1,579 research outputs found
A theoretical explanation for the Central Molecular Zone asymmetry
It has been known for more than thirty years that the distribution of
molecular gas in the innermost 300 parsecs of the Milky Way, the Central
Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of
molecular emission comes from positive longitudes, and only one quarter from
negative longitudes. However, despite much theoretical effort, the origin of
this asymmetry has remained a mystery. Here we show that the asymmetry can be
neatly explained by unsteady flow of gas in a barred potential. We use
high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art
chemical network. Despite the initial conditions and the bar potential being
point-symmetric with respect to the Galactic Centre, asymmetries develop
spontaneously due to the combination of a hydrodynamical instability known as
the "wiggle instability" and the thermal instability. The observed asymmetry
must be transient: observations made tens of megayears in the past or in the
future would often show an asymmetry in the opposite sense. Fluctuations of
amplitude comparable to the observed asymmetry occur for a large fraction of
the time in our simulations, and suggest that the present is not an exceptional
moment in the life of our Galaxy.Comment: Accepted for publication in MNRAS. Videos of the simulations are
available at http://www.ita.uni-heidelberg.de/~mattia/download.htm
Mutual information between geomagnetic indices and the solar wind as seen by WIND : implications for propagation time estimates
The determination of delay times of solar wind conditions at the sunward libration point to effects on Earth is investigated using mutual information. This measures the amount of information shared between two timeseries. We consider the mutual information content of solar wind observations, from WIND, and the geomagnetic indices. The success of five commonly used schemes for estimating interplanetary propagation times is examined. Propagation assuming a fixed plane normal at 45 degrees to the GSE x-axis (i.e. the Parker Spiral estimate) is found to give optimal mutual information. The mutual information depends on the point in space chosen as the target for the propagation estimate, and we find that it is maximized by choosing a point in the nightside rather than dayside magnetosphere. In addition, we employ recurrence plot analysis to visualize contributions to the mutual information, this suggests that it appears on timescales of hours rather than minutes
Non-homogeneous random walks, subdiffusive migration of cells and anomalous chemotaxis
This paper is concerned with a non-homogeneous in space and non-local in time
random walk model for anomalous subdiffusive transport of cells. Starting with
a Markov model involving a structured probability density function, we derive
the non-local in time master equation and fractional equation for the
probability of cell position. We show the structural instability of fractional
subdiffusive equation with respect to the partial variations of anomalous
exponent. We find the criteria under which the anomalous aggregation of cells
takes place in the semi-infinite domain.Comment: 18 pages, accepted for publicatio
Granulomatous Reactivation during the Course of a Leprosy Infection: Reaction or Relapse
Leprosy is a serious infectious disease whose treatment still poses some challenges. Patients are usually treated with a combination of antimicrobial drugs called multidrug therapy. Although this treatment is effective against Mycobacterium leprae, the bacillus that causes leprosy, patients may develop severe inflammatory reactions during treatment. These reactions may be either attributed to an improvement in the immunological reactivity of the patient along with the treatment, or to relapse of the disease due to the proliferation of remaining bacilli. In certain patients these two conditions may be difficult to differentiate. The present study addresses the histopathology picture of and the M. leprae bacilli in sequential biopsies taken from lesions of patients who presented such reactions aiming to improve the differentiation of the two conditions. This is important because these reactions are one of the major causes of the disabilities of the patients with leprosy, and should be treated early and appropriately. Our results show that the histopathology picture alone is not sufficient, and that bacilli's counting is necessary
Optical properties and charge-transfer excitations in edge-functionalized all-graphene nanojunctions
We investigate the optical properties of edge-functionalized graphene
nanosystems, focusing on the formation of junctions and charge transfer
excitons. We consider a class of graphene structures which combine the main
electronic features of graphene with the wide tunability of large polycyclic
aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we
show that, upon convenient choice of functional groups, low energy excitations
with remarkable charge transfer character and large oscillator strength are
obtained. These properties can be further modulated through an appropriate
width variation, thus spanning a wide range in the low-energy region of the
UV-Vis spectra. Our results are relevant in view of designing all-graphene
optoelectronic nanodevices, which take advantage of the versatility of
molecular functionalization, together with the stability and the electronic
properties of graphene nanostructures.Comment: J. Phys. Chem. Lett. (2011), in pres
Electrically tunable GHz oscillations in doped GaAs-AlAs superlattices
Tunable oscillatory modes of electric-field domains in doped semiconductor
superlattices are reported. The experimental investigations demonstrate the
realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the
temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes
is determined using an analytical and a numerical modeling of the dynamics of
domain formation. Three different oscillatory modes are found. Their presence
depends on the actual shape of the drift velocity curve, the doping density,
the boundary condition, and the length of the superlattice. For most bias
regions, the self-sustained oscillations are due to the formation, motion, and
recycling of the domain boundary inside the superlattice. For some biases, the
strengths of the low and high field domain change periodically in time with the
domain boundary being pinned within a few quantum wells. The dependency of the
frequency on the coupling leads to the prediction of a new type of tunable GHz
oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure
Auger Recombination in Semiconductor Quantum Wells
The principal mechanisms of Auger recombination of nonequilibrium carriers in
semiconductor heterostructures with quantum wells are investigated. It is shown
for the first time that there exist three fundamentally different Auger
recombination mechanisms of (i) thresholdless, (ii) quasi-threshold, and (iii)
threshold types. The rate of the thresholdless Auger process depends on
temperature only slightly. The rate of the quasi-threshold Auger process
depends on temperature exponentially. However, its threshold energy essentially
varies with quantum well width and is close to zero for narrow quantum wells.
