153 research outputs found
Pencil-Beam Surveys for Trans-Neptunian Objects: Limits on Distant Populations
Two populations of minor bodies in the outer Solar System remain particularly
elusive: Scattered Disk objects and Sedna-like objects. These populations are
important dynamical tracers, and understanding the details of their spatial-
and size-distributions will enhance our understanding of the formation and
on-going evolution of the Solar System. By using newly-derived limits on the
maximum heliocentric distances that recent pencil-beam surveys for
Trans-Neptunian Objects were sensitive to, we determine new upper limits on the
total numbers of distant SDOs and Sedna-like objects. While generally
consistent with populations estimated from wide-area surveys, we show that for
magnitude-distribution slopes of {\alpha} > 0.7-1.0, these pencil-beam surveys
provide stronger upper limits than current estimates in literature.Comment: Submitted to Icaru
Time-keeping and decision-making in living cells: Part I
To survive and reproduce, a cell must process information from its environment and its own internal state and respond accordingly, in terms of metabolic activity, gene expression, movement, growth, division and differentiation. These signal–response decisions are made by complex networks of interacting genes and proteins, which function as biochemical switches and clocks, and other recognizable information-processing circuitry. This theme issue of Interface Focus (in two parts) brings together articles on time-keeping and decision-making in living cells—work that uses precise mathematical modelling of underlying molecular regulatory networks to understand important features of cell physiology. Part I focuses on time-keeping: mechanisms and dynamics of biological oscillators and modes of synchronization and entrainment of oscillators, with special attention to circadian clocks
Time-keeping and decision-making in living cells: Part II
SCOPUS: re.jinfo:eu-repo/semantics/publishe
Phase Dynamics of Nearly Stationary Patterns in Activator-Inhibitor Systems
The slow dynamics of nearly stationary patterns in a FitzHugh-Nagumo model
are studied using a phase dynamics approach. A Cross-Newell phase equation
describing slow and weak modulations of periodic stationary solutions is
derived. The derivation applies to the bistable, excitable, and the Turing
unstable regimes. In the bistable case stability thresholds are obtained for
the Eckhaus and the zigzag instabilities and for the transition to traveling
waves. Neutral stability curves demonstrate the destabilization of stationary
planar patterns at low wavenumbers to zigzag and traveling modes. Numerical
solutions of the model system support the theoretical findings
Synchronization and Coarsening (without SOC) in a Forest-Fire Model
We study the long-time dynamics of a forest-fire model with deterministic
tree growth and instantaneous burning of entire forests by stochastic lightning
strikes. Asymptotically the system organizes into a coarsening self-similar
mosaic of synchronized patches within which trees regrow and burn
simultaneously. We show that the average patch length grows linearly with
time as t-->oo. The number density of patches of length L, N(L,t), scales as
^{-2}M(L/), and within a mean-field rate equation description we find
that this scaling function decays as e^{-1/x} for x-->0, and as e^{-x} for
x-->oo. In one dimension, we develop an event-driven cluster algorithm to study
the asymptotic behavior of large systems. Our numerical results are consistent
with mean-field predictions for patch coarsening.Comment: 5 pages, 4 figures, 2-column revtex format. To be submitted to PR
A mathematical framework for critical transitions: normal forms, variance and applications
Critical transitions occur in a wide variety of applications including
mathematical biology, climate change, human physiology and economics. Therefore
it is highly desirable to find early-warning signs. We show that it is possible
to classify critical transitions by using bifurcation theory and normal forms
in the singular limit. Based on this elementary classification, we analyze
stochastic fluctuations and calculate scaling laws of the variance of
stochastic sample paths near critical transitions for fast subsystem
bifurcations up to codimension two. The theory is applied to several models:
the Stommel-Cessi box model for the thermohaline circulation from geoscience,
an epidemic-spreading model on an adaptive network, an activator-inhibitor
switch from systems biology, a predator-prey system from ecology and to the
Euler buckling problem from classical mechanics. For the Stommel-Cessi model we
compare different detrending techniques to calculate early-warning signs. In
the epidemics model we show that link densities could be better variables for
prediction than population densities. The activator-inhibitor switch
demonstrates effects in three time-scale systems and points out that excitable
cells and molecular units have information for subthreshold prediction. In the
predator-prey model explosive population growth near a codimension two
bifurcation is investigated and we show that early-warnings from normal forms
can be misleading in this context. In the biomechanical model we demonstrate
that early-warning signs for buckling depend crucially on the control strategy
near the instability which illustrates the effect of multiplicative noise.Comment: minor corrections to previous versio
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The Physarum polycephalum Genome Reveals Extensive Use of Prokaryotic Two-Component and Metazoan-Type Tyrosine Kinase Signaling
Physarum polycephalum is a well-studied microbial eukaryote with unique experimental attributes relative to other experimental
model organisms. It has a sophisticated life cycle with several distinct stages including amoebal, flagellated, and plasmodial cells. It is
unusual in switching between open and closed mitosis according to specific life-cycle stages. Here we present the analysis of the
genome of this enigmatic and important model organism and compare it with closely related species. The genome is littered with
simple and complex repeats and the coding regions are frequently interrupted by introns with a mean size of 100 bases.
Complemented with extensive transcriptome data, we define approximately 31,000 gene loci, providing unexpected insights into
earlyeukaryoteevolution.Wedescribeextensiveuseofhistidinekinase-basedtwo-componentsystemsandtyrosinekinasesignaling,
the presence of bacterial and plant type photoreceptors (phytochromes, cryptochrome, and phototropin) and of plant-type pentatricopeptide
repeat proteins, as well as metabolic pathways, and a cell cycle control system typically found in more complex eukaryotes.
Our analysis characterizes P. polycephalum as a prototypical eukaryote with features attributed to the last common ancestor of
Amorphea, that is, the Amoebozoa and Opisthokonts. Specifically, the presence of tyrosine kinases inAcanthamoeba and Physarum
as representatives of two distantly related subdivisions ofAmoebozoa argues against the later emergence of tyrosine kinase signaling
in the opisthokont lineage and also against the acquisition by horizontal gene transfe
Constraining Strong Baryon-Dark Matter Interactions with Primordial Nucleosynthesis and Cosmic Rays
Self-interacting dark matter (SIDM) was introduced by Spergel & Steinhardt to
address possible discrepancies between collisionless dark matter simulations
and observations on scales of less than 1 Mpc. We examine the case in which
dark matter particles not only have strong self-interactions but also have
strong interactions with baryons. The presence of such interactions will have
direct implications for nuclear and particle astrophysics. Among these are a
change in the predicted abundances from big bang nucleosynthesis (BBN) and the
flux of gamma-rays produced by the decay of neutral pions which originate in
collisions between dark matter and Galactic cosmic rays (CR). From these
effects we constrain the strength of the baryon--dark matter interactions
through the ratio of baryon - dark matter interaction cross section to dark
matter mass, . We find that BBN places a weak upper limit to this ratio . CR-SIDM interactions, however, limit the possible DM-baryon cross
section to ; this rules out an energy-independent
interaction, but not one which falls with center-of-mass velocity as or steeper.Comment: 17 pages, 2 figures; plain LaTeX. To appear in PR
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