13,083 research outputs found
Mean First Passage Time in Periodic Attractors
The properties of the mean first passage time in a system characterized by
multiple periodic attractors are studied. Using a transformation from a high
dimensional space to 1D, the problem is reduced to a stochastic process along
the path from the fixed point attractor to a saddle point located between two
neighboring attractors. It is found that the time to switch between attractors
depends on the effective size of the attractors, , the noise, ,
and the potential difference between the attractor and an adjacent saddle point
as: ; the
ratio between the sizes of the two attractors affects . The
result is obtained analytically for small and confirmed by numerical
simulations. Possible implications that may arise from the model and results
are discussed.Comment: 14 pages, 3 figures, submitted to journal of physics
Star-forming galaxies in low-redshift clusters: Data and integrated galaxy properties
This paper is a continuation of an ongoing study of the evolutionary
processes affecting cluster galaxies. Both CCD R band and H alpha narrow-band
imaging was used to determine photometric parameters (m_(r), r_(24), H alpha
flux and equivalent width) and derive star formation rates for 227 CGCG
galaxies in 8 low-redshift clusters. The galaxy sample is a subset of CGCG
galaxies in an objective prism survey of cluster galaxies for H alpha emission.
It is found that detection of emission-line galaxies in the OPS is 85%, 70%,
and 50% complete at the mean surface brightness values of 1.25 x 10^(-19), 5.19
x 10^(-20), and 1.76 x 10^(-20) W m^(-2) arcsec^(-2), respectively, measured
within the R band isophote of 24 mag arcsec^(-2) for the galaxy. The CCD data,
together with matched data from a recent H alpha galaxy survey of UGC galaxies
within 3000 km s^(-1), will be used for a comparative study of R band and H
alpha surface photometry between cluster and field spirals.Comment: Accepted for publication in A&A. 11 pages, including 6 figure
Towards A Mean-Field Formulation Of The Babcock-Leighton Type Solar Dynamo. I. Alpha Coefficient Versus Durney's Double Ring Approach
We develop a model of the solar dynamo in which, on the one hand, we follow
the Babcock-Leighton approach to include surface processes like the production
of poloidal field from the decay of active regions, and, on the other hand, we
attempt to develop a mean field theory that can be studied in quantitative
detail. One of the main challenges in developing such models is to treat the
buoyant rise of toroidal field and the production of poloidal field from it
near the surface. We build up a dynamo model with two contrasting methods of
treating buoyancy. In one method, we incorporate the generation of the poloidal
field near the solar surface by Durney's procedure of double ring eruption. In
the second method, the poloidal field generation is treated by a positive
alpha-effect concentrated near the solar surface, coupled with an algorithm for
handling buoyancy. The two methods are found to give qualitatively similar
results.Comment: 32 pages, 27 figures, uses aastex.cls and epsfig.st
Simulations of a mortality plateau in the sexual Penna model for biological ageing
The Penna model is a strategy to simulate the genetic dynamics of
age-structured populations, in which the individuals genomes are represented by
bit-strings. It provides a simple metaphor for the evolutionary process in
terms of the mutation accumulation theory. In its original version, an
individual dies due to inherited diseases when its current number of
accumulated mutations, n, reaches a threshold value, T. Since the number of
accumulated diseases increases with age, the probability to die is zero for
very young ages (n = T). Here, instead
of using a step function to determine the genetic death age, we test several
other functions that may or may not slightly increase the death probability at
young ages (n < T), but that decreases this probability at old ones. Our
purpose is to study the oldest old effect, that is, a plateau in the mortality
curves at advanced ages. Imposing certain conditions, it has been possible to
obtain a clear plateau using the Penna model. However, a more realistic one
appears when a modified version, that keeps the population size fixed without
fluctuations, is used. We also find a relation between the birth rate, the
age-structure of the population and the death probability.Comment: submitted to Phys. Rev.
Memory Effects in Turbulent Dynamo: Generation and Propagation of Large Scale Magnetic Field
We are concerned with large scale magnetic field dynamo generation and
propagation of magnetic fronts in turbulent electrically conducting fluids. An
effective equation for the large scale magnetic field is developed here that
takes into account the finite correlation times of the turbulent flow. This
equation involves the memory integrals corresponding to the dynamo source term
describing the alpha-effect and turbulent transport of magnetic field. We find
that the memory effects can drastically change the dynamo growth rate, in
particular, non-local turbulent transport might increase the growth rate
several times compared to the conventional gradient transport expression.
