8,255 research outputs found
Modelling Accretion in Transitional Disks
Transitional disks are protoplanetary disk around young stars that display
inner holes in the dust distribution within a few AU, which is accompanied
nevertheless by some gas accretion onto the central star. These cavities could
possibly be created by the presence of one or more massive planets. If the gap
is created by planets and gas is still present in it, then there should be a
flow of gas past the planet into the inner region. It is our goal to study the
mass accretion rate into the gap and in particular the dependency on the
planet's mass and the thermodynamic properties of the disk. We performed 2D
hydro simulations for disks with embedded planets. We added radiative cooling
from the disk surfaces, radiative diffusion in the disk midplane, and stellar
irradiation to the energy equation to have more realistic models. The mass flow
rate into the gap region depends, for given disk thermodynamics,
non-monotonically on the mass of the planet. Generally, more massive planets
open wider and deeper gaps which would tend to reduce the mass accretion into
the inner cavity. However, for larger mass planets the outer disk becomes
eccentric and the mass flow rate is enhanced over the low mass cases. As a
result, for the isothermal disks the mass flow is always comparable to the
expected mass flow of unperturbed disks M_d, while for more realistic radiative
disks the mass flow is very small for low mass planets (<= 4 M_jup) and about
50% for larger planet masses. For the radiative disks that critical planet mass
for the disk to become eccentric is much larger that in the isothermal case.
Massive embedded planets can reduce the mass flow across the gap considerably,
to values of about an order of magnitude smaller than the standard disk
accretion rate, and can be responsible for opening large cavities. The
remaining mass flow into the central cavity is in good agreement with the
observations.Comment: 10 pages, 29 figures, accepted for publication in Astronomy &
Astrophysic
A mechanism for pair formation in strongly correlated systems
We start from a Hamiltonian describing non-interacting fermions and add
bosons to the model, with a Jaynes-Cummings-like interaction between the bosons
and fermions. Because of the specific form of the interaction the model can be
solved exactly. In the ground state, part of the electrons form bound pairs
with opposite momentum and spin. The model also shows a gap in the kinetic
energy of the fermions, but not in the spectrum of the full Hamiltonian. This
gap is not of a mean-field nature, but is due to the Pauli exclusion principle.Comment: 13 pages, corrected some notations and made some clarification
A topological classification of interaction-driven spin pumps
When adiabatically varied in time, certain one-dimensional band insulators
allow for the quantized noiseless pumping of spin even in the presence of
strong spin orbit scattering. These spin pumps are closely related to the
quantum spin Hall system, and their properties are protected by a time-reversal
restriction on the pumping cycle. In this paper we study pumps formed of
one-dimensional insulators with a time-reversal restriction on the pumping
cycle and a bulk energy gap which arises due to interactions. We find that the
correlated gapped phase can lead to novel pumping properties. In particular,
systems with different ground states can give rise to different
classes of spin pumps, including a trivial class which does not pump quantized
spin and non-trivial classes allowing for the pumping of quantized spin
on average per cycle, where . We discuss an example
of a spin pump that transfers on average spin without transferring
charge.Comment: 5 pages, 2 figure
Gaussian approximations for stochastic systems with delay: chemical Langevin equation and application to a Brusselator system
We present a heuristic derivation of Gaussian approximations for stochastic
chemical reaction systems with distributed delay. In particular we derive the
corresponding chemical Langevin equation. Due to the non-Markovian character of
the underlying dynamics these equations are integro-differential equations, and
the noise in the Gaussian approximation is coloured. Following on from the
chemical Langevin equation a further reduction leads to the linear-noise
approximation. We apply the formalism to a delay variant of the celebrated
Brusselator model, and show how it can be used to characterise noise-driven
quasi-cycles, as well as noise-triggered spiking. We find surprisingly
intricate dependence of the typical frequency of quasi-cycles on the delay
period.Comment: 14 pages, 9 figure
Framework for Analysis of Products Liability in Montana,
This article seeks to serve the needs of the Montana bench and bar by addressing the issues likely to be raised in products liability litigation. It will describe the history of products liability nationally and in Montana and will analyze major issues by examining current directions in case law. Finally, it will offer a framework for legal analysis of products liability to assist courts and counsel in avoiding some of the pitfalls encountered in development of products liability in other jurisdictions
A Framework for Analysis of Products Liability in Montana
A Framework For Analysis Of Products Liability In Montan
A Framework for Analysis of Products Liability in Montana
A Framework For Analysis Of Products Liability In Montan
Dynamic behavior of porous electrode systems final report
Mathematical model of flooded porous electrodes under dynamic and static conditions - Methods for measuring porous electrode reaction distributio
Approximate locality for quantum systems on graphs
In this Letter we make progress on a longstanding open problem of Aaronson
and Ambainis [Theory of Computing 1, 47 (2005)]: we show that if A is the
adjacency matrix of a sufficiently sparse low-dimensional graph then the
unitary operator e^{itA} can be approximated by a unitary operator U(t) whose
sparsity pattern is exactly that of a low-dimensional graph which gets more
dense as |t| increases. Secondly, we show that if U is a sparse unitary
operator with a gap \Delta in its spectrum, then there exists an approximate
logarithm H of U which is also sparse. The sparsity pattern of H gets more
dense as 1/\Delta increases. These two results can be interpreted as a way to
convert between local continuous-time and local discrete-time processes. As an
example we show that the discrete-time coined quantum walk can be realised as
an approximately local continuous-time quantum walk. Finally, we use our
construction to provide a definition for a fractional quantum fourier
transform.Comment: 5 pages, 2 figures, corrected typ
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