Refine
Year of publication
- 2001 (160) (remove)
Document Type
- Article (119)
- Doctoral Thesis (26)
- Monograph/Edited Volume (13)
- Habilitation Thesis (2)
Is part of the Bibliography
- yes (160)
Keywords
- chaos (2)
- synchronization (2)
- Absorptionsspektroskopie (1)
- Akkretionsscheiben (1)
- Autokorrelation (1)
- Chaos (1)
- Chaostheorie (1)
- Chaotische Sattel (1)
- Dynamische Systeme (1)
- Effekt (1)
Institute
- Institut für Physik und Astronomie (160) (remove)
Populärwissenschaftlicher Abstract: Bislang gibt es in der beobachtenden optischen Astronomie zwei verschiedene Herangehensweisen: Einerseits werden Objekte durch Kameras abbildend erfaßt, andererseits werden durch die wellenlängenabhängige Zerlegung ihres Lichtes Spektren gewonnen. Das Integral - Field - Verfahren ist eine relativ neue Technik, welche die genannten Beobachtungsmethoden vereint. Das Objektbild im Teleskopfokus wird in räumlich zerlegt und jedes Ortselement einem gemeinsamen Spektrografen zugeführt. Hierdurch wird das Objekt nicht nur zweidimensional räumlich erfaßt, sondern zusätzlich die spektrale Kompenente als dritte Dimension erhalten, weswegen das Verfahren auch als 3D-Methode bezeichnet wird. Anschaulich kann man sich das Datenresultat als eine Abbildung vorstellen, in der jeder einzelne Bildpunkt nicht mehr nur einen Intensitätswert enthält, sondern gleich ein ganzes Spektrum. Diese Technik ermöglicht es, ausgedehnte Objekte im Unterschied zu gängigen Spaltspektrografen komplett zu erfassen. Die besondere Stärke der Methode ist die Möglichkeit, die Hintergrundkontamination der unmittelbaren Umgebung des Objektes zu erfassen und in der Auswertung zu berücksichtigen. Durch diese Fähigkeit erscheint die 3D-Methode prädestiniert für den durch moderne Großteleskope erschlossenen Bereich der extragalaktischen Stellarastronomie. Die detaillierte Untersuchung aufgelöster stellare Populationen in nahegelegenen Galaxien ist erst seit kurzer Zeit dank der Fortschritte mit modernen Grossteleskopen und fortschrittlicher Instrumentierung möglich geworden. Wegen der Bedeutung für die Entstehung und Evolution von Galaxien werden diese Arbeiten zukünftig weiter an Bedeutung gewinnen. In der vorliegenden Arbeit wurde die Integral-Field-Spektroskopie an zwei planetarischen Nebeln in der nächstgelegenen großen Spiralgalaxie M31 (NGC 224) getestet, deren Helligkeiten und Koordinaten aus einer Durchmusterung vorlagen. Hierzu wurden Beobachtungen mit dem MPFS-Instrument am russischen 6m - Teleskop in Selentschuk/Kaukasus sowie mit INTEGRAL/WYFFOS am englischen William-Herschel-Teleskop auf La Palma gewonnen. Ein überraschendes Ergebnis war, daß eins der beiden Objekte falsch klassifiziert wurde. Sowohl die meßbare räumliche Ausdehnung des Objektes als auch das spektrale Erscheinungsbild schlossen die Identität mit einem planetarischen Nebel aus. Mit hoher Wahrscheinlichkeit handelt es sich um einen Supernovaüberrest, zumal im Rahmen der Fehler an gleicher Stelle eine vom Röntgensatelliten ROSAT detektierte Röntgenquelle liegt. Die in diesem Projekt verwendeten Integral-Field-Instrumente wiesen zwei verschiedene Bauweisen auf, die sich miteinander vergleichen ließen. Ein Hauptkritikpunkt der verwendeten Instrumente war ihre geringe Lichtausbeute. Die gesammelten Erfahrung fanden Eingang in das Konzept des derzeit in Potsdam in der Fertigung befindlichen 3D-Instruments PMAS (Potsdamer Multi - Apertur - Spektrophotometer), welcher zunächst für das 3.5m-Teleskop des Calar - Alto - Observatoriums in Südspanien vorgesehen ist. Um die Effizienz dieses Instrumentes zu verbessern, wurde in dieser Arbeit die Kopplung der zum Bildrasterung verwendeten Optik zu den Lichtleitfasern im Labor untersucht. Die Untersuchungen zur Maximierung von Lichtausbeute und Stabilität zeigen, daß sich die Effizienz durch Auswahl einer geeigneten Koppelmethode um etwa 20 Prozent steigern lässt.
