363 research outputs found
On the low dimensional dynamics of structured random networks
Using a generalized random recurrent neural network model, and by extending
our recently developed mean-field approach [J. Aljadeff, M. Stern, T. Sharpee,
Phys. Rev. Lett. 114, 088101 (2015)], we study the relationship between the
network connectivity structure and its low dimensional dynamics. Each
connection in the network is a random number with mean 0 and variance that
depends on pre- and post-synaptic neurons through a sufficiently smooth
function of their identities. We find that these networks undergo a phase
transition from a silent to a chaotic state at a critical point we derive as a
function of . Above the critical point, although unit activation levels are
chaotic, their autocorrelation functions are restricted to a low dimensional
subspace. This provides a direct link between the network's structure and some
of its functional characteristics. We discuss example applications of the
general results to neuroscience where we derive the support of the spectrum of
connectivity matrices with heterogeneous and possibly correlated degree
distributions, and to ecology where we study the stability of the cascade model
for food web structure.Comment: 16 pages, 4 figure
A self-consistent stellar and 3D nebular model for Planetary Nebula IC418
We present a coherent stellar and nebular model reproducing the observations
of the Planetary Nebula IC418. We want to test whether a stellar model obtained
by fitting the stellar observations is able to satisfactory ionize the nebula
and reproduce the nebular observations, which is by no mean evident. This
allows us to determine all the physical parameters of both the star and the
nebula, including the abundances and the distance. We used all the
observational material available (FUSE, IUE, STIS and optical spectra) to
constrain the stellar atmosphere model performed using the CMFGEN code. The
photoionization model is done with Cloudy_3D, and is based on CTIO, Lick, SPM,
IUE and ISO spectra as well as HST images. More than 140 nebular emission lines
are compared to the observed intensities. We reproduce all the observations for
the star and the nebula. The 3D morphology of the gas distribution is
determined. The effective temperature of the star is 36.7kK. Its luminosity is
7700 solar luminosity. We describe an original method to determine the distance
of the nebula using evolutionary tracks. No clumping factor is need to
reproduce the age-luminosity relation. The distance of 1.25 kpc is found in
very good agreement with recent determination using parallax method. The
chemical composition of both the star and the nebula are determined. Both are
Carbon-rich. The nebula presents evidence of depletion of elements Mg, Si, S,
Cl (0.5 dex lower than solar) and Fe (2.9 dex lower than solar). This is the
first self-consistent stellar and nebular model for a Planetary Nebula that
reproduces all the available observations ranging from IR to UV, showing that
the combined approach for the modeling process leads to more restrictive
constraints and, in principle, more trustworthy results.Comment: Accepted for publication in Astronomy and Astrophysics. V2: after
corrections language edito
Neural Decision Boundaries for Maximal Information Transmission
We consider here how to separate multidimensional signals into two
categories, such that the binary decision transmits the maximum possible
information transmitted about those signals. Our motivation comes from the
nervous system, where neurons process multidimensional signals into a binary
sequence of responses (spikes). In a small noise limit, we derive a general
equation for the decision boundary that locally relates its curvature to the
probability distribution of inputs. We show that for Gaussian inputs the
optimal boundaries are planar, but for non-Gaussian inputs the curvature is
nonzero. As an example, we consider exponentially distributed inputs, which are
known to approximate a variety of signals from natural environment.Comment: 5 pages, 3 figure
Detection of Fluorine in the Halo Planetary Nebula BoBn 1: Evidence For a Binary Progenitor Star
We have found the fluorine lines [F IV] 3996.92A,4059.90A in the extremely
metal-poor ([Ar/H] = -2.10+/-0.21) halo planetary nebula (PN) BoBn 1 in
high-dispersion spectra from the 8.2-m VLT/UVES archive. Chemical abundance
analysis shows that the fluorine abundance is [F/H] = +1.06+/-0.08, making BoBn
1 the most fluorine-enhanced and metal-poor PN among fluorine-detected PNe and
providing new evidence that fluorine is enhanced by nucleosynthesis in low mass
metal-poor stars. A comparison with the abundances of carbon-enhanced
metal-poor (CEMP) stars suggests that BoBn 1 shares their origin and evolution
with CEMP-s stars such as HE1305+0132. BoBn 1 might have evolved from a binary
consisting of ~2 Msun primary and ~0.8 Msun secondary stars.Comment: 13 pages, 3 figures, accepted by ApJ Letter
Enhancement of tunneling from a correlated 2D electron system by a many-electron Mossbauer-type recoil in a magnetic field
We consider the effect of electron correlations on tunneling from a 2D
electron layer in a magnetic field parallel to the layer. A tunneling electron
can exchange its momentum with other electrons, which leads to an exponential
increase of the tunneling rate compared to the single-electron approximation.
Explicit results are obtained for a Wigner crystal. They provide a qualitative
and quantitative explanation of the data on electrons on helium. We also
discuss tunneling in semiconductor heterostructures.Comment: published version, 4 pages, 2 figures, RevTeX 3.
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