158 research outputs found
Morphological properties of projection specific pyramidal neurons of primate anterior cingulate cortex
The anterior cingulate cortex is an important interface of cortical, motor, and limbic networks, and thus is a brain area uniquely situated to affect a wide variety of higher order functions. The aim of this study was to characterize the morphology of two distinct populations of anterior cingulate cortex (ACC) pyramidal neurons, a dorsal-caudal population projecting to the premotor cortex (PMC) and a ventral-rostral population projecting to the amygdala. Retrograde tracers injected into area 6DC of the “cognitive” premotor cortex, and into the basolateral nucleus of the “affective” amygdala were used to label distinct projection neurons in the ACC. Whole-cell patch clamp recording and intracellular filling techniques were used to fill the dendritic arbor of these labeled projection neurons. High resolution confocal microscopy and 3D neuronal reconstructions were used to quantify dendritic morphological parameters. Amygdala projecting neurons were more superficial than premotor projecting neurons, with an average soma-to-pia distance of 498 μm compared to 1,012 μm, respectively (amygdala projecting: 498 ± 139 μm vs. PMC projecting: 1012 ± 113 μm, p<.05). Overall, amygdala and PMC projection neurons had very similar average dendritic lengths, branch points, branch densities, and vertical and horizontal extensions in both apical and basal compartments. Amygdala projecting cells had greater apical tuft branch points than deep PMC projecting cells (8.25 vs. 3.3 apical tuft branch points, p<.05). Superficial PMC projecting cells had smaller total vertical and apical vertical extensions than deep PMC projecting cells (Total vertical: 304.98 vs. 750.96 μm, apical vertical: 241.78 vs 601.95 μm, p<.05). Sholl analyses revealed that the distribution of apical dendritic length as a function of distance from the soma of amygdala projections had bimodal peaks, while that of superficial and deep PMC cells had a single peak. Total spine number of amygdala projecting neurons was greater than PMC projecting cells (~17,000 spines vs. ~2,100 spines). Three major classes of morphology were visualized within the ACC neuron reconstructions dataset: regular-tufted, narrow-tufted, and untufted, with the regular-tufted cells containing more branch points than narrow tufted but less basal branch point density. The work in this study assessing cellular morphological properties of specific amygdala and PMC inputs and outputs within the ACC helps to characterize functional dynamics of both emotional and motor planning networks
Singular Laplacian Growth
The general equations of motion for two dimensional Laplacian growth are
derived using the conformal mapping method. In the singular case, all
singularities of the conformal map are on the unit circle, and the map is a
degenerate Schwarz-Christoffel map. The equations of motion describe the
motions of these singularities. Despite the typical fractal-like outcomes of
Laplacian growth processes, the equations of motion are shown to be not
particularly sensitive to initial conditions. It is argued that the sensitivity
of this system derives from a novel cause, the non-uniqueness of solutions to
the differential system. By a mechanism of singularity creation, every solution
can become more complex, even in the absence of noise, without violating the
growth law. These processes are permitted, but are not required, meaning the
equation of motion does not determine the motion, even in the small.Comment: 8 pages, Latex, 4 figures, Submitted to Phys. Rev.
Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
The early Solar System contained short-lived radionuclides such as 60Fe (t1/2
= 1.5 Myr) whose most likely source was a nearby supernova. Previous models of
Solar System formation considered a supernova shock that triggered the collapse
of the Sun's nascent molecular cloud. We advocate an alternative hypothesis,
that the Solar System's protoplanetary disk had already formed when a very
close (< 1 pc) supernova injected radioactive material directly into the disk.