Moreover, the integral turbulent transport term leads to a large decrease of
the speed of magnetic front propagation.Comment: 13 pages, 2 figure
Polar branches of stellar activity waves: dynamo models and observations
[Abridged abstract:] Stellar activity data provide evidence of activity wave
branches propagating polewards rather than equatorwards (the solar case).
Stellar dynamo theory allows polewards propagating dynamo waves for certain
governing parameters. We try to unite observations and theory, restricting our
investigation to the simplest mean-field dynamo models. We suggest a crude
preliminary systematization of the reported cases of polar activity branches.
Then we present results of dynamo model simulations which contain magnetic
structures with polar dynamo waves, and identify the models which look most
promising for explaining the latitudinal distribution of spots in dwarf stars.
Those models require specific features of stellar rotation laws, and so
observations of polar activity branches may constrain internal stellar
rotation. Specifically, we find it unlikely that a pronounced poleward branch
can be associated with a solar-like internal rotation profile, while it can be
more readily reproduced in the case of a cylindrical rotation law appropriate
for fast rotators. We stress the case of the subgiant component of the active
close binary HR 1099 which, being best investigated, presents the most severe
problems for a dynamo interpretation. Our best model requires dynamo action in
two layers separated in radius. Observations of polar activity branches provide
valuable information for understanding stellar activity mechanisms and internal
rotation, and thus deserve intensive observational and theoretical
investigation. Current stellar dynamo theory seems sufficiently robust to
accommodate the phenomenology.Comment: 13 pages, 10 figures, 4 tables, accepted by Astronomy and
Astrophysic
Physics of brain dynamics: Fokker-Planck analysis reveals changes in EEG delta-theta interactions in anaesthesia
We use drift and diffusion coefficients to reveal interactions between different oscillatory processes underlying a complex signal and apply the method to EEG delta and theta frequencies in the brain. By analysis of data recorded from rats during anaesthesia, we consider the stability and basins of attraction of fixed points in the phase portrait of the deterministic part of the retrieved stochastic process. We show that different classes of dynamics are associated with deep and light anaesthesia, and we demonstrate that the predominant directionality of the interaction is such that theta drives delt
An optical view of the filament region of Abell 85
We compare the distribution of optically and Halpha (Ha) selected galaxies in
the Southern half of the nearby Abell 85 (A85) cluster with the recently
discovered X-ray filament (XRF). We search for galaxies where star formation
(SF) may have been triggered by interactions with intracluster gas or tidal
pressure due to the cluster potential when entering the cluster. Our analysis
is based on images obtained with CFHT MegaPrime/MegaCam (1x1 deg2 field) in
four bands (ugri) and ESO 2.2mWFI (38'x36' field) in a narrow band filter
corresponding to the redshifted Halpha (Ha) line and in a broad R-band filter.
The LFs are estimated by statistically subtracting a reference field.
Background contamination is minimized by cutting out galaxies redder than the
observed red sequence in the g-i vs. i colour-magnitude diagram. The galaxy
distribution shows a significantly flattened cluster, whose principal axis is
slightly offset from the XRF. The analysis of the broad band LFs shows that the
filament region is well populated. The filament is also independently detected
as a gravitationally bound structure by the Serna & Gerbal hierarchical method.
101 galaxies are detected in Ha, among which 23 have spectroscopic redshifts in
the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have
photometric redshifts that tend to indicate that they are background
objects.The 23 galaxies with spectroscopic redshifts in the cluster are mostly
concentrated in the South part of the cluster and along the filament. We find a
number of galaxies showing evidence for SF in the XRF, and all our results are
consistent with the previous hypothesis that the XRF in A85 is a
gravitationally bound structure made of groups falling on to the main cluster.Comment: Accepted in A&A. 39 pages, 107 figures. Full resolution images
available at ftp://ftp.iap.fr/pub/from_users/gam/A85
Phase-Insensitive Scattering of Terahertz Radiation
The nonlinear interaction between Near-Infrared (NIR) and Terahertz pulses is
principally investigated as a means for the detection of radiation in the
hardly accessible THz spectral region. Most studies have targeted second-order
nonlinear processes, given their higher efficiencies, and only a limited number
have addressed third-order nonlinear interactions, mainly investigating
four-wave mixing in air for broadband THz detection. We have studied the
nonlinear interaction between THz and NIR pulses in solid-state media
(specifically diamond), and we show how the former can be frequency-shifted up
to UV frequencies by the scattering from the nonlinear polarisation induced by
the latter. Such UV emission differs from the well-known electric-field-induced
second harmonic (EFISH) one, as it is generated via a phase-insensitive
scattering, rather than a sum- or difference-frequency four-wave-mixing
process
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