1,419 research outputs found
Universal Cellular Automata and Class 4
Wolfram has provided a qualitative classification of cellular automata(CA)
rules according to which, there exits a class of CA rules (called Class 4)
which exhibit complex pattern formation and long-lived dynamical activity (long
transients). These properties of Class 4 CA's has led to the conjecture that
Class 4 rules are Universal Turing machines i.e. they are bases for
computational universality. We describe an embedding of a ``small'' universal
Turing machine due to Minsky, into a cellular automaton rule-table. This
produces a collection of cellular automata, all of which are
computationally universal. However, we observe that these rules are distributed
amongst the various Wolfram classes. More precisely, we show that the
identification of the Wolfram class depends crucially on the set of initial
conditions used to simulate the given CA. This work, among others, indicates
that a description of complex systems and information dynamics may need a new
framework for non-equilibrium statistical mechanics.Comment: Latex, 10 pages, 5 figures uuencode
Parametric ordering of complex systems
Cellular automata (CA) dynamics are ordered in terms of two global
parameters, computable {\sl a priori} from the description of rules. While one
of them (activity) has been used before, the second one is new; it estimates
the average sensitivity of rules to small configurational changes. For two
well-known families of rules, the Wolfram complexity Classes cluster
satisfactorily. The observed simultaneous occurrence of sharp and smooth
transitions from ordered to disordered dynamics in CA can be explained with the
two-parameter diagram
Atmospheric Circulation of Eccentric Hot Neptune GJ436b
GJ436b is a unique member of the transiting extrasolar planet population
being one of the smallest and least irradiated and possessing an eccentric
orbit. Because of its size, mass and density, GJ436b could plausibly have an
atmospheric metallicity similar to Neptune (20-60 times solar abundances),
which makes it an ideal target to study the effects of atmospheric metallicity
on dynamics and radiative transfer in an extrasolar planetary atmosphere. We
present three-dimensional atmospheric circulation models that include realistic
non-gray radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric
metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show
little day/night temperature variation and strong high-latitude jets. In
contrast, higher metallicity models (30 and 50 times solar) exhibit day/night
temperature variations and a strong equatorial jet. Spectra and light curves
produced from these simulations show strong orbital phase dependencies in the
50 times solar case and negligible variations with orbital phase in the 1 times
solar case. Comparisons between the predicted planet/star flux ratio from these
models and current secondary eclipse measurements support a high metallicity
atmosphere (30-50 times solar abundances) with disequilibrium carbon chemistry
at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b,
our models serve to illuminate how metallicity influences the atmospheric
circulation for a broad range of warm extrasolar planets.Comment: 25 pages, 13 figure
Effects of Initial Flow on Close-In Planet Atmospheric Circulation
We use a general circulation model to study the three-dimensional (3-D) flow
and temperature distributions of atmospheres on tidally synchronized extrasolar
planets. In this work, we focus on the sensitivity of the evolution to the
initial flow state, which has not received much attention in 3-D modeling
studies. We find that different initial states lead to markedly different
distributions-even under the application of strong forcing (large day-night
temperature difference with a short "thermal drag time") that may be
representative of close-in planets. This is in contrast with the results or
assumptions of many published studies. In general, coherent jets and vortices
(and their associated temperature distributions) characterize the flow, and
they evolve differently in time, depending on the initial condition. If the
coherent structures reach a quasi- stationary state, their spatial locations
still vary. The result underlines the fact that circulation models are
currently unsuitable for making quantitative predictions (e.g., location and
size of a "hot spot") without better constrained, and well posed, initial
conditions.Comment: Accepted for publication in the Astrophysical Journal; 23 pages, 9
figures
Gravity Waves on Hot Extrasolar Planets: I. Propagation and Interaction with the Background
We study the effects of gravity waves, or g-modes, on hot extrasolar planets.
These planets are expected to possess stably-stratified atmospheres, which
support gravity waves. In this paper, we review the derivation of the equation
that governs the linear dynamics of gravity waves and describe its application
to a hot extrasolar planet, using HD209458 b as a generic example. We find that
gravity waves can exhibit a wide range of behaviors, even for a single
atmospheric profile. The waves can significantly accelerate or decelerate the
background mean flow, depending on the difference between the wave phase and
mean flow speeds. In addition, the waves can provide significant heating (~100
to ~1000 K per planetary rotation), especially to the region of the atmosphere
above about 10 scale heights from the excitation region. Furthermore, by
propagating horizontally, gravity waves provide a mechanism for transporting
momentum and heat from the dayside of a tidally locked planet to its nightside.
