2,518 research outputs found
Hydro-dynamical models for the chaotic dripping faucet
We give a hydrodynamical explanation for the chaotic behaviour of a dripping
faucet using the results of the stability analysis of a static pendant drop and
a proper orthogonal decomposition (POD) of the complete dynamics. We find that
the only relevant modes are the two classical normal forms associated with a
Saddle-Node-Andronov bifurcation and a Shilnikov homoclinic bifurcation. This
allows us to construct a hierarchy of reduced order models including maps and
ordinary differential equations which are able to qualitatively explain prior
experiments and numerical simulations of the governing partial differential
equations and provide an explanation for the complexity in dripping. We also
provide a new mechanical analogue for the dripping faucet and a simple
rationale for the transition from dripping to jetting modes in the flow from a
faucet.Comment: 16 pages, 14 figures. Under review for Journal of Fluid Mechanic
Exact corrections for finite-time drift and diffusion coefficients
Real data are constrained to finite sampling rates, which calls for a
suitable mathematical description of the corrections to the finite-time
estimations of the dynamic equations. Often in the literature, lower order
discrete time approximations of the modeling diffusion processes are
considered. On the other hand, there is a lack of simple estimating procedures
based on higher order approximations. For standard diffusion models, that
include additive and multiplicative noise components, we obtain the exact
corrections to the empirical finite-time drift and diffusion coefficients,
based on It\^o-Taylor expansions. These results allow to reconstruct the real
hidden coefficients from the empirical estimates. We also derive higher-order
finite-time expressions for the third and fourth conditional moments, that
furnish extra theoretical checks for that class of diffusive models. The
theoretical predictions are compared with the numerical outcomes of some
representative artificial time-series.Comment: 18 pages, 5 figure
Shock excitation of the knots of Hen 3-1475
We present new optical STIS HST spectroscopic observations of the jets of the
proto-planetary nebula Hen 3-1475. The excitation conditions of the knots of
Hen 3-1475 are derived from the observed optical spectra, confirming that the
knots are shock excited. The shocked spectra are qualitatively reproduced by
simple ``3/2''D bow shock models. We present a set of bow shock models devoted
to planetary nebulae, and discuss the effects of the pre-ionization conditions,
the bow shock velocity, the bow shock shape and the chemical abundances on the
predicted spectra.
To explore the reliability of the ``3/2''D bow shock models, we also compare
the observed spectra of other three proto-planetary nebulae (M 1-92, M 2-56 and
CRL 618) to the predicted spectra.Comment: 13 pages. A&A (in press
Spin dynamics of the spin-Peierls compound CuGeO_3 under magnetic field
The magnetic field--driven transition in the spin-Peierls system CuGeO_3
associated with the closing of the spin gap is investigated numerically. The
field dependence of the spin dynamical structure factor (seen by inelastic
neutron scattering) and of the momentum dependent static susceptibility are
calculated. In the dimerized phase (H<H_c), we suggest that the strong field
dependence of the transverse susceptibility could be experimentally seen from
the low temperature spin-echo relaxation rate 1/T_{2G} or the second moment of
the NMR spectrum. Above H_c low energy spin excitations appear at
incommensurate wave vectors where the longitudinal susceptibility chi_{zz}(q)
peaks.Comment: 4 pages, LaTeX, postscript figures include
Locality of temperature
This work is concerned with thermal quantum states of Hamiltonians on spin
and fermionic lattice systems with short range interactions. We provide results
leading to a local definition of temperature, thereby extending the notion of
"intensivity of temperature" to interacting quantum models. More precisely, we
derive a perturbation formula for thermal states. The influence of the
perturbation is exactly given in terms of a generalized covariance. For this
covariance, we prove exponential clustering of correlations above a universal
critical temperature that upper bounds physical critical temperatures such as
the Curie temperature. As a corollary, we obtain that above the critical