It is shown that the thresholdless and the quasi-threshold Auger processes
dominate in narrow quantum wells, while the threshold and the quasi-threshold
processes prevail in wide quantum wells. The limiting case of a
three-dimensional (3D)Auger process is reached for infinitely wide quantum
wells. The critical quantum well width is found at which the quasi-threshold
and threshold Auger processes merge into a single 3D Auger process. Also
studied is phonon-assisted Auger recombination in quantum wells. It is shown
that for narrow quantum wells the act of phonon emission becomes resonant,
which in turn increases substantially the coefficient of phonon-assisted Auger
recombination. Conditions are found under which the direct Auger process
dominates over the phonon-assisted Auger recombination at various temperatures
and quantum well widths.Comment: 38 pages, 7 figure
Modular Robotic Vehicle
A modular robotic vehicle includes a chassis, driver input devices, an energy storage system (ESS), a power electronics module (PEM), modular electronic assemblies (eModules) connected to the ESS via the PEM, one or more master controllers, and various embedded controllers. Each eModule includes a drive wheel containing a propulsion-braking module, and a housing containing propulsion and braking control assemblies with respective embedded propulsion and brake controllers, and a mounting bracket covering a steering control assembly with embedded steering controllers. The master controller, which is in communication with each eModule and with the driver input devices, communicates with and independently controls each eModule, by-wire, via the embedded controllers to establish a desired operating mode. Modes may include a two-wheel, four-wheel, diamond, and omni-directional steering modes as well as a park mode. A bumper may enable docking with another vehicle, with shared control over the eModules of the vehicles
Optical Excitations and Field Enhancement in Short Graphene Nanoribbons
The optical excitations of elongated graphene nanoflakes of finite length are
investigated theoretically through quantum chemistry semi-empirical approaches.
The spectra and the resulting dipole fields are analyzed, accounting in full
atomistic details for quantum confinement effects, which are crucial in the
nanoscale regime. We find that the optical spectra of these nanostructures are
dominated at low energy by excitations with strong intensity, comprised of
characteristic coherent combinations of a few single-particle transitions with
comparable weight. They give rise to stationary collective oscillations of the
photoexcited carrier density extending throughout the flake, and to a strong
dipole and field enhancement. This behavior is robust with respect to width and
length variations, thus ensuring tunability in a large frequency range. The
implications for nanoantennas and other nanoplasmonic applications are
discussed for realistic geometries
Scaling up the effects of inbreeding depression from individuals to metapopulations
Abstract Inbreeding is common in nature, and many laboratory studies have documented that inbreeding depression can reduce the fitness of individuals. Demonstrating the consequences of inbreeding depression on the growth and persistence of populations is more challenging because populations are often regulated by density- or frequency-dependent selection and influenced by demographic and environmental stochasticity. A few empirical studies have shown that inbreeding depression can increase extinction risk of local populations. The importance of inbreeding depression at the metapopulation level has been conjectured based on population-level studies but has not been evaluated. We quantified the impact of inbreeding depression affecting the fitness of individuals on metapopulation persistence in heterogeneous habitat networks of different sizes and habitat configuration in a context of natural butterfly metapopulations. We developed a spatial individual-based simulation model of metapopulations with explicit genetics. We used Approximate Bayesian Computation to fit the model to extensive demographic, genetic, and life-history data available for the well-studied Glanville fritillary butterfly (Melitaea cinxia) metapopulations in the Åland islands in SW Finland. We compared 18 semi-independent habitat networks differing in size and fragmentation. The results show that inbreeding is more frequent in small habitat networks, and consequently, inbreeding depression elevates extinction risks in small metapopulations. Metapopulation persistence and neutral genetic diversity maintained in the metapopulations increase with the total habitat amount in and mean patch size of habitat networks. Dispersal and mating behavior interact with landscape structure to determine how likely it is to encounter kin while looking for mates. Inbreeding depression can decrease the viability of small metapopulations even when they are strongly influenced by stochastic extinction-colonization dynamics and density-dependent selection. The findings from this study support that genetic factors, in addition to demographic factors, can contribute to extinctions of small local populations and also of metapopulations. This article is protected by copyright. All rights reserved.Peer reviewe
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