1,225 research outputs found
Generalized Stacking Fault Energy Surfaces and Dislocation Properties of Silicon: A First-Principles Theoretical Study
The generalized stacking fault (GSF) energy surfaces have received
considerable attention due to their close relation to the mechanical properties
of solids. We present a detailed study of the GSF energy surfaces of silicon
within the framework of density functional theory. We have calculated the GSF
energy surfaces for the shuffle and glide set of the (111) plane, and that of
the (100) plane of silicon, paying particular attention to the effects of the
relaxation of atomic coordinates. Based on the calculated GSF energy surfaces
and the Peierls-Nabarro model, we obtain estimates for the dislocation
profiles, core energies, Peierls energies, and the corresponding stresses for
various planar dislocations of silicon.Comment: 9 figures (not included; send requests to [email protected]
Binary formation and mass function variations in fragmenting discs with short cooling times
Accretion discs at sub-pc distances around supermassive black holes are
likely to cool rapidly enough that self-gravity results in fragmentation. Here,
we use high-resolution hydrodynamic simulations of a simplified disc model to
study how the outcome of fragmentation depends upon numerical resolution and
cooling time, and to investigate the incidence of binary formation within
fragmenting discs. We investigate a range of cooling times, from the relatively
long cooling time-scales that are marginally unstable to fragmentation down to
highly unstable cooling on a time-scale that is shorter than the local
dynamical time. The characteristic mass of fragments decreases with reduced
cooling time, though the effect is modest and dependent upon details of how
rapidly bound clumps radiate. We observe a high incidence of capture binaries,
though we are unable to determine their final orbits or probability of
survival. The results suggest that faster cooling in the parent disc results in
an increased binary fraction, and that a high primordial binary fraction may
result from disc fragmentation. We discuss our results in terms of the young
massive stars close to the Galactic Centre, and suggest that observations of
some stellar binaries close to the Galactic Centre remain consistent with
formation in a fragmenting accretion disc.Comment: 10 pages, 5 figures. Accepted for publication in MNRAS. Figures 1 and
3 degraded to meet arXiv size limits - version with high resolution figures
available at http://www.strw.leidenuniv.nl/~rda/publications.htm
Maladaptation and the paradox of robustness in evolution
Background. Organisms use a variety of mechanisms to protect themselves
against perturbations. For example, repair mechanisms fix damage, feedback
loops keep homeostatic systems at their setpoints, and biochemical filters
distinguish signal from noise. Such buffering mechanisms are often discussed in
terms of robustness, which may be measured by reduced sensitivity of
performance to perturbations. Methodology/Principal Findings. I use a
mathematical model to analyze the evolutionary dynamics of robustness in order
to understand aspects of organismal design by natural selection. I focus on two
characters: one character performs an adaptive task; the other character
buffers the performance of the first character against perturbations. Increased
perturbations favor enhanced buffering and robustness, which in turn decreases
sensitivity and reduces the intensity of natural selection on the adaptive
character. Reduced selective pressure on the adaptive character often leads to
a less costly, lower performance trait. Conclusions/Significance. The paradox
of robustness arises from evolutionary dynamics: enhanced robustness causes an
evolutionary reduction in the adaptive performance of the target character,
leading to a degree of maladaptation compared to what could be achieved by
natural selection in the absence of robustness mechanisms. Over evolutionary
time, buffering traits may become layered on top of each other, while the
underlying adaptive traits become replaced by cheaper, lower performance
components. The paradox of robustness has widespread implications for
understanding organismal design
Gap Formation in the Dust Layer of 3D Protoplanetary Disks
We numerically model the evolution of dust in a protoplanetary disk using a
two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is
non-self-gravitating and locally isothermal. The code follows the three
dimensional distribution of dust in a protoplanetary disk as it interacts with
the gas via aerodynamic drag. In this work, we present the evolution of a disk
comprising 1% dust by mass in the presence of an embedded planet for two
different disk configurations: a small, minimum mass solar nebular (MMSN) disk
and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the
grain size and planetary mass to see how they effect the resulting disk
structure. We find that gap formation is much more rapid and striking in the
dust layer than in the gaseous disk and that a system with a given stellar,
disk and planetary mass will have a different appearance depending on the grain
size and that such differences will be detectable in the millimetre domain with
ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk
while not in the gas disk. We also note that dust accumulates at the external
edge of the planetary gap and speculate that the presence of a planet in the
disk may facilitate the growth of planetesimals in this high density region.Comment: 5 page, 4 figures. Accepted for publication in Astrophysics & Space
Scienc
Self-gravitating fragmentation of eccentric accretion disks
We consider the effects of eccentricity on the fragmentation of
gravitationally unstable accretion disks, using numerical hydrodynamics. We
find that eccentricity does not affect the overall stability of the disk
against fragmentation, but significantly alters the manner in which such
fragments accrete gas. Variable tidal forces around an eccentric orbit slow the
accretion process, and suppress the formation of weakly-bound clumps. The
"stellar" mass function resulting from the fragmentation of an eccentric disk
is found to have a significantly higher characteristic mass than that from a
corresponding circular disk. We discuss our results in terms of the disk(s) of
massive stars at ~0.1pc from the Galactic Center, and find that the
fragmentation of an eccentric accretion disk, due to gravitational instability,
is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap
Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre
Observations of massive stars within the central parsec of the Galaxy show
that, while most stars orbit within a well-defined disc, a significant fraction
have large eccentricities and / or inclinations with respect to the disc plane.