We discuss work that needs to be undertaken to incorporate these effects in
current atmosphere models of extrasolar planets.Comment: Accepted for publication in the Astrophysical Journal; 11 pages, 10
figures
Constraints on the Atmospheric Circulation and Variability of the Eccentric Hot Jupiter XO-3b
We report secondary eclipse photometry of the hot Jupiter XO-3b in the
4.5~m band taken with the Infrared Array Camera (IRAC) on the Spitzer
Space Telescope. We measure individual eclipse depths and center of eclipse
times for a total of twelve secondary eclipses. We fit these data
simultaneously with two transits observed in the same band in order to obtain a
global best-fit secondary eclipse depth of and a center of
eclipse phase of . We assess the relative magnitude of
variations in the dayside brightness of the planet by measuring the size of the
residuals during ingress and egress from fitting the combined eclipse light
curve with a uniform disk model and place an upper limit of 0.05. The new
secondary eclipse observations extend the total baseline from one and a half
years to nearly three years, allowing us to place an upper limit on the
periastron precession rate of degrees/day the tightest
constraint to date on the periastron precession rate of a hot Jupiter. We use
the new transit observations to calculate improved estimates for the system
properties, including an updated orbital ephemeris. We also use the large
number of secondary eclipses to obtain the most stringent limits to date on the
orbit-to-orbit variability of an eccentric hot Jupiter and demonstrate the
consistency of multiple-epoch Spitzer observations.Comment: 14 pages, 11 figures, published by Ap
Initial fixation placement in face images is driven by top-down guidance
The eyes are often inspected first and for longer period during face exploration. To examine whether this saliency of the eye region at the early stage of face inspection is attributed to its local structure properties or to the knowledge of its essence in facial communication, in this study we investigated the pattern of eye movements produced by rhesus monkeys (Macaca mulatta) as they free viewed images of monkey faces. Eye positions were recorded accurately using implanted eye coils, while images of original faces, faces with scrambled eyes, and scrambled faces except for the eyes were presented on a computer screen. The eye region in the scrambled faces attracted the same proportion of viewing time and fixations as it did in the original faces, even the scrambled eyes attracted substantial proportion of viewing time and fixations. Furthermore, the monkeys often made the first saccade towards to the location of the eyes regardless of image content. Our results suggest that the initial fixation placement in faces is driven predominantly by âtop-downâ or internal factors, such as the prior knowledge of the location of âeyesâ within the context of a face
Creation and Reproduction of Model Cells with Semipermeable Membrane
A high activity of reactions can be confined in a model cell with a
semipermeable membrane in the Schl\"ogl model. It is interpreted as a model of
primitive metabolism in a cell. We study two generalized models to understand
the creation of primitive cell systems conceptually from the view point of the
nonlinear-nonequilibrium physics. In the first model, a single-cell system with
a highly active state confined by a semipermeable membrane is spontaneously
created from an inactive homogeneous state by a stochastic jump process. In the
second model, many cell structures are reproduced from a single cell, and a
multicellular system is created.Comment: 11 pages, 7 figure
Direct Measure of Radiative and Dynamical Properties of an Exoplanet Atmosphere
Two decades after the discovery of 51 Peg b, the formation processes and atmospheres of short-period gas giants remain poorly understood. Observations of eccentric systems provide key insights on those topics as they can illuminate how a planet's atmosphere responds to changes in incident flux. We report here the analysis of multi-day multi-channel photometry of the eccentric (e ~ 0.93) hot Jupiter HD 80606 b obtained with the Spitzer Space Telescope. The planet's extreme eccentricity combined with the long coverage and exquisite precision of new periastron-passage observations allow us to break the degeneracy between the radiative and dynamical timescales of HD 80606 b's atmosphere and constrain its global thermal response. Our analysis reveals that the atmospheric layers probed heat rapidly (~4 hr radiative timescale) from <500 to 1400 K as they absorb ~ 20% of the incoming stellar flux during the periastron passage, while the planet's rotation period is 93_(-35)^(+85) hr, which exceeds the predicted pseudo-synchronous period (40 hr)
- âŠ