419 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
The Oxygen Abundance in the Solar Neighborhood
We present a homogeneous analysis of the oxygen abundance in five H II
regions and eight planetary nebulae (PNe) located at distances lower than 2 kpc
and with available spectra of high quality. We find that both the collisionally
excited lines and recombination lines imply that the PNe are overabundant in
oxygen by about 0.2 dex. An explanation that reconciles the oxygen abundances
derived with collisionally excited lines for H II regions and PNe with the
values found for B-stars, the Sun, and the diffuse ISM requires the presence in
H II regions of an organic refractory dust component that is not present in
PNe. This dust component has already been invoked to explain the depletion of
oxygen in molecular clouds and in the diffuse interstellar medium.Comment: 5 pages, 1 figure, accepted for publication in ApJ Letter
Abundances of s-process elements in planetary nebulae: Br, Kr & Xe
We identify emission lines of post-iron peak elements in very high
signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines
from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement
with the theories of s-process in AGB star. Surprisingly, we did not detect
lines from Br even though s-process calculations indicate that it should be
produced with Kr at detectable levels.Comment: 2 pages, 1 figure, to be published in the Proceedings of the IAU
Symposium 234: Planetary Nebulae in Our Galaxy and Beyond, eds. M.J. Barlow,
R.H. Mende
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
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