558 research outputs found
Landau-deGennes Theory of Biaxial Nematics Re-examined
Recent experiments report that the long looked for thermotropic biaxial
nematic phase has been finally detected in some thermotropic liquid crystalline
systems. Inspired by these experimental observations we concentrate on some
elementary theoretical issues concerned with the classical sixth-order
Landau-deGennes free energy expansion in terms of the symmetric and traceless
tensor order parameter . In particular, we fully explore the
stability of the biaxial nematic phase giving analytical solutions for all
distinct classes of the phase diagrams that theory allows. This includes
diagrams with triple- and (tri-)critical points and with multiple (reentrant)
biaxial- and uniaxial phase transitions. A brief comparison with predictions of
existing molecular theories is also given.Comment: 12 pages, 14 figure
Chiral Symmetry Breaking in Bent-Core Liquid Crystals
By molecular modeling we demonstrate that the nematic long-range order
discovered in bent-core liquid crystal systems should reveal further spatially
homogeneous phases. Two of them are identified as a tetrahedratic nematic
() phase with symmetry and a chiral tetrahedratic nematic
() phase with symmetry. These new phases were found for a lattice
model with quadrupolar and octupolar anisotropic interactions using Mean Field
theory and Monte Carlo simulations. The phase diagrams exhibit tetrahedratic
(), and phases, in addition to ordinary isotropic (),
uniaxial nematic () and biaxial nematic () phases. To our knowledge,
this is the first molecular model with spontaneous chiral symmetry breaking in
non-layered systems.Comment: 12 pages, 4 figures, submitted for publicatio
A Dynamical Study of the Black Hole X-ray Binary Nova Muscae 1991
We present a dynamical study of the Galactic black hole binary system Nova
Muscae 1991 (GS/GRS 1124-683). We utilize 72 high resolution Magellan
Echellette (MagE) spectra and 72 strictly simultaneous V-band photometric
observations; the simultaneity is a unique and crucial feature of this
dynamical study. The data were taken on two consecutive nights and cover the
full 10.4-hour orbital cycle. The radial velocities of the secondary star are
determined by cross-correlating the object spectra with the best-match template
spectrum obtained using the same instrument configuration. Based on our
independent analysis of five orders of the echellette spectrum, the
semi-amplitude of the radial velocity of the secondary is measured to be K_2 =
406.8+/-2.7 km/s, which is consistent with previous work, while the uncertainty
is reduced by a factor of 3. The corresponding mass function is f(M) =
3.02+/-0.06 M_\odot. We have also obtained an accurate measurement of the
rotational broadening of the stellar absorption lines (v sin i = 85.0+/-2.6
km/s) and hence the mass ratio of the system q = 0.079+/-0.007. Finally, we
have measured the spectrum of the non-stellar component of emission that veils
the spectrum of the secondary. In a future paper, we will use our
veiling-corrected spectrum of the secondary and accurate values of K_2 and q to
model multi-color light curves and determine the systemic inclination and the
mass of the black hole.Comment: ApJ accepted version; minor revision; added a subsection about
systematic uncertaintie
First astronomical unit scale image of the GW Ori triple. Direct detection of a new stellar companion
Young and close multiple systems are unique laboratories to probe the initial
dynamical interactions between forming stellar systems and their dust and gas
environment. Their study is a key building block to understanding the high
frequency of main-sequence multiple systems. However, the number of detected
spectroscopic young multiple systems that allow dynamical studies is limited.
GW Orionis is one such system. It is one of the brightest young T Tauri stars
and is surrounded by a massive disk. Our goal is to probe the GW Orionis
multiplicity at angular scales at which we can spatially resolve the orbit. We
used the IOTA/IONIC3 interferometer to probe the environment of GW Orionis with
an astronomical unit resolution in 2003, 2004, and 2005. By measuring squared
visibilities and closure phases with a good UV coverage we carry out the first
image reconstruction of GW Ori from infrared long-baseline interferometry. We
obtain the first infrared image of a T Tauri multiple system with astronomical
unit resolution. We show that GW Orionis is a triple system, resolve for the
first time the previously known inner pair (separation 1.4 AU) and
reveal a new more distant component (GW Ori C) with a projected separation of
8 AU with direct evidence of motion. Furthermore, the nearly equal (2:1)
H-band flux ratio of the inner components suggests that either GW Ori B is
undergoing a preferential accretion event that increases its disk luminosity or
that the estimate of the masses has to be revisited in favour of a more equal
mass-ratio system that is seen at lower inclination. Accretion disk models of
GW Ori will need to be completely reconsidered because of this outer companion
C and the unexpected brightness of companion B.Comment: 5 pages, 9 figures, accepted Astronomy and Astrophysics Letters. 201
Graph Neural Networks for temporal graphs: State of the art, open challenges, and opportunities
Graph Neural Networks (GNNs) have become the leading paradigm for learning on (static) graph-structured data. However, many real-world systems are dynamic in nature, since the graph and node/edge attributes change over time. In recent years, GNN-based models for temporal graphs have emerged as a promising area of research to extend the capabilities
of GNNs. In this work, we provide the first comprehensive overview of the current stateof-the-art of temporal GNN, introducing a rigorous formalization of learning settings and tasks and a novel taxonomy categorizing existing approaches in terms of how the temporal aspect is represented and processed. We conclude the survey with a discussion of the most relevant open challenges for the field, from both research and application perspectives
Roundoff-induced Coalescence of Chaotic Trajectories
Numerical experiments recently discussed in the literature show that
identical nonlinear chaotic systems linked by a common noise term (or signal)
may synchronize after a finite time. We study the process of synchronization as
function of precision of calculations. Two generic behaviors of the average
coalescence time are identified: exponential or linear. In both cases no
synchronization occurs if iterations are done with {\em infinite} precision.Comment: 6 pages, 3 postscript figures, to be published in Phys. Rev.
Cryptostroma corticale: malattia della corteccia fuligginosa dell'acero
Le segnalazioni di questa malattia che colpisce il genere Acer sono in crescita in tutta Europa. La sua recrudescenza pare legata al riscaldamento del clima. Oltre a portare a morte le piante e a comprometterne la stabilità meccanica, il fungo ascomicete che ne è causa, Cryptostoma corticale, minaccia la salute umana producendo una grande quantità di spore aeree che possono causare gravi infiammazioni polmonari
The structure of the distortion free-energy density in nematics: second-order elasticity and surface terms
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