical possibilities for large n
cryptography. An experiment corresponding to the scheme analysed in this paper
has been performed and is reported elsewhere.Comment: 16 pages. No figures. Accepted for publication in Phys. Rev. A. A
corresponding experiment is reported in quant-ph/040812
Scaling in Complex Systems: Analytical Theory of Charged Pores
In this paper we find an analytical solution of the equilibrium ion
distribution for a toroidal model of a ionic channel, using the Perfect
Screening Theorem (PST). The ions are charged hard spheres, and are treated
using a variational Mean Spherical Approximation (VMSA) .
Understanding ion channels is still a very open problem, because of the many
exquisite tuning details of real life channels. It is clear that the electric
field plays a major role in the channel behaviour, and for that reason there
has been a lot of work on simple models that are able to provide workable
theories. Recently a number of interesting papers have appeared that discuss
models in which the effect of the geometry, excluded volume and non-linear
behaviour is considered.
We present here a 3D model of ionic channels which consists of a charged,
deformable torus with a circular or elliptical cross section, which can be flat
or vertical (close to a cylinder). Extensive comparisons to MC simulations were
performed.
The new solution opens new possibilities, such as studying flexible pores,
and water phase transformations inside the pores using an approach similar to
that used on flat crystal surfaces
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