1,129 research outputs found
Description of Bocchus irwini sp. nov. from Madagascar (Hymenoptera Dryinidae)
Bocchus irwini sp. nov. is described from a male collected in Analagnambe forest,
Mahajanga Province, Madagascar. The new species is similar to Bocchus watshami Olmi 1987.
Keys to the Afrotropical species of Bocchus are modified to include the new species
Collective chaos in pulse-coupled neural networks
We study the dynamics of two symmetrically coupled populations of identical
leaky integrate-and-fire neurons characterized by an excitatory coupling. Upon
varying the coupling strength, we find symmetry-breaking transitions that lead
to the onset of various chimera states as well as to a new regime, where the
two populations are characterized by a different degree of synchronization.
Symmetric collective states of increasing dynamical complexity are also
observed. The computation of the the finite-amplitude Lyapunov exponent allows
us to establish the chaoticity of the (collective) dynamics in a finite region
of the phase plane. The further numerical study of the standard Lyapunov
spectrum reveals the presence of several positive exponents, indicating that
the microscopic dynamics is high-dimensional.Comment: 6 pages, 5 eps figures, to appear on Europhysics Letters in 201
Description of Anteon seramense (Hymenoptera. Dryinidae), a new species from Indonesia
Anteon seramense sp. nov. is described from Seram Island (Indonesia). Seram is an island
situated in a transition area between the Oriental and the Australian regions. Anteon seramense
can be distinguished from the related Australian species A. giluwense Olmi and
A. chelogynoides (Perkins) by the different sculpture of the scutum (granulated in A. seramense,
punctate and unsculptured among punctures in A. giluwense and A. chelogynoides)
and the different distal apex of the protarsal segment 5 (deeply hollow in A. seramense,
not hollow in A. giluwense and A. chelogynoides). Anteon seramense sp. nov. can be distinguished
from the related Oriental species A. heppneri Olmi and A. thai Olmi by the different
shape of the protarsal segment 5 (basal part slightly longer than distal part in A. seramense,
much longer in A. heppneri and A. thai)
Intermittent chaotic chimeras for coupled rotators
Two symmetrically coupled populations of N oscillators with inertia
display chaotic solutions with broken symmetry similar to experimental
observations with mechanical pendula. In particular, we report the first
evidence of intermittent chaotic chimeras, where one population is synchronized
and the other jumps erratically between laminar and turbulent phases. These
states have finite life-times diverging as a power-law with N and m. Lyapunov
analyses reveal chaotic properties in quantitative agreement with theoretical
predictions for globally coupled dissipative systems.Comment: 6 pages, 5 figures SUbmitted to Physical Review E, as Rapid
Communicatio
Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions
In the last decade, the relativistic magnetohydrodynamic (MHD) modelling of
pulsar wind nebulae, and of the Crab nebula in particular, has been highly
successful, with many of the observed dynamical and emission properties
reproduced down to the finest detail. Here, we critically discuss the results
of some of the most recent studies: namely the investigation of the origin of
the radio emitting particles and the quest for the acceleration sites of
particles of different energies along the termination shock, by using wisps
motion as a diagnostic tool; the study of the magnetic dissipation process in
high magnetization nebulae by means of new long-term three-dimensional
simulations of the pulsar wind nebula evolution; the investigation of the
relativistic tearing instability in thinning current sheets, leading to fast
reconnection events that might be at the origin of the Crab nebula gamma-ray
flares.Comment: 30 pages, 12 figure
Reliability Assessment of a Turbogenerator Coil Retaining Ring Based on Low Cycle Fatigue Data
Abstract
Turbogenerator coil retaining rings are shrunk-fitted onto the rotor over the coils, in order to restrain them against the centrifugal force. They are typically subjected to low cycle fatigue, with a cycle being completed at every machine switch-on and switch-off. The subject of this paper consists in the determination of the failure probability of a coil retaining ring. The failure mode of the ring cracking, when it swells in tension, due to the centrifugal force is here considered. The reliability assessment is preceded by the study of the input variables affecting the low-cycle fatigue load and of their stochastic distributions. This question is tackled by the experimental determination of the static, cyclic and fatigue curves of the involved material and by the application of a statistical model to compute related parameters and their standard deviations. Upon the determination of variable distributions, the probability of failure is estimated in the form of a cumulative distribution function by a computationally efficient methodology, based on the Advanced Mean Value approach. The obtained results account for the material response and the local stressstrain states at the most loaded coil retaining ring region. The determined probability at the end of the machine life, in the order of 10-12, is compatible with reference values for structures under fatigue in the mechanical and aeronautical fields
