13,172 research outputs found
A hybrid approach to space power control
Conventional control systems have traditionally been utilized for space-based power designs. However, the use of expert systems is becoming important for NASA applications. Rocketdyne has been pursuing the development of expert systems to aid and enhance control designs of space-based power systems. The need for integrated expert systems is vital for the development of autonomous power systems
Vortex crystals
Vortex crystals is one name in use for the subject of vortex patterns that move without change of shape or size. Most of what is known pertains to the case of arrays of parallel line vortices moving so as to produce an essentially two-dimensional flow. The possible patterns of points indicating the intersections of these vortices with a plane perpendicular to them have been studied for almost 150 years. Analog experiments have been devised, and experiments with vortices in a variety of fluids have been performed. Some of the states observed are understood analytically. Others have been found computationally to high precision. Our degree of understanding of these patterns varies considerably. Surprising connections to the zeros of 'special functions' arising in classical mathematical physics have been revealed. Vortex motion on two-dimensional manifolds, such as the sphere, the cylinder (periodic strip) and torus (periodic parallelogram) has also been studied, because of the potential applications, and some results are available regarding the problem of vortex crystals in such geometries. Although a large amount of material is available for review, some results are reported here for the first time. The subject seems pregnant with possibilities for further development.published or submitted for publicationis peer reviewe
Potentials for which the Radial Schr\"odinger Equation can be solved
In a previous paper, submitted to Journal of Physics A -- we presented an
infinite class of potentials for which the radial Schr\"odinger equation at
zero energy can be solved explicitely. For part of them, the angular momentum
must be zero, but for the other part (also infinite), one can have any angular
momentum. In the present paper, we study a simple subclass (also infinite) of
the whole class for which the solution of the Schr\"odinger equation is simpler
than in the general case. This subclass is obtained by combining another
approach together with the general approach of the previous paper. Once this is
achieved, one can then see that one can in fact combine the two approaches in
full generality, and obtain a much larger class of potentials than the class
found in ref. We mention here that our results are explicit, and when
exhibited, one can check in a straightforward manner their validity
Representations for Three-Body T-Matrix on Unphysical Sheets: Proofs
A proof is given for the explicit representations which have been formulated
in the author's previous work (nucl-th/9505028) for the Faddeev components of
three-body T-matrix continued analytically on unphysical sheets of the energy
Riemann surface. Also, the analogous representations for analytical
continuation of the three-body scattering matrices and resolvent are proved. An
algorithm to search for the three-body resonances on the base of the Faddeev
differential equations is discussed.Comment: 98 Kb; LaTeX; Journal-ref was added (the title changed in the
journal
Representations for Three-Body T-Matrix on Unphysical Sheets
Explicit representations are formulated for the Faddeev components of
three-body T-matrix continued analytically on unphysical sheets of the energy
Riemann surface. According to the representations, the T-matrix on unphysical
sheets is obviously expressed in terms of its components taken on the physical
sheet only. The representations for T-matrix are used then to construct similar
representations for analytical continuation of three-body scattering matrices
and resolvent. Domains on unphysical sheets are described where the
representations obtained can be applied.Comment: 123 Kb; LaTeX; Journal-ref was added (the title changed in the
journal
The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
Rotation is a directly-observable stellar property, and drives magnetic field
generation and activity through a magnetic dynamo. Main sequence stars with
masses below approximately 0.35Msun (mid-to-late M dwarfs) are
fully-convective, and are expected to have a different type of dynamo mechanism
than solar-type stars. Measurements of their rotation rates provide insights
into these mechanisms, but few rotation periods are available for these stars
at field ages. Using photometry from the MEarth transit survey, we measure
rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern
hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator
has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to
1%. We find no period-amplitude relation for stars below 0.25Msun and an
anti-correlation between period and amplitude for higher-mass M dwarfs. We
highlight the existence of older, slowly-rotating stars without H{\alpha}
emission that nevertheless have strong photometric variability. The Galactic
kinematics of our sample is consistent with the local population of G and K
dwarfs, and rotators have metallicities characteristic of the Solar
Neighborhood. We use the W space velocities and established age-velocity
relations to estimate that stars with P<10 days are on average <2 Gyrs, and