Nonlinear multistable systems under the influence of noise exhibit a plethora of interesting dynamical properties. A medium noise level causes hopping between the metastable states. This attractorhopping process is characterized through laminar motion in the vicinity of the attractors and erratic motion taking place on chaotic saddles, which are embedded in the fractal basin boundary. This leads to noise-induced chaos. The investigation of the dissipative standard map showed the phenomenon of preference of attractors through the noise. It means, that some attractors get a larger probability of occurrence than in the noisefree system. For a certain noise level this prefernce achieves a maximum. Other attractors are occur less often. For sufficiently high noise they are completely extinguished. The complexity of the hopping process is examined for a model of two coupled logistic maps employing symbolic dynamics. With the variation of a parameter the topological entropy, which is used together with the Shannon entropy as a measure of complexity, rises sharply at a certain value. This increase is explained by a novel saddle merging bifurcation, which is mediated by a snapback repellor. Scaling laws of the average time spend on one of the formerly disconnected parts and of the fractal dimension of the connected saddle describe this bifurcation in more detail. If a chaotic saddle is embedded in the open neighborhood of the basin of attraction of a metastable state, the required escape energy is lowered. This enhancement of noise-induced escape is demonstrated for the Ikeda map, which models a laser system with time-delayed feedback. The result is gained using the theory of quasipotentials. This effect, as well as the two scaling laws for the saddle merging bifurcation, are of experimental relevance.
One of the rules-of-thumb of colloid and surface physics is that most surfaces are charged when in contact with a solvent, usually water. This is the case, for instance, in charge-stabilized colloidal suspensions, where the surface of the colloidal particles are charged (usually with a charge of hundreds to thousands of e, the elementary charge), monolayers of ionic surfactants sitting at an air-water interface (where the water-loving head groups become charged by releasing counterions), or bilayers containing charged phospholipids (as cell membranes). In this work, we look at some model-systems that, although being a simplified version of reality, are expected to capture some of the physical properties of real charged systems (colloids and electrolytes). We initially study the simple double layer, composed by a charged wall in the presence of its counterions. The charges at the wall are smeared out and the dielectric constant is the same everywhere. The Poisson-Boltzmann (PB) approach gives asymptotically exact counterion density profiles around charged objects in the weak-coupling limit of systems with low-valent counterions, surfaces with low charge density and high temperature (or small Bjerrum length). Using Monte Carlo simulations, we obtain the profiles around the charged wall and compare it with both Poisson-Boltzmann (in the low coupling limit) and the novel strong coupling (SC) theory in the opposite limit of high couplings. In the latter limit, the simulations show that the SC leads in fact to asymptotically correct density profiles. We also compare the Monte Carlo data with previously calculated corrections to the Poisson-Boltzmann theory. We also discuss in detail the methods used to perform the computer simulations. After studying the simple double layer in detail, we introduce a dielectric jump at the charged wall and investigate its effect on the counterion density distribution. As we will show, the Poisson-Boltzmann description of the double layer remains a good approximation at low coupling values, while the strong coupling theory is shown to lead to the correct density profiles close to the wall (and at all couplings). For very large couplings, only systems where the