7,215 research outputs found
Linear and nonlinear analysis of orbital telescope/space shuttle dynamics and control
Work completed on the design and study of an annular suspension and pointing (ASP) system for the space shuttle was presented. This system makes use of a magnetically suspended vernier pointing assembly. The following objectives were pursued in this study: (1) development of a detailed mathematical model of the Space Shuttle/ASP system, (2) design of control laws in order to obtain the desired pointing performance, and (3) prediction of the statistical pointing accuracies in the presence of stochastic disturbances such as crew-motion, and sensor and actuator noise. The first two of these objectives are documented in this report
Instability of black hole formation under small pressure perturbations
We investigate here the spectrum of gravitational collapse endstates when
arbitrarily small perfect fluid pressures are introduced in the classic black
hole formation scenario as described by Oppenheimer, Snyder and Datt (OSD) [1].
This extends a previous result on tangential pressures [2] to the more
physically realistic scenario of perfect fluid collapse. The existence of
classes of pressure perturbations is shown explicitly, which has the property
that injecting any smallest pressure changes the final fate of the dynamical
collapse from a black hole to a naked singularity. It is therefore seen that
any smallest neighborhood of the OSD model, in the space of initial data,
contains collapse evolutions that go to a naked singularity outcome. This gives
an intriguing insight on the nature of naked singularity formation in
gravitational collapse.Comment: 7 pages, 1 figure, several modifications to match published version
on GR
Development of cavernous haemangioma following radical chemo-radiotherapy for nasopharyngeal carcinoma
Objective: We report an adult case of nasopharyngeal carcinoma treated with radical chemo-radiotherapy, with subsequent development of a histologically proved temporal cavernous haemangioma within the radiation field. Method: Case report and review of the current literature concerning radiation-induced, secondary, space-occupying lesions. Conclusion: The increasing role of radiotherapy in nasopharyngeal carcinoma treatment, together with improved patient survival, is likely to lead to radiation-induced, secondary, space-occupying lesions being encountered more frequently. We emphasise the need to be vigilant for this important but relatively rare complication, which has significant associated morbidity
Temperature dependence of transport spin polarization in NdNi5 measured using Point Contact Andreev reflection
We report a study in which Point contact Andreev reflection (PCAR)
spectroscopy using superconducting Nb tip has been carried out on NdNi5, a
ferromagnet with a Curie temperature of TC~7.7K. The measurements were carried
out over a temperature range of 2-9K which spans across the ferromagnetic
transition temperature. From an analysis of the spectra, we show that (i) the
temperature dependence of the extracted value of transport spin polarization
closely follows the temperature dependence of the spontaneous magnetization;
(ii) the superconducting quasiparticle lifetime shows a large decrease close to
the Curie temperature of the ferromagnet. We attribute the latter to the
presence of strong ferromagnetic spin fluctuations in the ferromagnet close to
the ferromagnetic transition temperature.Comment: pdf file including figures-Typographical error and errors in
references correcte
Library of Microcrystalline Tests for Novel Psychoactive Substances
A microcrystalline test is a precipitation reaction between a drug and a reagent, forming an insoluble drug-reagent complex that is unique to that specific test. These tests are quick, requiring minimal sample preparation and can be non-destructive. Therefore, they can be used as preliminary and confirmatory tests with expertise. Microcrystalline tests are one of the oldest analytical chemistry practices and their use for classic drugs such as cocaine, heroin and amphetamines is well-documented. However, there is very limited research on microcrystalline tests for the novel compounds encountered by law enforcement today. This research is an effort to increase understanding and promote use of microcrystalline tests for novel psychoactive substances
A Type-Theoretic Account of Neg-Raising Predicates in Tree Adjoining Grammars
International audienceNeg-Raising (NR) verbs form a class of verbs with a clausal complement that show the following behavior: when a negation syntactically attaches to the matrix predicate, it can semantically attach to the embedded predicate. This paper presents an account of NR predicates within Tree Adjoining Grammar (TAG). We propose a lexical semantic interpretation that heavily relies on a Montague-like semantics for TAG and on higher-order types
Strong correlations between text quality and complex networks features
Concepts of complex networks have been used to obtain metrics that were
correlated to text quality established by scores assigned by human judges.
