2,763 research outputs found
Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission
(Abridged) Aims. We intend to derive statistical properties of stochastic gas
motion inside the dense low mass star forming molecular cores traced by
NH3(1,1) and (2,2) emission lines. Methods. We use the spatial two-point
autocorrelation (ACF) and structure functions calculated from maps of the
radial velocity fields. Results. We find oscillating ACFs which eventually
decay to zero with increasing lags on scales of 0.04 <= l <= 0.5 pc. The
current paradigm supposes that the star formation process is controlled by the
interplay between gravitation and turbulence, the latter preventing molecular
cores from a rapid collapse due to their own gravity. Thus, oscillating ACFs
may indicate a damping of the developed turbulent flows surrounding the dense
but less turbulent core - a transition to dominating gravitational forces and,
hence, to gravitational collapse.Comment: 11 pages, 16 figures, 3 tables, to be published in Astronomy and
Astrophysic
Lattice two-point functions and conformal invariance
A new realization of the conformal algebra is studied which mimics the
behaviour of a statistical system on a discrete albeit infinite lattice. The
two-point function is found from the requirement that it transforms covariantly
under this realization. The result is in agreement with explicit lattice
calculations of the Ising model and the dimensional spherical
model. A hard core is found which is not present in the continuum. For a
semi-infinite lattice, profiles are also obtained.Comment: 5 pages, plain Tex with IOP macros, no figure
Exactly solvable models through the empty interval method, for more-than-two-site interactions
Single-species reaction-diffusion systems on a one-dimensional lattice are
considered, in them more than two neighboring sites interact. Constraints on
the interaction rates are obtained, that guarantee the closedness of the time
evolution equation for 's, the probability that consecutive sites
are empty at time . The general method of solving the time evolution
equation is discussed. As an example, a system with next-nearest-neighbor
interaction is studied.Comment: 19 pages, LaTeX2
Ageing, dynamical scaling and its extensions in many-particle systems without detailed balance
Recent studies on the phenomenology of ageing in certain many-particle
systems which are at a critical point of their non-equilibrium steady-states,
are reviewed. Examples include the contact process, the parity-conserving
branching-annihilating random walk, two exactly solvable particle-reaction
models and kinetic growth models. While the generic scaling descriptions known
from magnetic system can be taken over, some of the scaling relations between
the ageing exponents are no longer valid. In particular, there is no obvious
generalization of the universal limit fluctuation-dissipation ratio. The form
of the scaling function of the two-time response function is compared with the
prediction of the theory of local scale-invariance.Comment: Latex2e with IOP macros, 32 pages; extended discussion on contact
process and new section on kinetic growth processe
Local scale invariance and strongly anisotropic equilibrium critical systems
A new set of infinitesimal transformations generalizing scale invariance for
strongly anisotropic critical systems is considered. It is shown that such a
generalization is possible if the anisotropy exponent \theta =2/N, with N=1,2,3
... Differential equations for the two-point function are derived and
explicitly solved for all values of N. Known special cases are conformal
invariance (N=2) and Schr\"odinger invariance (N=1). For N=4 and N=6, the
results contain as special cases the exactly known scaling forms obtained for
the spin-spin correlation function in the axial next nearest neighbor spherical
(ANNNS) model at its Lifshitz points of first and second order.Comment: 4 pages Revtex, no figures, with file multicol.sty, to appear in PR
Noise-induced dynamical transition in systems with symmetric absorbing states
We investigate the effect of noise strength on the macroscopic ordering
dynamics of systems with symmetric absorbing states. Using an explicit
stochastic microscopic model, we present evidence for a phase transition in the
coarsening dynamics, from an Ising-like to a voter-like behavior, as the noise
strength is increased past a nontrivial critical value. By mapping to a thermal
diffusion process, we argue that the transition arises due to locally-absorbing
states being entered more readily in the high-noise regime, which in turn
prevents surface tension from driving the ordering process.Comment: v2 with improved introduction and figures, to appear in PRL. 4 pages,
4 figure
Magnetic noise around metallic microstructures
We compute the local spectrum of the magnetic field near a metallic
microstructure at finite temperature. Our main focus is on deviations from a
plane-layered geometry for which we review the main properties. Arbitrary
geometries are handled with the help of numerical calculations based on surface
integral equations. The magnetic noise shows a significant polarization
anisotropy above flat wires with finite lateral width, in stark contrast to an
infinitely wide wire. Within the limits of a two-dimensional setting, our
results provide accurate estimates for loss and dephasing rates in so-called
`atom chip traps' based on metallic wires. A simple approximation based on the
incoherent summation of local current elements gives qualitative agreement with
the numerics, but fails to describe current correlations among neighboring
objects.Comment: 10 pages, 9 figures, accepted for publication in J Appl Phys; figures
plotted for slightly smaller structur
Ageing in the critical contact process: a Monte Carlo study
The long-time dynamics of the critical contact process which is brought
suddenly out of an uncorrelated initial state undergoes ageing in close analogy
with quenched magnetic systems. In particular, we show through Monte Carlo
simulations in one and two dimensions and through mean-field theory that
time-translation invariance is broken and that dynamical scaling holds. We find
that the autocorrelation and autoresponse exponents lambda_{Gamma} and lambda_R
are equal but, in contrast to systems relaxing to equilibrium, the ageing
exponents a and b are distinct. A recent proposal to define a non-equilibrium
temperature through the short-time limit of the fluctuation-dissipation ratio
is therefore not applicable.Comment: 18 pages, 7 figures, Latex2e with IOP macros; final for
Autonomous models solvable through the full interval method
The most general exclusion single species one dimensional reaction-diffusion
models with nearest-neighbor interactions which are both autonomous and can be
solved exactly through full interval method are introduced. Using a generating
function method, the general solution for, , the probability that
consecutive sites be full, is obtained. Some other correlation functions of
number operators at nonadjacent sites are also explicitly obtained. It is shown
that for a special choice of initial conditions some correlation functions of
number operators called full intervals remain uncorrelated
Concentration for One and Two Species One-Dimensional Reaction-Diffusion Systems
We look for similarity transformations which yield mappings between different
one-dimensional reaction-diffusion processes. In this way results obtained for
special systems can be generalized to equivalent reaction-diffusion models. The
coagulation (A + A -> A) or the annihilation (A + A -> 0) models can be mapped
onto systems in which both processes are allowed. With the help of the
coagulation-decoagulation model results for some death-decoagulation and
annihilation-creation systems are given. We also find a reaction-diffusion
system which is equivalent to the two species annihilation model (A + B ->0).
Besides we present numerical results of Monte Carlo simulations. An accurate
description of the effects of the reaction rates on the concentration in
one-species diffusion-annihilation model is made. The asymptotic behavior of
the concentration in the two species annihilation system (A + B -> 0) with
symmetric initial conditions is studied.Comment: 20 pages latex, uuencoded figures at the en
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