2,282 research outputs found
The interplay between radiation pressure and the photoelectric instability in optically thin disks of gas and dust
Previous theoretical works have shown that in optically thin disks, dust
grains are photoelectrically stripped of electrons by starlight, heating nearby
gas and possibly creating a dust clumping instability, the photoelectric
instability (PeI), that significantly alters global disk structure. In the
current work, we use the Pencil Code to perform the first numerical models of
the PeI that include stellar radiation pressure on dust grains in order to
explore the parameter regime in which the instability operates. In models with
gas surface densities greater than ,
we see a variety of dust structures, including sharp concentric rings and
non-axisymmetric arcs and clumps that represent dust surface density
enhancements of factors of depending on the run parameters. The
gas distributions show various structures as well, including clumps and arcs
formed from spiral arms. In models with lower gas surface densities, vortices
and smooth spiral arms form in the gas distribution, but the dust is too weakly
coupled to the gas to be significantly perturbed. In one high gas surface
density model, we include a large, low-order gas viscosity, and, in agreement
with previous radiation pressure-free models, find that it observably smooths
the structures that form in the gas and dust, suggesting that resolved images
of a given disk may be useful for deriving constraints on the effective
viscosity of its gas. Broadly, our models show that radiation pressure does not
preclude the formation of complex structure from the PeI, but the qualitative
manifestation of the PeI depends strongly on the parameters of the system. The
PeI may provide an explanation for unusual disk morphologies such as the moving
blobs of the AU Mic disk, the asymmetric dust distribution of the 49 Ceti disk,
and the rings and arcs found in the disk around HD 141569A.Comment: 13 pages, 13 figures; submitted to Ap
On shocks driven by high-mass planets in radiatively inefficient disks. I. Two-dimensional global disk simulations
Recent observations of gaps and non-axisymmetric features in the dust
distributions of transition disks have been interpreted as evidence of embedded
massive protoplanets. However, comparing the predictions of planet-disk
interaction models to the observed features has shown far from perfect
agreement. This may be due to the strong approximations used for the
predictions. For example, spiral arm fitting typically uses results that are
based on low-mass planets in an isothermal gas. In this work, we describe
two-dimensional, global, hydrodynamical simulations of disks with embedded
protoplanets, with and without the assumption of local isothermality, for a
range of planet-to-star mass ratios 1-10 M_jup for a 1 M_sun star. We use the
Pencil Code in polar coordinates for our models. We find that the inner and
outer spiral wakes of massive protoplanets (M>5 M_jup) produce significant
shock heating that can trigger buoyant instabilities. These drive sustained
turbulence throughout the disk when they occur. The strength of this effect
depends strongly on the mass of the planet and the thermal relaxation
timescale; for a 10 M_jup planet embedded in a thin, purely adiabatic disk, the
spirals, gaps, and vortices typically associated with planet-disk interactions
are disrupted. We find that the effect is only weakly dependent on the initial
radial temperature profile. The spirals that form in disks heated by the
effects we have described may fit the spiral structures observed in transition
disks better than the spirals predicted by linear isothermal theory.Comment: 10 pages, 8 figures. ApJ, accepte
Dynamical and quasistatic structural relaxation paths in Pd_(40)Ni_(40)P_(20) glass
By sequential heat treatment of a Pd_(40)Ni_(40)P_(20) metallic glass at temperatures and durations for which
α-relaxation is not possible, dynamic, and quasistatic relaxation paths below the glass transition are
identified via ex situ ultrasonic measurements following each heat treatment. The dynamic
relaxation paths are associated with hopping between nonequilibrium potential energy states of the
glass, while the quasistatic relaxation path is associated with reversible β-relaxation events toward
quasiequilibrium states. These quasiequilibrium states are identified with secondary potential energy
minima that exist within the inherent energy minimum of the glass, thereby supporting the concept
of the sub-basin/metabasin organization of the potential-energy landscape
Comparison of the IOFFE neutral particle analyser with the Princeton analyser on the Mega Amp Spherical Tokamak
On the relationship between structure and dynamics in a supercooled liquid
We present the dynamic propensity distribution as an explicit measure of the
degree to which the dynamics in a liquid over the time scale of structural
relaxation is determined by the initial configuration. We then examine, for a
binary mixture of soft discs in two dimensions, the correlation between the
spatial distribution of propensity and that of two localmeasures of
configuration structure: the local composition and local free volume. While the
small particles dominate the high propensity population,we find no strong
correlation between either the local composition or the local free volume and
the propensity. It is argued that this is a generic failure of purely local
structural measures to capture the inherently non-local character of collective
behaviour.Comment: Published, see below or
http://www.iop.org/EJ/abstract/0953-8984/17/49/001/ Editing comments have
been remove
Hole-burning experiments within solvable glassy models
We reproduce the results of non-resonant spectral hole-burning experiments
with fully-connected (equivalently infinite-dimensional) glassy models that are
generalizations of the mode-coupling approach to nonequilibrium situations. We
show that an ac-field modifies the integrated linear response and the
correlation function in a way that depends on the amplitude and frequency of
the pumping field. We study the effect of the waiting and recovery-times and
the number of oscillations applied. This calculation will help descriminating
which results can and which cannot be attributed to dynamic heterogeneities in
real systems.Comment: 4 pages, 8 figures, RevTe
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