We conduct the first numerical simulations designed to answer two questions
related to this hypothesis: will the disk be destroyed by such a close
supernova; and will any of the ejecta be mixed into the disk? Our simulations
demonstrate that the disk does not absorb enough momentum from the shock to
escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss
occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the
disk, which also mix into the disk about 1% of the intercepted ejecta. These
low efficiencies of destruction and injectation are due to the fact that the
high disk pressures prevent the ejecta from penetrating far into the disk
before stalling. Injection of gas-phase ejecta is too inefficient to be
consistent with the abundances of radionuclides inferred from meteorites. On
the other hand, the radionuclides found in meteorites would have condensed into
dust grains in the supernova ejecta, and we argue that such grains will be
injected directly into the disk with nearly 100% efficiency. The meteoritic
abundances of the short-lived radionuclides such as 60Fe therefore are
consistent with injection of grains condensed from the ejecta of a nearby (< 1
pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure
Morphological diagram of diffusion driven aggregate growth in plane: competition of anisotropy and adhesion
Two-dimensional structures grown with Witten and Sander algorithm are
investigated. We analyze clusters grown off-lattice and clusters grown with
antenna method with and 8 allowed growth directions. With
the help of variable probe particles technique we measure fractal dimension of
such clusters as a function of their size . We propose that in the
thermodynamic limit of infinite cluster size the aggregates grown with high
degree of anisotropy () tend to have fractal dimension equal
to 3/2, while off-lattice aggregates and aggregates with lower anisotropy
() have . Noise-reduction procedure results in the
change of universality class for DLA. For high enough noise-reduction value
clusters with have fractal dimension going to when
.Comment: 6 pages, 8 figures, conference CCP201
The Crustal Rigidity of a Neutron Star, and Implications for PSR 1828-11 and other Precession Candidates
We calculate the crustal rigidity parameter, b, of a neutron star (NS), and
show that b is a factor 40 smaller than the standard estimate due to Baym &
Pines (1971). For a NS with a relaxed crust, the NS's free-precession frequency
is directly proportional to b. We apply our result for b to PSR 1828-11, a 2.5
Hz pulsar that appears to be precessing with period 511 d. Assuming this 511-d
period is set by crustal rigidity, we show that this NS's crust is not relaxed,
and that its reference spin (roughly, the spin for which the crust is most
relaxed) is 40 Hz, and that the average spindown strain in the crust is 5
\times 10^{-5}. We also briefly describe the implications of our b calculation
for other well-known precession candidates.Comment: 44 pages, 10 figures, submitted to Ap
Chandra View of the Dynamically Young Cluster of Galaxies A1367 I. Small-Scale Structures
The 40 ks \emph{Chandra} ACIS-S observation of A1367 provides new insights
into small-scale structures and point sources in this dynamically young
cluster. Here we concentrate on small-scale extended structures. A ridge-like
structure around the center (``the ridge'') is significant in the \chandra\
image. The ridge, with a projected length of 8 arcmin (or 300
h kpc), is elongated from northwest (NW) to southeast (SE), as is
the X-ray surface brightness distribution on much larger scales ( 2
h Mpc). The ridge is cooler than its western and southern
surroundings while the differences from its eastern and northern surroundings
are small. We also searched for small-scale structures with sizes
arcmin. Nine extended features, with sizes from 0.5 to 1.5, were
detected at significance levels above 4 . Five of the nine features are
located in the ridge and form local crests. The nine extended features can be
divided into two types. Those associated with galaxies (NGC 3860B, NGC 3860 and
UGC 6697) are significantly cooler than their surroundings (0.3 - 0.9 keV vs. 3
- 4.5 keV). The masses of their host galaxies are sufficient to bind the
extended gas. These extended features are probably related to thermal halos or
galactic superwinds of their host galaxies. The existence of these relatively
cold halos imply that galaxy coronae can survive in cluster environment (e.g.,
Vikhlinin et al. 2001). Features of the second type are not apparently
associated with galaxies. Their temperatures may not be significantly different
from those of their surroundings. This class of extended features may be
related to the ridge. We consider several possibilities for the ridge and the
second type of extended features. The merging scenario is preferred.Comment: To appear in ApJ, Vol 576, 2002, Sep., a high-resolution version is
in http://cfa160.harvard.edu/~sunm/a1367_a.ps.g
Model Simulations of a Shock-Cloud Interaction in the Cygnus Loop
We present optical observations and 2D hydrodynamic modeling of an isolated
shocked ISM cloud. H images taken in 1992.6 and 2003.7 of a small
optical emission cloud along the southwestern limb of the Cygnus Loop were used
to measure positional displacements of 0 \farcs 1 yr for
