18,785 research outputs found
Phase Transitions in Lyotropic Nematic Gels
In this paper, we discuss the equilibrium phases and collapse transitions of
a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel
consists of a cross-linked polymer network with rod-like molecules embedded in
it. Upon decreasing the quality of the solvent, we find that a lyotropic
nematic gel undergoes a discontinuous volume change accompanied by an
isotropic-nematic transition. We also present phase diagrams that these systems
may exhibit. In particular, we show that coexistence of two isotropic phases,
of two nematic phases, or of an isotropic and a nematic phase can occur.Comment: 13 pages Revtex, 10 figures, submitted to EPJ
The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction
Exploring the ICM power spectrum can help us to probe the physics of galaxy
clusters. Using high-resolution 3D plasma simulations, we study the statistics
of the velocity field and its relation with the thermodynamic perturbations.
The normalization of the ICM spectrum (density, entropy, or pressure) is
linearly tied to the level of large-scale motions, which excite both gravity
and sound waves due to stratification. For low 3D Mach number M~0.25, gravity
waves mainly drive entropy perturbations, traced by preferentially tangential
turbulence. For M>0.5, sound waves start to significantly contribute, passing
the leading role to compressive pressure fluctuations, associated with
isotropic (or slightly radial) turbulence. Density and temperature fluctuations
are then characterized by the dominant process: isobaric (low M), adiabatic
(high M), or isothermal (strong conduction). Most clusters reside in the
intermediate regime, showing a mixture of gravity and sound waves, hence
drifting towards isotropic velocities. Remarkably, regardless of the regime,
the variance of density perturbations is comparable to the 1D Mach number. This
linear relation allows to easily convert between gas motions and ICM
perturbations, which can be exploited by Chandra, XMM data and by the
forthcoming Astro-H. At intermediate and small scales (10-100 kpc), the
turbulent velocities develop a Kolmogorov cascade. The thermodynamic
perturbations act as effective tracers of the velocity field, broadly
consistent with the Kolmogorov-Obukhov-Corrsin advection theory. Thermal
conduction acts to damp the gas fluctuations, washing out the filamentary
structures and steepening the spectrum, while leaving unaltered the velocity
cascade. The ratio of the velocity and density spectrum thus inverts the
downtrend shown by the non-diffusive models, allowing to probe the presence of
significant conductivity in the ICM.Comment: Accepted by A&A; 15 pages, 10 figures; added insights and references
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Internal Gravity Waves Modulate the Apparent Misalignment of Exoplanets around Hot Stars
We propose that the observed misalignment between extra-solar planets and
their hot host stars can be explained by angular momentum transport within the
host star. Observations have shown that this misalignment is preferentially
around hot stars, which have convective cores and extended radiative envelopes.
This situation is amenable to substantial angular momentum transport by
internal gravity waves (IGW) generated at the convective-radiative interface.
Here we present numerical simulations of this process and show that IGW can
modulate the surface rotation of the star. With these two- dimensional
simulations we show that IGW could explain the retrograde orbits observed in
systems such as HAT-P-6 and HAT-P-7, however, extension to high obliquity
objects will await future three- dimensional simulations. We note that these
results also imply that individual massive stars should show temporal
variations in their v sini measurements.Comment: 6 pages, 2 figures, Accepted for publication in ApJ
Combining Thesaurus Knowledge and Probabilistic Topic Models
In this paper we present the approach of introducing thesaurus knowledge into
probabilistic topic models. The main idea of the approach is based on the
assumption that the frequencies of semantically related words and phrases,
which are met in the same texts, should be enhanced: this action leads to their
larger contribution into topics found in these texts. We have conducted
experiments with several thesauri and found that for improving topic models, it
is useful to utilize domain-specific knowledge. If a general thesaurus, such as
WordNet, is used, the thesaurus-based improvement of topic models can be
achieved with excluding hyponymy relations in combined topic models.Comment: Accepted to AIST-2017 conference (http://aistconf.ru/). The final
publication will be available at link.springer.co
Quantifying properties of ICM inhomogeneities
We present a new method to identify and characterize the structure of the
intracluster medium (ICM) in simulated galaxy clusters. The method uses the
median of gas properties, such as density and pressure, which we show to be
very robust to the presence of gas inhomogeneities. In particular, we show that
the radial profiles of median gas properties are smooth and do not exhibit
fluctuations at locations of massive clumps in contrast to mean and mode
properties. It is shown that distribution of gas properties in a given radial
shell can be well described by a log-normal PDF and a tail. The former
corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the
volume, while the tail corresponds to high density inhomogeneities. We show
that this results in a simple and robust separation of the diffuse and clumpy
components of the ICM. The FWHM of the density distribution grows with radius
and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed
clusters. The small scatter in the width between relaxed clusters suggests that
the degree of inhomogeneity is a robust characteristic of the ICM. It broadly
agrees with the amplitude of density perturbations in the Coma cluster. We
discuss the origin of ICM density variations in spherical shells and show that
less than 20% of the width can be attributed to the triaxiality of the cluster
gravitational potential. As a link to X-ray observations of real clusters we
evaluated the ICM clumping factor with and without high density
inhomogeneities. We argue that these two cases represent upper and lower limits
on the departure of the observed X-ray emissivity from the median value. We
find that the typical value of the clumping factor in the bulk component of
relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in
broad agreement with recent observations.Comment: 16 pages, 12 figure, accepted to MNRA
On the Canonical Reduction of Spherically Symmetric Gravity
In a thorough paper Kuchar has examined the canonical reduction of the most
general action functional describing the geometrodynamics of the maximally
extended Schwarzschild geometry. This reduction yields the true degrees of
freedom for (vacuum) spherically symmetric general relativity. The essential
technical ingredient in Kuchar's analysis is a canonical transformation to a
certain chart on the gravitational phase space which features the Schwarzschild
mass parameter , expressed in terms of what are essentially
Arnowitt-Deser-Misner variables, as a canonical coordinate. In this paper we
discuss the geometric interpretation of Kuchar's canonical transformation in
terms of the theory of quasilocal energy-momentum in general relativity given
by Brown and York. We find Kuchar's transformation to be a ``sphere-dependent
boost to the rest frame," where the ``rest frame'' is defined by vanishing
quasilocal momentum. Furthermore, our formalism is general enough to cover the
case of (vacuum) two-dimensional dilaton gravity. Therefore, besides reviewing
Kucha\v{r}'s original work for Schwarzschild black holes from the framework of
hyperbolic geometry, we present new results concerning the canonical reduction
of Witten-black-hole geometrodynamics.Comment: Revtex, 35 pages, no figure
Entanglement and the nonlinear elastic behavior of forests of coiled carbon nanotubes
Helical or coiled nanostructures have been object of intense experimental and
theoretical studies due to their special electronic and mechanical properties.
Recently, it was experimentally reported that the dynamical response of
foamlike forest of coiled carbon nanotubes under mechanical impact exhibits a
nonlinear, non-Hertzian behavior, with no trace of plastic deformation. The
physical origin of this unusual behavior is not yet fully understood. In this
work, based on analytical models, we show that the entanglement among
neighboring coils in the superior part of the forest surface must be taken into
account for a full description of the strongly nonlinear behavior of the impact
response of a drop-ball onto a forest of coiled carbon nanotubes.Comment: 4 pages, 3 figure
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