Texts produced by high-school students in Portuguese were represented as
scale-free networks (word adjacency model), from which typical network features
such as the in/outdegree, clustering coefficient and shortest path were
obtained. Another metric was derived from the dynamics of the network growth,
based on the variation of the number of connected components. The scores
assigned by the human judges according to three text quality criteria
(coherence and cohesion, adherence to standard writing conventions and theme
adequacy/development) were correlated with the network measurements. Text
quality for all three criteria was found to decrease with increasing average
values of outdegrees, clustering coefficient and deviation from the dynamics of
network growth. Among the criteria employed, cohesion and coherence showed the
strongest correlation, which probably indicates that the network measurements
are able to capture how the text is developed in terms of the concepts
represented by the nodes in the networks. Though based on a particular set of
texts and specific language, the results presented here point to potential
applications in other instances of text analysis.Comment: 8 pages, 8 figure
Radiating black holes in Einstein-Yang-Mills theory and cosmic censorship
Exact nonstatic spherically symmetric black-hole solution of the higher
dimensional Einstein-Yang-Mills equations for a null dust with Yang-Mills gauge
charge are obtained by employing Wu-Yang \textit{ansatz}, namely, HD-EYM Vaidya
solution. It is interesting to note that gravitational contribution of YM gauge
charge for this ansatz is indeed opposite (attractive rather than repulsive)
that of Maxwell charge. It turns out that the gravitational collapse of null
dust with YM gauge charge admit strong curvature shell focusing naked
singularities violating cosmic censorship. However, there is significant
shrinkage of the initial data space for a naked singularity of the HD-Vaidya
collapse due to presence of YM gauge charge. The effect of YM gauge charge on
structure and location of the apparent and event horizons is also discussed.Comment: 9 RevTeX pages, 1 figur
Mass Segregation in Globular Clusters
We present the results of a new study of mass segregation in two-component
star clusters, based on a large number of numerical N-body simulations using
our recently developed dynamical Monte Carlo code. Specifically, we follow the
dynamical evolution of clusters containing stars with individual masses m_1 as
well as a tracer population of objects with individual masses m_2=\mu m_1,
using N=10^5 total stars. For heavy tracers, which could represent stellar
remnants such as neutron stars or black holes in a globular cluster, we
characterize in a variety of ways the tendency for these objects to concentrate
in or near the cluster core. In agreement with simple theoretical arguments, we
find that the characteristic time for this mass segregation process varies as
1/\mu. For models with very light tracers (\mu <~ 10^-2), which could represent
free-floating planets or brown dwarfs, we find the expected depletion of light
objects in the cluster core, but also sometimes a significant enhancement in
the halo. Using these results we estimate the optical depth to gravitational
microlensing by planetary mass objects or brown dwarfs in typical globular
clusters. For some initial conditions, the optical depth in the halo due to
very low-mass objects could be much greater than that of luminous stars. If we
apply our results to M22, using the recent null detection of Sahu, Anderson, &
King (2001), we find an upper limit of ~25% at the 63% confidence level for the
current mass fraction of M22 in the form of very low-mass objects.Comment: Accepted for publication in ApJ. Minor revisions reflecting the new
results of Sahu et al. on M22. 13 pages in emulateapj style, including 9
figures and 3 table
Binaries and Globular Cluster Dynamics
We summarize the results of recent theoretical work on the dynamical
evolution of globular clusters containing primordial binaries. Even a very
small initial binary fraction (e.g., 10%) can play a key role in supporting a
cluster against gravothermal collapse for many relaxation times. Inelastic
encounters between binaries and single stars or other binaries provide a very
significant energy source for the cluster. These dynamical interactions also
lead to the production of large numbers of exotic systems such as ultracompact
X-ray binaries, recycled radio pulsars, double degenerate systems, and blue
stragglers. Our work is based on a new parallel supercomputer code implementing
Henon's Monte Carlo method for simulating the dynamical evolution of dense
stellar systems in the Fokker-Planck approximation. This new code allows us to
calculate very accurately the evolution of a cluster containing a realistic
number of stars (N ~ 10^5 - 10^6) in typically a few hours to a few days of
computing time. The discrete, star-by-star representation of the cluster in the
simulation makes it possible to treat naturally a number of important
processes, including single and binary star evolution, all dynamical
interactions of single stars and binaries, and tidal interactions with the
Galaxy.Comment: 15 pages, to appear in `The Influence of Binaries on Stellar
Population Studies', ed. D. Vanbeveren (Kluwer
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