Here, we investigate whether this dynamically hot component could have arisen
via scattering from an initially cold disc -- the expected initial condition if
the stars formed from the fragmentation of an accretion disc. Using N-body
methods, we evolve a variety of flat, cold, stellar systems, and study the
effects of initial disc eccentricity, primordial binaries, very massive stars
and intermediate mass black holes. We find, consistent with previous results,
that a circular disc does not become eccentric enough unless there is a
significant population of undetected 100--1000 Msun objects. However, since
fragmentation of an eccentric disc can readily yield eccentric stellar orbits,
the strongest constraints come from inclinations. We show that_none_ of our
initial conditions yield the observed large inclinations, regardless of the
initial disc eccentricity or the presence of massive objects. These results
imply that the orbits of the young massive stars in the Galactic Centre are
largely primordial, and that the stars are unlikely to have formed as a
dynamically cold disc.Comment: 5 pages, 6 colour figures. MNRAS Letters in press. (v2: very minor
changes
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Air-Stimulated ATP Release from Keratinocytes Occurs through Connexin Hemichannels
Cutaneous ATP release plays an important role in both epidermal stratification and chronic pain, but little is known about ATP release mechanisms in keratinocytes that comprise the epidermis. In this study, we analyzed ATP release from cultured human neonatal keratinocytes briefly exposed to air, a process previously demonstrated to trigger ATP release from these cells. We show that exposing keratinocytes to air by removing media for 15 seconds causes a robust, long-lasting ATP release. This air-stimulated ATP release was increased in calcium differentiated cultures which showed a corresponding increase in connexin 43 mRNA, a major component of keratinocyte hemichannels. The known connexin hemichannel inhibitors 1-octanol and carbenoxolone both significantly reduced air-stimulated ATP release, as did two drugs traditionally used as ABC transporter inhibitors (glibenclamide and verapamil). These same 4 inhibitors also prevented an increase in the uptake of a connexin permeable dye induced by air exposure, confirming that connexin hemichannels are open during air-stimulated ATP release. In contrast, activity of the MDR1 ABC transporter was reduced by air exposure and the drugs that inhibited air-stimulated ATP release had differential effects on this transporter. These results indicate that air exposure elicits non-vesicular release of ATP from keratinocytes through connexin hemichannels and that drugs used to target connexin hemichannels and ABC transporters may cross-inhibit. Connexins represent a novel, peripheral target for the treatment of chronic pain and dermatological disease
Dust filtration at gap edges: Implications for the spectral energy distributions of discs with embedded planets
The spectral energy distributions (SEDs) of some T Tauri stars display a
deficit of near-IR flux that could be a consequence of an embedded Jupiter-mass
planet partially clearing an inner hole in the circumstellar disc. Here, we use
two-dimensional numerical simulations of the planet-disc interaction, in
concert with simple models for the dust dynamics, to quantify how a planet
influences the dust at different radii within the disc. We show that pressure
gradients at the outer edge of the gap cleared by the planet act as a filter -
letting particles smaller than a critical size through to the inner disc while
holding back larger particles in the outer disc. The critical particle size
depends upon the disc properties, but is typically of the order of 10 microns.
This filtration process will lead to discontinuous grain populations across the
planet's orbital radius, with small grains in the inner disc and an outer
population of larger grains. We show that this type of dust population is
qualitatively consistent with SED modelling of systems that have optically thin
inner holes in their circumstellar discs. This process can also produce a very
large gas-to-dust ratio in the inner disc, potentially explaining those systems
with optically thin inner cavities that still have relatively high accretion
rates.Comment: 9 pages, 7 figures, Accepted fir publication in MNRA
Dynamic ductile to brittle transition in a one-dimensional model of viscoplasticity
We study two closely related, nonlinear models of a viscoplastic solid. These
models capture essential features of plasticity over a wide range of strain
rates and applied stresses. They exhibit inelastic strain relaxation and steady
flow above a well defined yield stress. In this paper, we describe a first step
in exploring the implications of these models for theories of fracture and
related phenomena. We consider a one dimensional problem of decohesion from a
substrate of a membrane that obeys the viscoplastic constitutive equations that
we have constructed. We find that, quite generally, when the yield stress
becomes smaller than some threshold value, the energy required for steady
decohesion becomes a non-monotonic function of the decohesion speed. As a
consequence, steady state decohesion at certain speeds becomes unstable. We
believe that these results are relevant to understanding the ductile to brittle
transition as well as fracture stability.Comment: 10 pages, REVTeX, 12 postscript figure
Excitonic condensate and quasiparticle transport in electron-hole bilayer systems
Bilayer electron-hole systems undergo excitonic condensation when the
distance d between the layers is smaller than the typical distance between
particles within a layer. All excitons in this condensate have a fixed dipole
moment which points perpendicular to the layers, and therefore this condensate
of dipoles couples to external electromagnetic fields. We study the transport
properties of this dipolar condensate system based on a phenomenological model
which takes into account contributions from the condensate and quasiparticles.
We discuss, in particular, the drag and counterflow transport, in-plane
Josephson effect, and noise in the in-plane currents in the condensate state
which provides a direct measure of the superfluid collective-mode velocity.Comment: 7 pages, 3 figure
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