surrounding Balmer dominated emission filaments and 0\farcs025 - \farcs055
yr for internal cloud emission features. These measurements imply
transverse velocities of 250 km s and 80 -- 140 km
s for ambient ISM and internal cloud shocks respectively. The complex
shock structure visible within the cloud indicates that the cloud's internal
density distribution is two phased: a smoothly varying background density which
is populated by higher density clumps. We present model results for a shock
interacting with a non-uniform ISM cloud. We find that this cloud can be well
modeled by a smoothly varying power law core surrounded by a low density
envelope with a Lorentzian profile. The lack of sharp density gradients in such
a model inhibits the growth of Kelvin-Helmholtz instabilities, consistent with
the cloud's appearance. Our model results also suggest that cloud clumps have
densities 10 times the ambient ISM density and account for 30% of
the total cloud volume. Moreover, the observed spacing of internal cloud shocks
and model simulations indicate that the distance between clumps is 4
clump radii.Comment: To be published in Ap
Tails of the Unexpected: The Interaction of an Isothermal Shell with a Cloud
A new mechanism for the formation of cometary tails behind dense clouds or
globules is discussed. Numerical hydrodynamical models show that when a dense
shell of swept-up matter overruns a cloud, material in the shell is focussed
behind the cloud to form a tail. This mode of tail formation is completely
distinct from other methods, which involve either the removal of material from
the cloud, or shadowing from a strong, nearby source of ionization. This
mechanism is relevant to the cometary tails seen in planetary nebulae and to
the interaction of superbubble shells with dense clouds.Comment: 6 pages, 6 figures, accepted for publication in MNRAS letter
Multiscale Finite-Difference-Diffusion-Monte-Carlo Method for Simulating Dendritic Solidification
We present a novel hybrid computational method to simulate accurately
dendritic solidification in the low undercooling limit where the dendrite tip
radius is one or more orders of magnitude smaller than the characteristic
spatial scale of variation of the surrounding thermal or solutal diffusion
field. The first key feature of this method is an efficient multiscale
diffusion Monte-Carlo (DMC) algorithm which allows off-lattice random walkers
to take longer and concomitantly rarer steps with increasing distance away from
the solid-liquid interface. As a result, the computational cost of evolving the
large scale diffusion field becomes insignificant when compared to that of
calculating the interface evolution. The second key feature is that random
walks are only permitted outside of a thin liquid layer surrounding the
interface. Inside this layer and in the solid, the diffusion equation is solved
using a standard finite-difference algorithm that is interfaced with the DMC
algorithm using the local conservation law for the diffusing quantity. Here we
combine this algorithm with a previously developed phase-field formulation of
the interface dynamics and demonstrate that it can accurately simulate
three-dimensional dendritic growth in a previously unreachable range of low
undercoolings that is of direct experimental relevance.Comment: RevTeX, 16 pages, 10 eps figures, submitted to J. Comp. Phy
Starburst-Driven Galactic Winds: Filament Formation and Emission Processes
We have performed a series of three-dimensional simulations of the
interaction of a supersonic wind with a non-spherical radiative cloud. These
simulations are motivated by our recent three-dimensional model of a
starburst-driven galactic wind interacting with an inhomogeneous disk, which
show that an optically emitting filament can be formed by the break-up and
acceleration of a cloud into a supersonic wind. In this study we consider the
evolution of a cloud with two different geometries (fractal and spherical) and
investigate the importance of radiative cooling on the cloud's survival. We
have also undertaken a comprehensive resolution study in order to ascertain the
effect of the assumed numerical resolution on the results. We find that the
ability of the cloud to radiate heat is crucial for its survival. While an
adiabatic cloud is destroyed over a short period of time, a radiative cloud is
broken up via the Kelvin-Helmholtz instability into numerous small, dense
cloudlets, which are drawn into the flow to form a filamentary structure. The
degree of fragmentation is highly dependent on the resolution of the
simulation, with the number of cloudlets formed increasing as the
Kelvin-Helmholtz instability is better resolved. Nevertheless, there is a clear
qualitative trend, with the filamentary structure still persistent at high
resolution. We confirm the mechanism behind the formation of the H-alpha
emitting filaments found in our global simulations of a starburst-driven wind.
Based on our resolution study, we conclude that bow shocks around accelerated
gas clouds, and their interaction, are the main source of the soft X-ray
emission observed in these galactic-scale winds. [ABRIDGED]Comment: Accepted to ApJ, 39 pages, 21 figures, movie file can obtained at
http://www.mso.anu.edu.au/~jcooper/movie/halpha.mo
- …