difference between the dielectric constants of the wall and of the solvent is small are shown to be well described by SC. Another experimentally relevant modification to the simple double layer is to make the charges at the plane discrete. The counterions are still assumed to be point-like, but we constraint the distance of approach between ions in the plane and counterions to a minimum distance D. The ratio between D and the distance between neighboring ions in the plane is, as we will see, one of the important quantities in determining the influence of the discrete nature of the charges at the wall over the density profiles. Another parameter that plays an important role, as in the previous case, is the coupling as we will demonstrate, systems with higher coupling are more subject to discretization effects than systems with low coupling parameter. After studying the isolated double layer, we look at the interaction between two double layers. The system is composed by two equally charged walls at distance d, with the counterions confined between them. The charge at the walls is smeared out and the dielectric constant is the same everywhere. Using Monte-Carlo simulations we obtain the inter-plate pressure in the global parameter space, and the pressure is shown to be negative (attraction) at certain conditions. The simulations also show that the equilibrium plate separation (where the pressure changes from attractive to repulsive) exhibits a novel unbinding transition. We compare the Monte Carlo results with the strong-coupling theory, which is shown to describe well the bound states of systems with moderate and high couplings. The regime where the two walls are very close to each other is also shown to be well described by the SC theory. Finally, Using a field-theoretic approach, we derive the exact low-density ("virial") expansion of a binary mixture of positively and negatively charged hard spheres (two-component hard-core plasma, TCPHC). The free energy obtained is valid for systems where the diameters d_+ and d_- and the charge valences q_+ and q_- of positive and negative ions are unconstrained, i.e., the same expression can be used to treat dilute salt solutions (where typically d_+ ~ d_- and q_+ ~ q_-) as well as colloidal suspensions (where the difference in size and valence between macroions and counterions can be very large). We also discuss some applications of our results.
Chemisch dotiertes Polypyrrol gilt als Modellsubstanz für leitfähige Polymere mit nichtdegeneriertem Grundzustand. Das elektrische Transportverhalten in dotiertem Polypyrrol wird durch lokalisierte Ladungsträger, Bipolaronen und Polaronen, bestimmt. Es besteht dabei eine enge gegenseitige Wechselwirkung zwischen der Struktur der Polymerkette und den Eigenschaften der Ladungsträger. Die in dieser Arbeit vorgestellte Kombination von Hochdruckmethodik und optischer Spektroskopie vertieft das Verständnis der Beziehung zwischen der molekularen und supramolekularen Struktur und den elektronischen und optischen Eigenschaften. Durch spezifische Synthesemethoden lassen sich unterschiedliche Strukturen in der polymeren Probe induzieren, die sich durch den Anteil an hochgeordneten Polymerketten unterscheiden. Die gezielte Veränderung dieser Strukturen durch Druckexperimente ermöglicht das Studium des Einflusses der Synthesemethoden auf die Ladungsträgereigenschaften. Für diese Studien wurden herkömmlich synthetisierte Polypyrrol-Filme und Filme, die sich aus Polypyrrol-Nanoröhren zusammensetzen (Synthese in Kernspur-membranen, "Template-Synthese") bei ansonsten gleichen Syntheseparametern untersucht. Raman- und Infrarotspektroskopie sowie UV-Vis-NIR-Absorptionsspektroskopie, die jeweils für die Hochdruckmethodik adaptiert wurden, dienten der Charakterisierung der Proben. Zusätzlich wurden temperatur- und druckabhängige Messungen des elektrischen Widerstands an den Template-Proben durchgeführt. Die Morphologie template-synthetisierter Polypyrrol-Nanoröhren und die filmbildenden Eigenschaften sowie der