8,423 research outputs found
Entropy-based parametric estimation of spike train statistics
We consider the evolution of a network of neurons, focusing on the asymptotic
behavior of spikes dynamics instead of membrane potential dynamics. The spike
response is not sought as a deterministic response in this context, but as a
conditional probability : "Reading out the code" consists of inferring such a
probability. This probability is computed from empirical raster plots, by using
the framework of thermodynamic formalism in ergodic theory. This gives us a
parametric statistical model where the probability has the form of a Gibbs
distribution. In this respect, this approach generalizes the seminal and
profound work of Schneidman and collaborators. A minimal presentation of the
formalism is reviewed here, while a general algorithmic estimation method is
proposed yielding fast convergent implementations. It is also made explicit how
several spike observables (entropy, rate, synchronizations, correlations) are
given in closed-form from the parametric estimation. This paradigm does not
only allow us to estimate the spike statistics, given a design choice, but also
to compare different models, thus answering comparative questions about the
neural code such as : "are correlations (or time synchrony or a given set of
spike patterns, ..) significant with respect to rate coding only ?" A numerical
validation of the method is proposed and the perspectives regarding spike-train
code analysis are also discussed.Comment: 37 pages, 8 figures, submitte
An HI interstellar bubble surrounding WR85 and RCW118
We analyze the distribution of the interstellar matter in the environs of the
Wolf-Rayet star LSS3982 (= WR85, WN6+OB?) linked to the optical ring nebula
RCW118. Our study is based on neutral hydrogen 21cm-line data belonging to the
Southern Galactic Plane Survey (SGPS).
The analysis of the HI data allowed the identification of a neutral hydrogen
interstellar bubble related to WR 85 and the 25' diameter ring nebula RCW118.
The HI bubble was detected at a systemic velocity of -21.5 km/s, corresponding
to a kinematical distance of 2.8+/-1.1 kpc, compatible with the stellar
distance. The neutral stucture is about 25' in radius or 21+/-8 pc, and is
expanding at 9+/-2 km/s. The associated ionized and neutral masses amount to
3000 Mo. The CO emission distribution depicts a region lacking CO coincident in
position and velocity with the HI structure. The 9'.3 diameter inner optical
nebula appears to be related to the approaching part of the neutral atomic
shell. The HI void and shell are the neutral gas counterparts of the optical
bubble and have very probably originated in the action of the strong stellar
wind of the central star during the O-type and WR phases on the surrounding
interstellar medium. The HI bubble appears to be in the momentun conserving
stage.Comment: 9 pages, 7 figures, accepted in MNRA
Non-radial Oscillation Modes of Compact Stars with a Crust
Oscillation modes of isolated compact stars can, in principle, be a
fingerprint of the equation of state (EoS) of dense matter. We study the
non-radial high-frequency l=2 spheroidal modes of neutron stars and strange
quark stars, adopting a two-component model (core and crust) for these two
types of stars. Using perturbed fluid equations in the relativistic Cowling
approximation, we explore the effect of a strangelet or hadronic crust on the
oscillation modes of strange stars. The results differ from the case of neutron
stars with a crust. In comparison to fluid-only configurations, we find that a
solid crust on top of a neutron star increases the p-mode frequency slightly
with little effect on the f-mode frequency, whereas for strange stars, a
strangelet crust on top of a quark core significantly increases the f-mode
frequency with little effect on the p-mode frequency.Comment: 10 pages, 6 figure
Effects of lattice distortion and Jahn–Teller coupling on the magnetoresistance of La0.7Ca0.3MnO3 and La0.5Ca0.5CoO3 epitaxial films
Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T → TC, TC being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T ≪ TC/, the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn–Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn–Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn–Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites
Mixed-integer-linear-programming-based energy management system for hybrid PV-wind-battery microgrids: Modeling, design, and experimental verification
© 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksMicrogrids are energy systems that aggregate distributed energy resources, loads, and power electronics devices in a stable and balanced way. They rely on energy management systems to schedule optimally the distributed energy resources. Conventionally, many scheduling problems have been solved by using complex algorithms that, even so, do not consider the operation of the distributed energy resources. This paper presents the modeling and design of a modular energy management system and its integration to a grid-connected battery-based microgrid. The scheduling model is a power generation-side strategy, defined as a general mixed-integer linear programming by taking into account two stages for proper charging of the storage units. This model is considered as a deterministic problem that aims to minimize operating costs and promote self-consumption based on 24-hour ahead forecast data. The operation of the microgrid is complemented with a supervisory control stage that compensates any mismatch between the offline scheduling process and the real time microgrid operation. The proposal has been tested experimentally in a hybrid microgrid at the Microgrid Research Laboratory, Aalborg University.Peer ReviewedPostprint (author's final draft
A decentralized scalable approach to voltage control of DC islanded microgrids
We propose a new decentralized control scheme for DC Islanded microGrids
(ImGs) composed by several Distributed Generation Units (DGUs) with a general
interconnection topology. Each local controller regulates to a reference value
the voltage of the Point of Common Coupling (PCC) of the corresponding DGU.
