21,542 research outputs found
A twin-mirrored galvanometer laser light sheet generator
A galvanometer mirror-based laser light sheet system has been developed for use in the Basic Aerodynamics Research Tunnel at NASA Langley. This system generates and positions single or multiple light sheets over aeronautical research models being tested in the low speed tunnel. This report describes a twin mirrored galvanometer laser light sheet generator and shows typical light sheet arrangements in use. With this system, illumination of smoke entrained in the flow over a delta wing model reveals the vortical flow produced by the separation of the flow at the leading edge of the model. The light sheet system has proven to be very adaptable and easy to use in sizing and positioning light sheets in wind tunnel applications
Employment Changes Play Major Role in Access to Employer Health Coverage
Highlights findings on the factors that drive short-term changes in employer-sponsored health insurance coverage, including the rising cost of health insurance and changes in employment rates and availability of better jobs during macroeconomic cycles
Keplerian Motion of Broad-Line Region Gas as Evidence for Supermassive Black Holes in Active Galactic Nuclei
Emission-line variability data on NGC 5548 argue strongly for the existence
of a mass of order 7 x 10^7 solar masses within the inner few light days of the
nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the
emission lines to continuum variations is used to infer the size of the
line-emitting region, and these determinations are combined with measurements
of the Doppler widths of the variable line components to estimate a virial
mass. The data for several different emission lines spanning an order of
magnitude in distance from the central source show the expected V proportional
to r^{-1/2} correlation and are consistent with a single value for the mass.Comment: 9 pages, 2 Figures. accepted by ApJ Letter
Ground-State Properties of a Rotating Bose-Einstein Condensate with Attractive Interaction
The ground state of a rotating Bose-Einstein condensate with attractive
interaction in a quasi-one-dimensional torus is studied in terms of the ratio
of the mean-field interaction energy per particle to the
single-particle energy-level spacing. The plateaus of quantized circulation are
found to appear if and only if with the lengths of the plateaus
reduced due to hybridization of the condensate over different angular-momentum
states.Comment: 4 pages, 2 figures, Accepted for publication in Physical Reveiw
Letter
A Dusty Disk Around WD1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks
We report the discovery of excess K-band radiation from a metal-rich DAV
white dwarf star, WD1150-153. Our near infrared spectroscopic observations show
that the excess radiation cannot be explained by a (sub)stellar companion, and
is likely to be caused by a debris disk similar to the other DAZ white dwarfs
with circumstellar debris disks. We find that the fraction of DAZ white dwarfs
with detectable debris disks is at least 14%. We also revisit the problem of
explaining the metals in white dwarf photospheres by accretion from the
interstellar medium (ISM). We use the observed interstellar column densities
toward stars in close angular proximity and similar distance as DAZ white
dwarfs to constrain the contribution of accretion from the ISM. We find no
correlation between the accretion density required to supply metals observed in
DAZs with the densities observed in their interstellar environment, indicating
that ISM accretion alone cannot explain the presence of metals in nearby DAZ
white dwarfs. Although ISM accretion will certainly contribute, our analysis
indicates that it is not the dominant source of metals for most DAZ white
dwarfs. Instead, the growing number of circumstellar debris disks around DAZs
suggests that circumstellar material may play a more dominant role in polluting
the white dwarf atmospheres.Comment: ApJ, in pres
Mumford dendrograms and discrete p-adic symmetries
In this article, we present an effective encoding of dendrograms by embedding
them into the Bruhat-Tits trees associated to -adic number fields. As an
application, we show how strings over a finite alphabet can be encoded in
cyclotomic extensions of and discuss -adic DNA encoding. The
application leads to fast -adic agglomerative hierarchic algorithms similar
to the ones recently used e.g. by A. Khrennikov and others. From the viewpoint
of -adic geometry, to encode a dendrogram in a -adic field means
to fix a set of -rational punctures on the -adic projective line
. To is associated in a natural way a
subtree inside the Bruhat-Tits tree which recovers , a method first used by
F. Kato in 1999 in the classification of discrete subgroups of
.
Next, we show how the -adic moduli space of
with punctures can be applied to the study of time series of
dendrograms and those symmetries arising from hyperbolic actions on
. In this way, we can associate to certain classes of dynamical
systems a Mumford curve, i.e. a -adic algebraic curve with totally
degenerate reduction modulo .
Finally, we indicate some of our results in the study of general discrete
actions on , and their relation to -adic Hurwitz spaces.Comment: 14 pages, 6 figure
Axistationary perfect fluids -- a tetrad approach
Stationary axisymmetric perfect fluid space-times are investigated using the
curvature description of geometries. Attention is focused on space-times with a
vanishing electric part of the Weyl tensor. It is shown that the only
incompressible axistationary magnetic perfect fluid is the interior
Schwarzschild solution. The existence of a rigidly rotating perfect fluid,
generalizing the interior Schwarzschild metric is proven. Theorems are stated
on Petrov types and electric/magnetic Weyl tensors.Comment: 12 page
Ranking Templates for Linear Loops
We present a new method for the constraint-based synthesis of termination
arguments for linear loop programs based on linear ranking templates. Linear
ranking templates are parametrized, well-founded relations such that an
assignment to the parameters gives rise to a ranking function. This approach
generalizes existing methods and enables us to use templates for many different
ranking functions with affine-linear components. We discuss templates for
multiphase, piecewise, and lexicographic ranking functions. Because these
ranking templates require both strict and non-strict inequalities, we use
Motzkin's Transposition Theorem instead of Farkas Lemma to transform the
generated -constraint into an -constraint.Comment: TACAS 201
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