mögliche Aufbau von Schichtarchitekturen wurden mit transmissions- und rasterelektronenmikroskopischen Techniken untersucht. Die aus den Hochdruckexperimenten gewonnenen Daten werden in der Arbeit im Hinblick auf die Stabilität der Ladungsträger interpretiert. Im Ergebnis bewirkt die Druckerhöhung eine Dissoziation der Bipolaronen in den untersuchten Proben. Das Ladungsträger-gleichgewicht verschiebt sich dadurch mit steigendem Druck zu Zuständen mit höherem Anteil an polaronischen Ladungsträgern. Die Template-Synthese bewirkt gegenüber herkömmlich synthetisierten Proben einen höheren Anteil an Polaronen bereits bei Normaldruck, und eine Lage des Systems näher bei einem Isolator-Metall-Übergang. Die Dissoziationsrate der Bipolaronen ist für Template- und herkömmlich synthetisierte Proben vergleichbar groß und unabhängig vom Initialzustand nach der Synthese. Dieses Verhalten der Ladungsträger wird weitergehend im Rahmen eines Modells untersucht, bei dem der Einfluß benachbarter Polymerketten und der Dotandionen berücksichtigt wird. Dementsprechend können sich die Wellenfunktionen der Ladungsträger unter bestimmten Bedingungen auch auf benachbarte Ketten erstrecken (transversale Polaronen bzw. Bipolaronen). Eine solche Ausdehnung der Wellenfunktionen unter Mitwirkung der Dotandionen wurde in den untersuchten Proben nicht festgestellt. Die Wellenfunktionen der Ladungsträger besitzen demnach hauptsächlich Komponenten entlang der Polymerkette (longitudinale Polaronen bzw. Bipolaronen). Aus der Änderungsrate druckabhängiger spektraler Charakteristiken lassen sich Aussagen über den Ordnungszustand der Probe ableiten. Diese auf experimentellem Wege gefundenen Ergebnisse liefern somit Hinweise für die bisher kontrovers diskutierte Koexistenz der beiden Ladungsträgerarten Polaronen und Bipolaronen und die Größe ihrer jeweiligen Bindungsenergien. Druckerhöhung und Template-Synthese bewirken analoge Änderungen der Polymerstruktur. Sowohl höherer Druck wie auch die Template-Synthese lassen sich mit einem höheren Ordnungsgrad in den Template-Proben korrelieren. Der Ladungstransport in den Proben kann durch ein Mott Variable Range Hopping-Modell mit druckabhängiger charakteristischer Dimension beschrieben werden. Die Erhöhung des Drucks bewirkt einen Anstieg der Dimension, eine bessere Überlappung der Wellenfunktionen der Ladungsträger und eine Vergrößerung der Lokalisierungslänge der Ladungsträger. Die druckinduzierte Dissoziation der Bipolaronen beeinflußt den Ladungstransport zusätzlich durch Erhöhung der Anzahl unabhängiger Ladungsträger und verbessert diesen aufgrund stärkerer Überlappung der Wellenfunktionen. Template-Proben niedriger Synthesetemperatur zeigen bei Normaldruck eine höhere Dimension des Mott Variable Range Hoppings und eine größere Lokalisierungslänge gegenüber bei Raumtemperatur synthetisierten Proben. Kürzere Synthesezeiten bewirken einen Anstieg der Dimension bei Normaldruck und eine Verschiebung des Dimensionscrossovers zu niedrigeren Temperaturen. Template-Proben kurzer Synthesezeit zeigen geringere druckinduzierte Änderungen als solche mit langer Synthesezeit. Es wurde ein kontinuierliches Ordnungsmodell der Polypyrrol-Nanoröhren entwickelt, das dieses Verhalten beschreibt. Die Morphologie und die mechanischen Eigenschaften der Nanoröhren werden durch spezifische Syntheseparameter, wie Temperatur und Dauer, beeinflußt und können mit Transmissions- und Rasterelektronenmikroskopie beobachtet werden. Die filmbildenden Eigenschaften der Röhren hängen stark von diesen mechanischen Eigenschaften ab. Die Struktur der Filme kann dabei von einer unregelmäßigen Anordnung der Röhren bis zu nahezu parallel ausgerichteten Röhren variieren. Es wurden Möglichkeiten untersucht, die Röhren in den Filmen zu orientieren und aus diesen Filmen durch Schichtung makroskopische Architekturen mit einem hohen Grad an orientierten Röhren aufzubauen. Solche Architekturen können für verschiedene Anwendungen, z.B. in elektronischen Bauteilen oder mikroskopischen Bioreaktoren, von Interesse sein.
