43,360 research outputs found
Eulerian Statistically Preserved Structures in Passive Scalar Advection
We analyze numerically the time-dependent linear operators that govern the
dynamics of Eulerian correlation functions of a decaying passive scalar
advected by a stationary, forced 2-dimensional Navier-Stokes turbulence. We
show how to naturally discuss the dynamics in terms of effective compact
operators that display Eulerian Statistically Preserved Structures which
determine the anomalous scaling of the correlation functions. In passing we
point out a bonus of the present approach, in providing analytic predictions
for the time-dependent correlation functions in decaying turbulent transport.Comment: 10 pages, 10 figures. Submitted to Phys. Rev.
The Schwinger mechanism and graphene
The Schwinger mechanism, the production of charged particle-antiparticle
pairs in a macroscopic external electric field, is derived for 2+1 dimensional
theories. The rate of pair production per unit area for four species of
massless fermions, with charge , in a constant electric field is given
by where is
the speed of light for the two-dimensional system. To the extent undoped
graphene behaves like the quantum field-theoretic vacuum for massless fermions
in 2+1 dimensions, the Schwinger mechanism should be testable experimentally. A
possible experimental configuration for this is proposed. Effects due to
deviations from this idealized picture of graphene are briefly considered. It
is argued that with present day samples of graphene, tests of the Schwinger
formula may be possible.Comment: Extensive revisions. The distinction between the vacuum decay rate
and the pair production rate in the Schwinger mechanism is now stressed. The
discussion of quality of sample needed for a viable experimental test has
been significantly expanded. References adde
An improved lower bound for (1,<=2)-identifying codes in the king grid
We call a subset of vertices of a graph a -identifying
code if for all subsets of vertices with size at most , the sets
are distinct. The concept of
identifying codes was introduced in 1998 by Karpovsky, Chakrabarty and Levitin.
Identifying codes have been studied in various grids. In particular, it has
been shown that there exists a -identifying code in the king grid
with density 3/7 and that there are no such identifying codes with density
smaller than 5/12. Using a suitable frame and a discharging procedure, we
improve the lower bound by showing that any -identifying code of
the king grid has density at least 47/111
Model Independent Extraction of Without Heavy Quark Symmetry
A new method to extract is proposed based on a sum--rule for
semileptonic decays of the meson. The method relies on much weaker
assumptions than previous approaches which are based on heavy--quark symmetry.
This sum--rule only relies on the assumption that the virtual
pair content of the meson can be neglected. The extraction of the CKM
matrix element also requires that the sum--rule saturates in the kinematically
accessible region.Comment: 10 pages revtex3 manuscript. No figures, U. of MD PP #94--086. With
our apologies, some innocuous errors corrected and some references added that
had been brought to our attentio
Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars
We present MERLIN maps of the 22-GHz H2O masers around four low-mass
late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular
resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O
masers are found in thick expanding shells with inner radii ~ 6 to 16 au and
outer radii four times larger. The expansion velocity increases radially
through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9.
IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset
between the near and far sides of the shell, which suggests that the masers are
distributed in oblate spheroids inclined to the line of sight. U Ori and U Her
have elongated poorly-filled shells with indications that the masers at the
inner edge have been compressed by shocks; these stars also show OH maser
flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4
au and filling factors of only ~ 0.01 with respect to the whole H2O maser
shells. The CSE velocity structure gives additional evidence the maser clouds
are density bounded. Masing clouds can be identified over a similar timescale
to their sound crossing time (~2 yr) but not longer. The sizes and observed
lifetimes of these clouds are an order of magnitude smaller than those around
red supergiants, similar to the ratio of low-mass:high-mass stellar masses and
sizes. This suggests that cloud size is determined by stellar properties, not
local physical phenomena in the wind.Comment: 21 pages, including 14 figures and 8 tables. Accepted for publication
in MNRA
Random Walks in Local Dynamics of Network Losses
We suggest a model for data losses in a single node of a packet-switched
network (like the Internet) which reduces to one-dimensional discrete random
walks with unusual boundary conditions. The model shows critical behavior with
an abrupt transition from exponentially small to finite losses as the data
arrival rate increases. The critical point is characterized by strong
fluctuations of the loss rate. Although we consider the packet arrival being a
Markovian process, the loss rate exhibits non-Markovian power-law correlations
in time at the critical point.Comment: 4 pages, 2 figure
On Toroidal Horizons in Binary Black Hole Inspirals
We examine the structure of the event horizon for numerical simulations of
two black holes that begin in a quasicircular orbit, inspiral, and finally
merge. We find that the spatial cross section of the merged event horizon has
spherical topology (to the limit of our resolution), despite the expectation
that generic binary black hole mergers in the absence of symmetries should
result in an event horizon that briefly has a toroidal cross section. Using
insight gained from our numerical simulations, we investigate how the choice of
time slicing affects both the spatial cross section of the event horizon and
the locus of points at which generators of the event horizon cross. To ensure
the robustness of our conclusions, our results are checked at multiple
numerical resolutions. 3D visualization data for these resolutions are
available for public access online. We find that the structure of the horizon
generators in our simulations is consistent with expectations, and the lack of
toroidal horizons in our simulations is due to our choice of time slicing.Comment: Submitted to Phys. Rev.
NICMOS Imaging of the Dusty Microjansky Radio Source VLA J123642+621331 at z = 4.424
We present the discovery of a radio galaxy at a likely redshift of z = 4.424
in one of the flanking fields of the Hubble Deep Field. Radio observations with
the VLA and MERLIN centered on the HDF yielded a complete sample of microjansky
radio sources, of which about 20% have no optical counterpart to I < 25 mag. In
this Letter, we address the possible nature of one of these sources, through
deep HST NICMOS images in the F110W (J) and F160W (H) filters. VLA
J123642+621331 has a single emission line at 6595-A, which we identify with
Lyman-alpha at z = 4.424. We argue that this faint (H = 23.9 mag), compact (r =
0.2 arcsec), red (I - K = 2.0) object is most likely a dusty, star-forming
galaxy with an embedded active nucleus.Comment: Accepted for publication in Astrophysical Journal Letters. 11 pages,
4 figures, uses aastex v5.0 and psfi
Moller operators and Lippmann-Schwinger equations for step-like potentials
The Moller operators and the asociated Lippman-Schwinger equations obtained
from different partitionings of the Hamiltonian for a step-like potential
barrier are worked out, compared and related.Comment: 15 pages, 1 inlined figure, iopart.cl
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