809,167 research outputs found
Differentiation and Replication of Spots in a Reaction Diffusion System with Many Chemicals
The replication and differentiation of spots in reaction diffusion equations
are studied by extending the Gray-Scott model with self-replicating spots to
include many degrees of freedom needed to model systems with many chemicals. By
examining many possible reaction networks, the behavior of this model is
categorized into three types: replication of homogeneous fixed spots,
replication of oscillatory spots, and differentiation from `m ultipotent
spots'. These multipotent spots either replicate or differentiate into other
types of spots with different fixed-point dynamics, and as a result, an
inhomogeneous pattern of spots is formed. This differentiation process of spots
is analyzed in terms of the loss of chemical diversity and decrease of the
local Kolmogorov-Sinai entropy. The relevance of the results to developmental
cell biology and stem cells is also discussed.Comment: 8 pages, 12 figures, Submitted to EP
Understanding plastic deformation in thermal glasses from single-soft-spot dynamics
By considering the low-frequency vibrational modes of amorphous solids,
Manning and Liu [Phys. Rev. Lett. 107, 108302 (2011)] showed that a population
of "soft spots" can be identified that are intimately related to plasticity at
zero temperature under quasistatic shear. In this work we track individual soft
spots with time in a two-dimensional sheared thermal Lennard Jones glass at
temperatures ranging from deep in the glassy regime to above the glass
transition temperature. We show that the lifetimes of individual soft spots are
correlated with the timescale for structural relaxation. We additionally
calculate the number of rearrangements required to destroy soft spots, and show
that most soft spots can survive many rearrangements. Finally, we show that
soft spots are robust predictors of rearrangements at temperatures well into
the super-cooled regime. Altogether, these results pave the way for mesoscopic
theories of plasticity of amorphous solids based on dynamical behavior of
individual soft spots.Comment: 9 pages, 6 figure
In-Situ Particle Acceleration in Extragalactic Radio Hot Spots: Observations Meet Expectations
We discuss, in terms of particle acceleration, the results from optical VLT
observations of hot spots associated with radio galaxies. On the basis of
observational and theoretical grounds, it is shown that:
1. relatively low radio-radio power hot spots are the optimum candidates for
being detected at optical waves. This is supported by an unprecedented optical
detection rate of 70% out of a sample of low radio power hot spots.
2. the shape of the synchrotron spectrum of hot spots is mainly determined by
the strength of the magnetic field in the region. In particular, the break
frequency, related to the age of the oldest electrons in the hot spots, is
found to increase with decreasing synchrotron power and magnetic field
strength.
Both observational results are in agreement with an in-situ particle
acceleration scenario.Comment: 5 pages, TeX (or Latex, etc), 4 figures, to appear in MNRAS Letter,
Updated reference
Meteorology of Jupiter's Equatorial Hot Spots and Plumes from Cassini
We present an updated analysis of Jupiter's equatorial meteorology from
Cassini observations. For two months preceding the spacecraft's closest
approach, the Imaging Science Subsystem (ISS) onboard regularly imaged the
atmosphere. We created time-lapse movies from this period in order to analyze
the dynamics of equatorial hot spots and their interactions with adjacent
latitudes. Hot spots are quasi-stable, rectangular dark areas on
visible-wavelength images, with defined eastern edges that sharply contrast
with surrounding clouds, but diffuse western edges serving as nebulous
boundaries with adjacent equatorial plumes. Hot spots exhibit significant
variations in size and shape over timescales of days and weeks. Some of these
changes correspond with passing vortex systems from adjacent latitudes
interacting with hot spots. Strong anticyclonic gyres present to the south and
southeast of the dark areas appear to circulate into hot spots. Impressive,
bright white plumes occupy spaces in between hot spots. Compact cirrus-like
'scooter' clouds flow rapidly through the plumes before disappearing within the
dark areas. These clouds travel at 150-200 m/s, much faster than the 100 m/s
hot spot and plume drift speed. This raises the possibility that the scooter
clouds may be more illustrative of the actual jet stream speed at these
latitudes. Most previously published zonal wind profiles represent the drift
speed of the hot spots at their latitude from pattern matching of the entire
longitudinal image strip. If a downward branch of an equatorially-trapped
Rossby waves controls the overall appearance of hot spots, however, the
westward phase velocity of the wave leads to underestimates of the true jet
stream speed.Comment: 33 pages, 11 figures; accepted for publication in Icarus; for
supplementary movies, please contact autho
Spot deformation and replication in the two-dimensional Belousov-Zhabotinski reaction in water-in-oil microemulsion
In the limit of large diffusivity ratio, spot-like solutions in the
two-dimensional Belousov-Zhabotinski reaction in water-in-oil microemulsion are
studied. It is shown analytically that such spots undergo an instability as the
diffusivity ratio is decreased. An instability threshold is derived. For spots
of small radius, it is shown that this instability leads to a spot splitting
into precisely two spots. For larger spots, it leads to deformation, fingering
patterns and space-filling curves. Numerical simulations are shown to be in
close agreement with the analytical predictions.Comment: To appear, PR
Local Current Distribution and "Hot Spots" in the Integer Quantum Hall Regime
In a recent experiment, the local current distribution of a two-dimensional
electron gas in the quantum Hall regime was probed by measuring the variation
of the conductance due to local gating. The main experimental finding was the
existence of "hot spots", i.e. regions with high degree of sensitivity to local
gating, whose density increases as one approaches the quantum Hall transition.