Line driven winds are accelerated by the momentum transfer from photons to a plasma, by absorption and scattering in numerous spectral lines. Line driving is most efficient for ultraviolet radiation, and at plasma temperatures from 10^4 K to 10^5 K. Astronomical objects which show line driven winds include stars of spectral type O, B, and A, Wolf-Rayet stars, and accretion disks over a wide range of scales, from disks in young stellar objects and cataclysmic variables to quasar disks. It is not yet possible to solve the full wind problem numerically, and treat the combined hydrodynamics, radiative transfer, and statistical equilibrium of these flows. The emphasis in the present writing is on wind hydrodynamics, with severe simplifications in the other two areas. I consider three topics in some detail, for reasons of personal involvement. 1. Wind instability, as caused by Doppler de-shadowing of gas parcels. The instability causes the wind gas to be compressed into dense shells enclosed by strong shocks. Fast clouds occur in the space between shells, and collide with the latter. This leads to X-ray flashes which may explain the observed X-ray emission from hot stars. 2. Wind runaway, as caused by a new type of radiative waves. The runaway may explain why observed line driven winds adopt fast, critical solutions instead of shallow (or breeze) solutions. Under certain conditions the wind settles on overloaded solutions, which show a broad deceleration region and kinks in their velocity law. 3. Magnetized winds, as launched from accretion disks around stars or in active galactic nuclei. Line driving is assisted by centrifugal forces along co-rotating poloidal magnetic field lines, and by Lorentz forces due to toroidal field gradients. A vortex sheet starting at the inner disk rim can lead to highly enhanced mass loss rates.
One of the classical ways to describe the dynamics of nonlinear systems is to analyze theur Fourier spectra. For periodic and quasiperiodic processes the Fourier spectrum consists purely of discrete delta-functions. On the contrary, the spectrum of a chaotic motion is marked by the presence of the continuous component. In this work, we describe the peculiar, neither regular nor completely chaotic state with so called singular-continuous power spectrum. Our investigations concern various cases from most different fields, where one meets the singular continuous (fractal) spectra. The examples include both the physical processes which can be reduced to iterated discrete mappings or even symbolic sequences, and the processes whose description is based on the ordinary or partial differential equations.
Electrically conducting fluids in motion can act as self-excited dynamos. The magnetic fields of celestial bodies like the Earth and the Sun are generated by such dynamos. Their theory aims at modeling and understanding both the kinematic and dynamic aspects of the underlying processes. Kinematic dynamo models, in which for a prescribed flow the linear induction equation is solved and growth rates of the magnetic field are calculated, have been studied for many decades. But in order to get consistent models and to take into account the back-reaction of the magnetic field on the fluid motion, the full nonlinear system of the magnetohydrodynamic (MHD) equations has to be studied. It is generally accepted that these equations, i.e. the Navier-Stokes equation (NSE) and the induction equation, provide a theoretical basis for the explanation of the dynamo effect. The general idea is that mechanical energy pumped into the fluid by heating or other mechanisms is transferred to the magnetic field by nonlinear interactions. For two special helical flows which are known to be effective kinematic dynamos and which can be produced by appropriate external mechanical forcing, we review the nonlinear dynamo properties found in the framework of the full MHD equations. Specifically, we deal with the ABC flow (named after Arnold, Beltrami and Childress) and the Roberts flow (after G.~O. Roberts). The appearance of generic dynamo effects is demonstrated. Applying special numerical bifurcation-analysis techniques to high-dimensional approximations in Fourier space and varying the Reynolds number (or the strength of the forcing) as the relevant control parameter, qualitative changes in the dynamics are investigated. We follow the bifurcation sequences until chaotic states are reached. The transitions from the primary flows with vanishing magnetic field to dynamo-active states are described in particular detail. In these processes the stagnation points of the flows and their heteroclinic connections play a promoting role for the magnetic field generation. By the example of the Roberts flow we demonstrate how the break up of the heteroclinic lines after the primary bifurcation leads to a complicated intersection of stable and unstable manifolds forming a chaotic web which is in turn correlated with the spatial appearance of the dynamo.
