2,000 research outputs found
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
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
Unveiling shocks in planetary nebulae
The propagation of a shock wave into a medium is expected to heat the
material beyond the shock, producing noticeable effects in intensity line
ratios such as [O III]/Halpha. To investigate the occurrence of shocks in
planetary nebulae (PNe), we have used all narrowband [O III] and Halpha images
of PNe available in the HST archive to build their [O III]/Halpha ratio maps
and to search for regions where this ratio is enhanced. Regions with enhanced
[O III]/Halpha emission ratio can be ascribed to two different types of
morphological structures: bow-shock structures produced by fast collimated
outflows and thin skins enveloping expanding nebular shells. Both collimated
outflows and expanding shells are therefore confirmed to generate shocks in
PNe. We also find regions with depressed values of the [O III]/Halpha ratio
which are found mostly around density bounded PNe, where the local contribution
of [N II] emission into the F656N Halpha filter cannot be neglected.Comment: 13 pages, 9 figures, 3 tables; To appear in Astronomy & Astrophysic
The complex structure of HH 110 as revealed from Integral Field Spectroscopy
HH 110 is a rather peculiar Herbig-Haro object in Orion that originates due
to the deflection of another jet (HH 270) by a dense molecular clump, instead
of being directly ejected from a young stellar object. Here we present new
results on the kinematics and physical conditions of HH 110 based on Integral
Field Spectroscopy. The 3D spectral data cover the whole outflow extent (~4.5
arcmin, ~0.6 pc at a distance of 460 pc) in the spectral range 6500-7000 \AA.
We built emission-line intensity maps of H, [NII] and [SII] and of
their radial velocity channels. Furthermore, we analysed the spatial
distribution of the excitation and electron density from [NII]/H,
[SII]/H, and [SII] 6716/6731 integrated line-ratio maps, as well as
their behaviour as a function of velocity, from line-ratio channel maps. Our
results fully reproduce the morphology and kinematics obtained from previous
imaging and long-slit data. In addition, the IFS data revealed, for the first
time, the complex spatial distribution of the physical conditions (excitation
and density) in the whole jet, and their behaviour as a function of the
kinematics. The results here derived give further support to the more recent
model simulations that involve deflection of a pulsed jet propagating in an
inhomogeneous ambient medium. The IFS data give richer information than that
provided by current model simulations or laboratory jet experiments. Hence,
they could provide valuable clues to constrain the space parameters in future
theoretical works.Comment: 12 pages, 15 figures Accepted in MNRA
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
Trapped surfaces and emergent curved space in the Bose-Hubbard model
A Bose-Hubbard model on a dynamical lattice was introduced in previous work as a spin system analogue of emergent geometry and gravity. Graphs with regions of high connectivity in the lattice were identified as candidate analogues of spacetime geometries that contain trapped surfaces. We carry out a detailed study of these systems and show explicitly that the highly connected subgraphs trap matter. We do this by solving the model in the limit of no back-reaction of the matter on the lattice, and for states with certain symmetries that are natural for our problem. We find that in this case the problem reduces to a one-dimensional Hubbard model on a lattice with variable vertex degree and multiple edges between the same two vertices. In addition, we obtain a (discrete) differential equation for the evolution of the probability density of particles which is closed in the classical regime. This is a wave equation in which the vertex degree is related to the local speed of propagation of probability. This allows an interpretation of the probability density of particles similar to that in analogue gravity systems: matter inside this analogue system sees a curved spacetime. We verify our analytic results by numerical simulations. Finally, we analyze the dependence of localization on a gradual, rather than abrupt, falloff of the vertex degree on the boundary of the highly connected region and find that matter is localized in and around that region
Absolute Maximal Entanglement and Quantum Secret Sharing
We study the existence of absolutely maximally entangled (AME) states in
quantum mechanics and its applications to quantum information. AME states are
characterized by being maximally entangled for all bipartitions of the system
and exhibit genuine multipartite entanglement. With such states, we present a
novel parallel teleportation protocol which teleports multiple quantum states
between groups of senders and receivers. The notable features of this protocol
are that (i) the partition into senders and receivers can be chosen after the
state has been distributed, and (ii) one group has to perform joint quantum
operations while the parties of the other group only have to act locally on
their system. We also prove the equivalence between pure state quantum secret
sharing schemes and AME states with an even number of parties. This equivalence
implies the existence of AME states for an arbitrary number of parties based on
known results about the existence of quantum secret sharing schemes.Comment: 5 pages, 2 figure
Modelling nitrogen and phosphorus loads in a Mediterranean river catchment (La Tordera, NE Spain)
Human activities have resulted in increased nutrient levels in many rivers all over Europe. Sustainable management of river basins demands an assessment of the causes and consequences of human alteration of nutrient flows, together with an evaluation of management options. In the context of an integrated and interdisciplinary environmental assessment (IEA) of nutrient flows, we present and discuss the application of the nutrient emission model MONERIS (MOdelling Nutrient Emissions into River Systems) to the Catalan river basin, La Tordera (north-east Spain), for the period 1996-2002. After a successful calibration and verification process (Nash-Sutcliffe efficiencies E=0.85 for phosphorus and E=0.86 for nitrogen), the application of the model MONERIS proved to be useful in estimating nutrient loads. Crucial for model calibration, in-stream retention was estimated to be about 50 % of nutrient emissions on an annual basis. Through this process, we identified the importance of point sources for phosphorus emissions (about 94% for 1996-2002), and diffuse sources, especially inputs via groundwater, for nitrogen emissions (about 31% for 1996-2002). Despite hurdles related to model structure, observed loads, and input data encountered during the modelling process, MONERIS provided a good representation of the major interannual and spatial patterns in nutrient emissions. An analysis of the model uncertainty and sensitivity to input data indicates that the model MONERIS, even in data-starved Mediterranean catchments, may be profitably used by water managers for evaluating quantitative nutrient emission scenarios for the purpose of managing river basins. As an example of scenario modelling, an analysis of the changes in nutrient emissions through two different future scenarios allowed the identification of a set of relevant measures to reduce nutrient loads
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