However, the direct connection between these "hot spots" and regions of high
current flow is not clear. Here, based on a recent model for the quantum Hall
transition consisting of a mixture of perfect and quantum links, the relation
between the "hot spots" and the current distribution in the sample has been
investigated. The model reproduces the observed dependence of the number and
sizes of "hot spots" on the filling factor. It is further demonstrated that
these "hot spots" are not located in regions where most of the current flows,
but rather, in places where the currents flow both when injected from the left
or from the right. A quantitative measure, the harmonic mean of these currents
is introduced and correlates very well with the "hot spots" positions
Interferometry of chemically peculiar stars: theoretical predictions vs. modern observing facilities
By means of numerical experiments we explore the application of
interferometry to the detection and characterization of abundance spots in
chemically peculiar (CP) stars using the brightest star eps~Uma as a case
study. We find that the best spectral regions to search for spots and stellar
rotation signatures are in the visual domain. The spots can clearly be detected
already at a first visibility lobe and their signatures can be uniquely
disentangled from that of rotation. The spots and rotation signatures can also
be detected in NIR at low spectral resolution but baselines longer than 180~m
are needed for all potential CP candidates. According to our simulations, an
instrument like VEGA (or its successor e.g., FRIEND) should be able to detect,
in the visual, the effect of spots and spots+rotation, provided that the
instrument is able to measure , and/or closure phase. In
infrared, an instrument like AMBER but with longer baselines than the ones
available so far would be able to measure rotation and spots. Our study
provides necessary details about strategies of spot detection and the
requirements for modern and planned interferometric facilities essential for CP
star research.Comment: Accepted by NMRAS, 18 pages, 11 figures, 2 table
Properties of starspots on CoRoT-2
As a planet eclipses its parent star, a dark spot on the surface of the star
may be occulted, causing a detectable variation in the light curve. A total of
77 consecutive transit light curves of CoRoT-2 were observed with a high
temporal resolution of 32 s, corresponding to an uninterrupted period of 134
days. By analyzing small intensity variations in the transit light curves, it
was possible to detect and characterize spots at fixed positions (latitude and
longitude) on the surface of the star. The model used simulates planetary
transits and enables the inclusion of spots on the stellar surface with
different sizes, intensities (i.e. temperatures), and positions. Fitting the
data by this model, it is possible to infer the spots physical characteristics.
The fits were either in spot longitude and radius, with a fixed intensity, or
in spots longitude and intensity, for spots of constant size. Before the
modeling of the spots were performed, the planetary radius relative to the star
radius was estimated by fitting the deepest transit to minimize the effect of
spots. A slightly larger (3%) radius, 0.172 Rstar, resulted instead of the
previously reported 0.1667 Rstar . The fitting of the transits yield spots, or
spot groups, with sizes of ranging from 0.2 to 0.7 planet radius, Rp, with a
mean of (0.41 +/- 0.13) Rp (~100,000 km), resulting in a stellar area covered
by spots within the transit latitudes of 10-20%. The intensity varied from 0.4
to 0.9 of the disk center intensity, Ic, with a mean of (0.60 +/- 0.19) Ic,
which can be converted to temperature by assuming an effective temperature of
5625 K for the stellar photosphere, the spots temperature ranges mainly from
3600 to 5000 K. The results from the spot modeling are in agreement with those
found for magnetic activity analysis from out of transit data of the same star.Comment: 7 pages, 11 figure
- …