22,285 research outputs found
Electromagnetic containerless processing requirements and recommended facility concept and capabilities for space lab
Containerless melting, reaction, and solidification experiments and processes which potentially can lead to new understanding of material science and production of new or improved materials in the weightless space environment are reviewed in terms of planning for spacelab. Most of the experiments and processes discussed are amenable to the employment of electromagnetic position control and electromagnetic induction or electron beam heating and melting. The spectrum of relevant properties of materials, which determine requirements for a space laboratory electromagnetic containerless processing facility are reviewed. Appropriate distributions and associated coil structures are analyzed and compared on the basis of efficiency, for providing the functions of position sensing, control, and induction heating. Several coil systems are found capable of providing these functions. Exchangeable modular coils in appropriate sizes are recommended to achieve the maximum power efficiencies, for a wide range of specimen sizes and resistivities, in order to conserve total facility power
Astrometric Microlensing Constraints on a Massive Body in the Outer Solar System with Gaia
A body in Solar orbit beyond the Kuiper belt exhibits an annual parallax that
exceeds its apparent proper motion by up to many orders of magnitude. Apparent
motion of this body along the parallactic ellipse will deflect the angular
position of background stars due to astrometric microlensing ("induced
parallax"). By synoptically sampling the astrometric position of background
stars over the entire sky, constraints on the existence (and basic properties)
of a massive nearby body may be inferred. With a simple simulation, we estimate
the signal-to-noise for detecting such a body -- as function of mass,
heliocentric distance, and ecliptic latitude -- using the anticipated
sensitivity and temporal cadences from Gaia (launch 2011). A Jupiter-mass
(M_Jup) object at 2000 AU is detectable by Gaia over the whole sky above
5-sigma, with even stronger constraints if it lies near the ecliptic plane.
Hypotheses for the mass (~3M_Jup), distance (~20,000 AU) and location of the
proposed perturber ("Planet X") which gives rise to long-period comets may be
testable.Comment: 17 pages, 6 figures. Figures revised, new figure added, minor text
revisions. Accepted to ApJ, to appear in the Dec 10, 2005 issue (v635
Resolving The ISM Surrounding GRBs with Afterglow Spectroscopy
We review current research related to spectroscopy of gamma-ray burst (GRB)
afterglows with particular emphasis on the interstellar medium (ISM) of the
galaxies hosting these high redshift events. These studies reveal the physical
conditions of star-forming galaxies and yield clues to the nature of the GRB
progenitor. We offer a pedagogical review of the experimental design and review
current results. The majority of sightlines are characterized by large HI
column densities, negligible molecular fraction, the ubiquitous detection of UV
pumped fine-structure transitions, and metallicities ranging from 1/100 to
nearly solar abundance.Comment: Conference procedings for Gamma Ray Bursts 2007 November 5-9, 2007
Santa Fe, New Mexico (8 pages, 4 figures
Molecular Clock on a Neutral Network
The number of fixed mutations accumulated in an evolving population often
displays a variance that is significantly larger than the mean (the
overdispersed molecular clock). By examining a generic evolutionary process on
a neutral network of high-fitness genotypes, we establish a formalism for
computing all cumulants of the full probability distribution of accumulated
mutations in terms of graph properties of the neutral network, and use the
formalism to prove overdispersion of the molecular clock. We further show that
significant overdispersion arises naturally in evolution when the neutral
network is highly sparse, exhibits large global fluctuations in neutrality, and
small local fluctuations in neutrality. The results are also relevant for
elucidating the topological structure of a neutral network from empirical
measurements of the substitution process.Comment: 10 page
Convergence of random zeros on complex manifolds
We show that the zeros of random sequences of Gaussian systems of polynomials
of increasing degree almost surely converge to the expected limit distribution
under very general hypotheses. In particular, the normalized distribution of
zeros of systems of m polynomials of degree N, orthonormalized on a regular
compact subset K of C^m, almost surely converge to the equilibrium measure on K
as the degree N goes to infinity.Comment: 16 page
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