that those with P>70 days are about 5 Gyrs. The period distribution is mass
dependent: as the mass decreases, the slowest rotators at a given mass have
longer periods, and the fastest rotators have shorter periods. We find a lack
of stars with intermediate rotation periods. [Abridged]Comment: Accepted to ApJ. Machine readable tables and additional figures are
available in the published article or on reques
Viscous evolution of point vortex equilibria: The collinear state
When point vortex equilibria of the 2D Euler equations are used as initial
conditions for the corre- sponding Navier-Stokes equations (viscous), typically
an interesting dynamical process unfolds at short and intermediate time scales,
before the long time single peaked, self-similar Oseen vortex state dom-
inates. In this paper, we describe the viscous evolution of a collinear three
vortex structure that cor- responds to an inviscid point vortex fixed
equilibrium. Using a multi-Gaussian 'core-growth' type of model, we show that
the system immediately begins to rotate unsteadily, a mechanism we attribute to
a 'viscously induced' instability. We then examine in detail the qualitative
and quantitative evolution of the system as it evolves toward the long-time
asymptotic Lamb-Oseen state, showing the sequence of topological bifurcations
that occur both in a fixed reference frame, and in an appropriately chosen
rotating reference frame. The evolution of passive particles in this viscously
evolving flow is shown and interpreted in relation to these evolving streamline
patterns.Comment: 17 pages, 15 figure
A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100 Hour Spitzer Campaign
We present the results of a search for additional bodies in the GJ 1132
system through two methods: photometric transits and transit timing variations
of the known planet. We collected 21 transit observations of GJ 1132b with the
MEarth-South array since 2015. We obtained 100 near-continuous hours of
observations with the Space Telescope, including two transits of GJ
1132b and spanning 60\% of the orbital phase of the maximum period at which
bodies coplanar with GJ 1132b would pass in front of the star. We exclude
transits of additional Mars-sized bodies, such as a second planet or a moon,
with a confidence of 99.7\%. When we combine the mass estimate of the star
(obtained from its parallax and apparent band magnitude) with the stellar
density inferred from our high-cadence light curve (assuming zero
eccentricity), we measure the stellar radius of GJ 1132 to be
, and we refine the radius measurement of
GJ 1132b to . Combined with HARPS RV measurements, we
determine the density of GJ 1132b to be \ g cm, with the
mass determination dominating this uncertainty. We refine the ephemeris of the
system and find no evidence for transit timing variations, which would be
expected if there was a second planet near an orbital resonance with GJ 1132b.Comment: 29 pages, 4 Tables, 8 Figures, Submitted to ApJ. Comments welcom
Completed cohomology of Shimura curves and a p-adic Jacquet-Langlands correspondence
We study indefinite quaternion algebras over totally real fields F, and give
an example of a cohomological construction of p-adic Jacquet-Langlands
functoriality using completed cohomology. We also study the (tame) levels of
p-adic automorphic forms on these quaternion algebras and give an analogue of
Mazur's `level lowering' principle.Comment: Updated version. Contains some minor corrections compared to the
published versio
A study of Feshbach resonances and the unitary limit in a model of strongly correlated nucleons
A model of strongly interacting and correlated hadrons is developed. The
interaction used contains a long range attraction and short range repulsive
hard core. Using this interaction and various limiting situations of it, a
study of the effect of bound states and Feshbach resonances is given. The
limiting situations are a pure square well interaction, a delta-shell potential
and a pure hard core potential. The limit of a pure hard core potential are
compared with results for a spinless Bose and Fermi gas. The limit of many
partial waves for a pure hard core interaction is also considered and result in
expressions involving the hard core volume. This feature arises from a scaling
relation similar to that for hard sphere scattering with diffractive
corrections. The role of underlying isospin symmetries associated with the
strong interaction of protons and neutrons in this two component model is
investigated. Properties are studied with varying proton fraction. An analytic
expression for the Beth Uhlenbeck continuum integral is developed which closely
approximates exact results based on the potential model considered. An analysis
of features associated with a unitary limit is given. In the unitary limit of
very large scattering length, the ratio of effective range to thermal
wavelength appears as a limiting scale. Thermodynamic quantities such as the
entropy and compressibility are also developed. The effective range corrections
to the entropy vary as the cube of this ratio for low temperatures and are
therefore considerably reduced compared to the corrections to the interaction
energy which varies linearly with this ratio. Effective range corrections to
the compressibility are also linear in the ratio.Comment: 39 pages, 15 figures, 2 table
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