Notably, off-line control design is conducted in a Plug-and-Play (PnP) fashion
meaning that (i) the possibility of adding/removing a DGU without spoiling
stability of the overall ImG is checked through an optimization problem; (ii)
when a DGU is plugged in or out at most neighbouring DGUs have to update their
controllers and (iii) the synthesis of a local controller uses only information
on the corresponding DGU and lines connected to it. This guarantee total
scalability of control synthesis as the ImG size grows or DGU gets replaced.
Yes, under mild approximations of line dynamics, we formally guarantee
stability of the overall closed-loop ImG. The performance of the proposed
controllers is analyzed simulating different scenarios in PSCAD.Comment: arXiv admin note: text overlap with arXiv:1405.242
Plug-and-play and coordinated control for bus-connected AC islanded microgrids
This paper presents a distributed control architecture for voltage and
frequency stabilization in AC islanded microgrids. In the primary control
layer, each generation unit is equipped with a local controller acting on the
corresponding voltage-source converter. Following the plug-and-play design
approach previously proposed by some of the authors, whenever the
addition/removal of a distributed generation unit is required, feasibility of
the operation is automatically checked by designing local controllers through
convex optimization. The update of the voltage-control layer, when units plug
-in/-out, is therefore automatized and stability of the microgrid is always
preserved. Moreover, local control design is based only on the knowledge of
parameters of power lines and it does not require to store a global microgrid
model. In this work, we focus on bus-connected microgrid topologies and enhance
the primary plug-and-play layer with local virtual impedance loops and
secondary coordinated controllers ensuring bus voltage tracking and reactive
power sharing. In particular, the secondary control architecture is
distributed, hence mirroring the modularity of the primary control layer. We
validate primary and secondary controllers by performing experiments with
balanced, unbalanced and nonlinear loads, on a setup composed of three
bus-connected distributed generation units. Most importantly, the stability of
the microgrid after the addition/removal of distributed generation units is
assessed. Overall, the experimental results show the feasibility of the
proposed modular control design framework, where generation units can be
added/removed on the fly, thus enabling the deployment of virtual power plants
that can be resized over time
Molecular gas associated with IRAS 10361-5830
We analyze the distribution of the molecular gas and the dust in the
molecular clump linked to IRAS 10361-5830, located in the environs of the
bubble-shaped HII region Gum 31 in the Carina region, with the aim of
determining the main parameters of the associated material and investigating
the evolutionary state of the young stellar objects identified there.
Using the APEX telescope, we mapped the molecular emission in the J=3-2
transition of three CO isotopologues, 12CO, 13CO and C18O, over a 1.5' x 1.5'
region around the IRAS position. We also observed the high density tracers CS
and HCO+ toward the source. The cold dust distribution was analyzed using
submillimeter continuum data at 870 \mu\ obtained with the APEX telescope.
Complementary IR and radio data at different wavelengths were used to complete
the study of the ISM.
The molecular gas distribution reveals a cavity and a shell-like structure of
~ 0.32 pc in radius centered at the position of the IRAS source, with some
young stellar objects (YSOs) projected onto the cavity. The total molecular
mass in the shell and the mean H volume density are ~ 40 solar masses and
~(1-2) x 10 cm, respectively. The cold dust counterpart of the
molecular shell has been detected in the far-IR at 870 \mu\ and in Herschel
data at 350 \mu. Weak extended emission at 24 \mu\ from warm dust is projected
onto the cavity, as well as weak radio continuum emission.
A comparison of the distribution of cold and warm dust, and molecular and
ionized gas allows us to conclude that a compact HII region has developed in
the molecular clump, indicating that this is an area of recent massive star
formation. Probable exciting sources capable of creating the compact HII region
are investigated. The 2MASS source 10380461-5846233 (MSX G286.3773-00.2563)
seems to be responsible for the formation of the HII region.Comment: Accepted in A&A. 11 pages, 10 Postscript figure
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