We study the dynamo properties of asymmetric square patterns in Boussinesq Rayleigh-B'enard convection in a plane horizontal layer. Cases without rotation and with weak rotation about a vertical axis are considered. There exist different types of solutions distinguished by their symmetry, among them such with flows possessing a net helicity and being capable of kinematic dynamo action in the presence as well as in the absence of rotation. In the nonrotating case these flows are, however, always only kinematic, not nonlinear dynamos. Nonlinearly the back-reaction of the magnetic field then forces the solution into the basin of attraction of a roll pattern incapable of dynamo action. But with rotation added parameter regions are found where the Coriolis force counteracts the Lorentz force in such a way that the asymmetric squares are also nonlinear dynamos.
Using the incompressible magnetohydrodynamic equations, we have numerically studied the dynamo effect in electrically conducting fluids. The necessary energy input into the system was modeled either by an explicit forcing term in the Navier-Stokes equation or fully selfconsistently by thermal convection in a fluid layer heated from below. If the fluid motion is capable of dynamo action, the dynamo effect appears in the form of a phase transition or bifurcation at some critical strength of the forcing. Both the dynamo bifurcation and subsequent bifurcations that occur when the strength of the forcing is further raised were studied, including the transition to chaotic states. Special attention was paid to the helicity of the flow as well as to the symmetries of the system and symmetry breaking in the bifurcations. The magnetic field tends to be accumulated in special regions of the flow, notably in the vicinity of stagnation points or near the boundaries of convection cells.
We numerically investigate nonlinear asymmetric square patterns in a horizontal convection layer with up-down reflection symmetry. As a novel feature we find the patterns to appear via the skewed varicose instability of rolls. The time-independent nonlinear state is generated by two unstable checkerboard (symmetric square) patterns and their nonlinear interaction. As the bouyancy forces increase the interacting modes give rise to bifurcations leading to a periodic alternation between a nonequilateral hexagonal pattern and the square pattern or to different kinds of standing oscillations.
We investigate the notion of Bose-Einstein condensation of interacting particles. The definition of the condensate is based on the existence of the dominant eigenvalue of the single-particle density matrix. The statistical properies and the characteristics temperature are computed exactly in the soluble models of two interacting atoms.
A transport theory for atomic matter waves in low-dimensional waveguides is outlined. The thermal fluctuation spectrum of magnetic near fields leaking out of metallic microstructures is estimated. The corresponding scattering rate for paramagnetic atoms turns out to be quite large in micrometer-sized waveguides (approx. 100/s). Analytical estimate for the heating and decoherence of a cold atom cloud are given. We finally discuss numerical and analytical results for the scattering from static potential imperfections and the ensuing spatial diffusion process.
Coherent transport
(2001)
We discuss the transport of matter waves in low-dimensional waveguides. Due to scattering from uncontrollable noise fields, the spatial coherence gets reduced and eventually lost. We develop a description of this decoherence process in terms of transport equations for the atomic Wigner function. We outline its derivation and discuss the special case of white noise where an analytical solution can be found.
Coherence properties of thermal near fields : implications for nanometer-scale radiative transfer
(2001)
With the recent development of local (optical and thermal) probe microscopy and the advent of nanotechnology, it seems necessary to revisit the old subject of coherence properies of thermal sources of light. Concerning temporal coherence, we show that thermal sources may produce quasi-monochromatic near fields. In light of this result, the possibility of perfoming near-field solid-state spectroscopy and of designing near-field infrared sources is discussed. The problem of radiative transfer between two thermal sources held at subwavelength distance is studied. The radiative flux may be enhanced by several orders of magnitude due to the excitation of resonant surface waves, and this may occur at particular frequencies. Finally, we study the spatial coherence of thermal sources and the substantial influence of the near field. Surface waves may induce long-range spatial correlation, on a scale much larger than the wavelength. Conversely, quasi-static contributions, as well as skin-layer currents, induce arbitary small correlations. With the recent development of local (optical and thermal) probe microscopy and the advent of nanotechnology, it seems necessary to revisit the old subject of coherence of thermal fields.