Explosive synchronization in weighted complex networks
The emergence of dynamical abrupt transitions in the macroscopic state of a
system is currently a subject of the utmost interest. Given a set of phase
oscillators networking with a generic wiring of connections and displaying a
generic frequency distribution, we show how combining dynamical local
information on frequency mismatches and global information on the graph
topology suggests a judicious and yet practical weighting procedure which is
able to induce and enhance explosive, irreversible, transitions to
synchronization. We report extensive numerical and analytical evidence of the
validity and scalability of such a procedure for different initial frequency
distributions, for both homogeneous and heterogeneous networks, as well as for
both linear and non linear weighting functions. We furthermore report on the
possibility of parametrically controlling the width and extent of the
hysteretic region of coexistence of the unsynchronized and synchronized states
Relativistic MHD modeling of magnetized neutron stars, pulsar winds, and their nebulae
Neutron stars are among the most fascinating astrophysical sources, being
characterized by strong gravity, densities about the nuclear one or even above,
and huge magnetic fields. Their observational signatures can be extremely
diverse across the electromagnetic spectrum, ranging from the periodic and
low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray
flares of magnetars, where energies of ~10^46 erg are released in a few
seconds. Fast-rotating and highly magnetized neutron stars are expected to
launch powerful relativistic winds, whose interaction with the supernova
remnants gives rise to the non-thermal emission of pulsar wind nebulae, which
are known cosmic accelerators of electrons and positrons up to PeV energies. In
the extreme cases of proto-magnetars (magnetic fields of ~10^15 G and
millisecond periods), a similar mechanism is likely to provide a viable engine
for the still mysterious gamma-ray bursts. The key ingredient in all these
spectacular manifestations of neutron stars is the presence of strong magnetic
fields in their constituent plasma. Here we will present recent updates of a
couple of state-of-the-art numerical investigations by the high-energy
astrophysics group in Arcetri: a comprehensive modeling of the steady-state
axisymmetric structure of rotating magnetized neutron stars in general
relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and
their associated nebulae.Comment: EPS 44th Conference on Plasma Physics (June 2017, Belfast), paper
accepted for publication on Plasma Physics and Controlled Fusio
The structure and dynamics of the dense cores in the Perseus molecular cloud complex
We have produced wide-field (≳ 0.1 deg^2) images of the molecular gas around the dense cores observed by Ladd et al. (1994) in the Perseus cloud complex in various CO (CO(1-0), ^(13)CO(1-0), C^(18)O(1-0)) and CS (CS(2-1), C^(34)S(2-1)) isotopomers, and N_2H^+(1-0), using the 16-element focal plane array operating at a wavelength of 3 mm at the Five College Radio Astronomy Observatory. We also performed mosaic observations in the N_2H^+(1-0) line and in the adjacent 3 mm continuum with the OVRO interferometer. Only within one of the observed cores we unambiguously detected a 3 mm continuum compact source with the interferometer. The single-dish large-scale maps of the densest gas, which in Perseus is concentrated within two large filamentary structures roughly aligned along a NE-SW axis, allowed us to analyse the spatial and kinematical properties of the cores and of the surrounding ambient gas. In the PER4/PER5 and PER7 regions we find that the large-scale and core velocity gradients have the same sign and similar magnitudes. In at least three cases we then find pairs of nearby cores with differences in the CS and N_2H^+ emission and in the line profile, which may have been caused by evolutionary effects. The small fraction of cores with compact continuum sources, the chemical differentiation and the inward motions observed suggest that we are observing objects in a phase preceeding the collapse and the formation of stars
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