6,031 research outputs found
Thermodynamics of the dead-zone inner edge in protoplanetary disks
In protoplanetary disks, the inner boundary between the turbulent and laminar
regions could be a promising site for planet formation, thanks to the trapping
of solids at the boundary itself or in vortices generated by the Rossby wave
instability. At the interface, the disk thermodynamics and the turbulent
dynamics are entwined because of the importance of turbulent dissipation and
thermal ionization. Numerical models of the boundary, however, have neglected
the thermodynamics, and thus miss a part of the physics. The aim of this paper
is to numerically investigate the interplay between thermodynamics and dynamics
in the inner regions of protoplanetary disks by properly accounting for
turbulent heating and the dependence of the resistivity on the local
temperature. Using the Godunov code RAMSES, we performed a series of 3D global
numerical simulations of protoplanetary disks in the cylindrical limit,
including turbulent heating and a simple prescription for radiative cooling. We
find that waves excited by the turbulence significantly heat the dead zone, and
we subsequently provide a simple theoretical framework for estimating the wave
heating and consequent temperature profile. In addition, our simulations reveal
that the dead-zone inner edge can propagate outward into the dead zone, before
staling at a critical radius that can be estimated from a mean-field model. The
engine driving the propagation is in fact density wave heating close to the
interface. A pressure maximum appears at the interface in all simulations, and
we note the emergence of the Rossby wave instability in simulations with
extended azimuth. Our simulations illustrate the complex interplay between
thermodynamics and turbulent dynamics in the inner regions of protoplanetary
disks. They also reveal how important activity at the dead-zone interface can
be for the dead-zone thermodynamic structure.Comment: 16 pages, 16 figures. Accepted in Astronomy and Astrophysic
Distributed anonymous function computation in information fusion and multiagent systems
We propose a model for deterministic distributed function computation by a
network of identical and anonymous nodes, with bounded computation and storage
capabilities that do not scale with the network size. Our goal is to
characterize the class of functions that can be computed within this model. In
our main result, we exhibit a class of non-computable functions, and prove that
every function outside this class can at least be approximated. The problem of
computing averages in a distributed manner plays a central role in our
development
Distributed anonymous discrete function computation
We propose a model for deterministic distributed function computation by a
network of identical and anonymous nodes. In this model, each node has bounded
computation and storage capabilities that do not grow with the network size.
Furthermore, each node only knows its neighbors, not the entire graph. Our goal
is to characterize the class of functions that can be computed within this
model. In our main result, we provide a necessary condition for computability
which we show to be nearly sufficient, in the sense that every function that
satisfies this condition can at least be approximated. The problem of computing
suitably rounded averages in a distributed manner plays a central role in our
development; we provide an algorithm that solves it in time that grows
quadratically with the size of the network
Residence time of symmetric random walkers in a strip with large reflective obstacles
We study the effect of a large obstacle on the so called residence time,
i.e., the time that a particle performing a symmetric random walk in a
rectangular (2D) domain needs to cross the strip. We observe a complex
behavior, that is we find out that the residence time does not depend
monotonically on the geometric properties of the obstacle, such as its width,
length, and position. In some cases, due to the presence of the obstacle, the
mean residence time is shorter with respect to the one measured for the
obstacle--free strip. We explain the residence time behavior by developing a 1D
analog of the 2D model where the role of the obstacle is played by two defect
sites having a smaller probability to be crossed with respect to all the other
regular sites. The 1D and 2D models behave similarly, but in the 1D case we are
able to compute exactly the residence time finding a perfect match with the
Monte Carlo simulations
Continuous-time average-preserving opinion dynamics with opinion-dependent communications
We study a simple continuous-time multi-agent system related to Krause's
model of opinion dynamics: each agent holds a real value, and this value is
continuously attracted by every other value differing from it by less than 1,
with an intensity proportional to the difference.
We prove convergence to a set of clusters, with the agents in each cluster
sharing a common value, and provide a lower bound on the distance between
clusters at a stable equilibrium, under a suitable notion of multi-agent system
stability.
To better understand the behavior of the system for a large number of agents,
we introduce a variant involving a continuum of agents. We prove, under some
conditions, the existence of a solution to the system dynamics, convergence to
clusters, and a non-trivial lower bound on the distance between clusters.
Finally, we establish that the continuum model accurately represents the
asymptotic behavior of a system with a finite but large number of agents.Comment: 25 pages, 2 figures, 11 tex files and 2 eps file
Manipulation of Giant Faraday Rotation in Graphene Metasurfaces
Faraday rotation is a fundamental magneto-optical phenomenon used in various
optical control and magnetic field sensing techniques. Recently, it was shown
that a giant Faraday rotation can be achieved in the low-THz regime by a single
monoatomic graphene layer. Here, we demonstrate that this exceptional property
can be manipulated through adequate nano-patterning, notably achieving giant
rotation up to 6THz with features no smaller than 100nm. The effect of the
periodic patterning on the Faraday rotation is predicted by a simple physical
model, which is then verified and refined through accurate full-wave
simulations.Comment: 4 pages, 5 figures, submitted to Applied Physics Letter
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