A series of new poly-1,3,4-oxadiazoles has been synthesized by polycondensation reaction of hydrazine sulfate with a mixture of a dicarboxylic acid containing unsaturated bonds and a dicarboxylic acid containing silicon, by using methanesulfonic acid/phosphorus pentoxide as a reaction medium. These polymers were highly thermostable but they were only soluble in strong inorganic acids such as sulfuric or methanesulfonic ones. An alternative way was followed by using the corresponding dihydrazides containing unsaturated bonds and the corresponding diacid chloride containing silicon that reacted in N-methylpyrrolidinone (NMP) to give soluble silicon-containing unsaturated polyhydrazides, which were cyclodehydrated either by thermal or chemical treatment to give the corresponding polyoxadiazoles. Very thin coatings of polyhydrazides and polyoxadiazoles were deposited onto silicon wafers and they showed a very smooth surface, free of pinholes, when studied by atomic force microscopy (AFM). Some polyoxadiazole films showed strong blue photoluminescence.
In the roughly 20 years of its existence as an observational science, gravitational lensing has established itself as a valuable tool in many astrophysical fields. In the introduction of this review we briefly present the basics of lensing. Then it is shown that the two propagation effects, lensing and scintillation, have a number of properties in common. In the main part various lensing phenomena are discussed with emphasis on recent observations.
So far, the evolution of post-AGB stars is not fully understood. In particular the formation of hydrogen- deficient and hydrogen-free Central Stars of Planetary Nebulae (CSPN) is unsettled. New evolution models, which allow for the consistent treatment of the physics of late thermal pulses, promise new insights to the formation of these stars. In this paper we summarize the results of non-LTE analyses of CSPN with wind. By comparing these results with the predictions of the new evolution models, open questions concerning the evolution of the stars might be answered. In addition we discuss the driving mechanism of the winds of Wolf-Rayet CSPN. New models, which account for millions of iron lines, support the assumptions that these winds are driven by radiation.
We propose a technique to calculate large-scale dimension densities in both higher-dimensional spatio-temporal systems and low-dimensional systems from only a few data points, where known methods usually have an unsatisfactory scaling behavior. This is mainly due to boundary and finite size effects. With our rather simple method we normalize boundary effects and get a significant correction of the dimension estimate. This straightforward approach is basing on rather general assumptions. So even weak coherent structures obtained from small spatial couplings can be detected with this method, what is impossible by using the Lyapunov-dimension density. We demonstrate the efficiency of our technique for coupled logistic maps, coupled tent maps, the Lorenz-attractor and the Roessler-attractor.
We present a measure of quantum entanglement which is capable of quantifying the degree of entanglement of a multi-partite quantum system. This measure, which is based on a generalization of the Schmidt rank of a pure state, is defined on the full state space and is shown to be an entanglement monotone, that is, it cannot increase under local quantum operations with classical communication and under mixing. For a large class of mixed states this measure of entanglement can be calculated exactly, and it provides a detailed classification of mixed states.
We have recently reported the phenomenon of doubly stochastic resonance [Phys. Rev. Lett. 85, 227 (2000)], a synthesis of noise-induced transition and stochastic resonance. The essential feature of this phenomenon is that multiplicative noise induces a bimodality and additive noise causes stochastic resonance behavior in the induced structure. In the present paper we outline possible applications of this effect and design a simple lattice of electronic circuits for the experimental realization of doubly stochastic resonance.
We have obtained time-resolved observations of line-profile variations of the two Wolf-Rayet stars WR 135 and WR 111. The spectra, taken during two consecutive nights, cover a broad range from 4470 to 6590 Ang. The profile variability of the C iii emission line at 5696 Ang in WR 135 is shown in detail. The principal difficulties to constrain the velocity law from the frequency drift of discrete spectral features is discussed, emphasizing the crucial dependence on the adopted location of the line-emission region, and the possible necessity to distinguish between the motion of structures and the flow of the matter. - Full access to the observational data is provided via anonymous file transfer.