temperature, thermal states are stable against distant Hamiltonian
perturbations. Moreover, our results imply that above the critical temperature,
local expectation values can be approximated efficiently in the error and the
system size.Comment: 11 pages + 6 pages appendix, 6 figures; proof of the clustering
theorem corrected, improved presentatio
Influence of the anion potential on the charge ordering in quasi-one dimensional charge transfer salts
We examine the various instabilities of quarter-filled strongly correlated
electronic chains in the presence of a coupling to the underlying lattice. To
mimic the physics of the (TMTTF)X Bechgaard-Fabre salts we also include
electrostatic effects of intercalated anions. We show that small displacements
of the anion can stabilize new mixed Charged Density Wave-Bond Order Wave
phases in which central symmetry centers are suppressed. This finding is
discussed in the context of recent experiments. We suggest that the recently
observed charge ordering is due to a cooperative effect between the Coulomb
interaction and the coupling of the electronic stacks to the anions. On the
other hand, the Spin-Peierls instability at lower temperature requires a
Peierls-like lattice coupling.Comment: Latex, 4 pages, 4 postscript figure
Time-dependent ejection velocity model for the outflow of Hen 3--1475
We present 2D axisymmetric and 3D numerical simulations of the
proto-planetary nebula Hen 3-1475, which is characterized by a remarkably
highly collimated optical jet, formed by a string of shock-excited knots along
the axis of the nebula. It has recently been suggested that the kinematical and
morphological properties of the Hen 3-1475 jet could be the result of an
ejection variability of the central source (Riera et al. 2003). The
observations suggest a periodic variability of the ejection velocity
superimposed on a smoothly increasing ejection velocity ramp. From our
numerical simulations, we have obtained intensity maps (for different optical
emission lines) and position-velocity diagrams, in order to make a direct
comparison with the HST observations of this object. Our numerical study allows
us to conclude that a model of a precessing jet with a time-dependent ejection
velocity, which is propagating into an ISM previously perturbed by an AGB wind,
can succesfully explain both the morphological and the kinematical
characteristics of this proto-planetary nebula.Comment: Astronomy and Astrophysics (accepted) (8 figures
Hole-Pairs in a Spin Liquid: Influence of Electrostatic Hole-Hole Repulsion
The stability of hole bound states in the t-J model including short-range
Coulomb interactions is analyzed using computational techniques on ladders with
up to sites. For a nearest-neighbors (NN) hole-hole repulsion,
the two-holes bound state is surprisingly robust and breaks only when the
repulsion is several times the exchange . At hole doping the
pairs break only for a NN-repulsion as large as . Pair-pair
correlations remain robust in the regime of hole binding. The results support
electronic hole-pairing mechanisms on ladders based on holes moving in
spin-liquid backgrounds. Implications in two dimensions are also presented. The
need for better estimations of the range and strength of the Coulomb
interaction in copper-oxides is remarked.Comment: Revised version with new figures. 4 pages, 5 figure
3-D Kinematics of the HH 110 jet
We present new results on the kinematics of the jet HH 110. New proper motion
measurements have been calculated from [SII] CCD images obtained with a time
baseline of nearly fifteen years. HH 110 proper motions show a strong asymmetry
with respect to the outflow axis, with a general trend of pointing towards the
west of the axis direction. Spatial velocities have been obtained by combining
the proper motions and radial velocities from Fabry-Perot data. Velocities
decrease by a factor ~3 over a distance of ~10 cm, much shorter than the
distances expected for the braking caused by the jet/environment interaction.
Our results show evidence of an anomalously strong interaction between the
outflow and the surrounding environment, and are compatible with the scenario
in which HH 110 emerges from a deflection in a jet/cloud collision.Comment: (1)Universitat de Barcelona; (2)UNAM; (3)UPC; (4)University of
Hawaii; (5)Southern Astrophysical Research Telescope. 9 pages; 7 Figures
Accepted by A&
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