41,732 research outputs found
On the He(plus) triplet line intensities
The theoretical calculations of helium triplet line strengths, including collisional enhancement, are compared to astronomical observations. Both are plotted on an I(10830)/I(5876) vs I(5876)/I(4471) plane. It appears that the theory of helium triplet line strengths agrees with present observations, and that the question of an additional depopulation mechanism for the 2 3S population is probably predicted correctly within 30%
A plausible energy source and structure for quasi-stellar objects
If a collision of two large, massive, fast gas clouds occurs, their kinetic energy is converted to radiation in a pair of shock fronts at their interface. The resulting structure is described, and the relevance of this as a radiation source for quasi-stellar objects is considered
A conceptual design study for the secondary mirror drive of the shuttle infrared telescope facility (SIRTF)
Various conceptual designs for the secondary mirror actuator system to be used in the Shuttle Infrared Telescope Facility (SIRTF) were evaluated. In addition, a set of design concepts was developed to assist in the solution of problems crucial for optimum performance of the secondary mirror actuator system. A specific conceptual approach was presented along with a plan for developing that approach and identifying issues of critical importance in the developmental effort
Sum of Two Squares - Pair Correlation and Distribution in Short Intervals
In this work we show that based on a conjecture for the pair correlation of
integers representable as sums of two squares, which was first suggested by
Connors and Keating and reformulated here, the second moment of the
distribution of the number of representable integers in short intervals is
consistent with a Poissonian distribution, where "short" means of length
comparable to the mean spacing between sums of two squares. In addition we
present a method for producing such conjectures through calculations in prime
power residue rings and describe how these conjectures, as well as the above
stated result, may by generalized to other binary quadratic forms. While
producing these pair correlation conjectures we arrive at a surprising result
regarding Mertens' formula for primes in arithmetic progressions, and in order
to test the validity of the conjectures, we present numericalz computations
which support our approach.Comment: 3 figure
Revealing local failed supernovae with neutrino telescopes
We study the detectability of neutrino bursts from nearby direct black
hole-forming collapses (failed supernovae) at Megaton detectors. Due to their
high energetics, these bursts could be identified - by the time coincidence of
N >= 2 or N >= 3 events within a ~ 1 s time window - from as far as ~ 4-5 Mpc
away. This distance encloses several supernova-rich galaxies, so that failed
supernova bursts could be detected at a rate of up to one per decade,
comparable to the expected rate of the more common, but less energetic, neutron
star-forming collapses. Thus, the detection of a failed supernova within the
lifetime of a Mt detector is realistic. It might give the first evidence of
direct black hole formation, with important implications on the physics of this
phenomenon.Comment: LaTeX, 4 pages, 4 figures; minor changes to the text, results
unchange
Parrondo-like behavior in continuous-time random walks with memory
The Continuous-Time Random Walk (CTRW) formalism can be adapted to encompass
stochastic processes with memory. In this article we will show how the random
combination of two different unbiased CTRWs can give raise to a process with
clear drift, if one of them is a CTRW with memory. If one identifies the other
one as noise, the effect can be thought as a kind of stochastic resonance. The
ultimate origin of this phenomenon is the same of the Parrondo's paradox in
game theoryComment: 8 pages, 3 figures, revtex; enlarged and revised versio
Benchmarking the performance of Density Functional Theory and Point Charge Force Fields in their Description of sI Methane Hydrate against Diffusion Monte Carlo
High quality reference data from diffusion Monte Carlo calculations are
presented for bulk sI methane hydrate, a complex crystal exhibiting both
hydrogen-bond and dispersion dominated interactions. The performance of some
commonly used exchange-correlation functionals and all-atom point charge force
fields is evaluated. Our results show that none of the exchange-correlation
functionals tested are sufficient to describe both the energetics and the
structure of methane hydrate accurately, whilst the point charge force fields
perform badly in their description of the cohesive energy but fair well for the
dissociation energetics. By comparing to ice Ih, we show that a good prediction
of the volume and cohesive energies for the hydrate relies primarily on an
accurate description of the hydrogen bonded water framework, but that to
correctly predict stability of the hydrate with respect to dissociation to ice
Ih and methane gas, accuracy in the water-methane interaction is also required.
Our results highlight the difficulty that density functional theory faces in
describing both the hydrogen bonded water framework and the dispersion bound
methane.Comment: 8 pages, 4 figures, 1 table. Minor typos corrected and clarification
added in Method
Cell surface localization of tissue transglutaminase is dependent on a fibronectin-binding site in its N-terminal beta-sandwich domain
Increasing evidence indicates that tissue transglutaminase (tTG) plays a role in the assembly and remodeling of extracellular matrices and promotes cell adhesion. Using an inducible system we have previously shown that tTG associates with the extracellular matrix deposited by stably transfected 3T3 fibroblasts overexpressing the enzyme. We now show by confocal microscopy that tTG colocalizes with pericellular fibronectin in these cells, and by immunogold electron microscopy that the two proteins are found in clusters at the cell surface. Expression vectors encoding the full-length tTG or a N-terminal truncated tTG lacking the proposed fibronectin-binding site (fused to the bacterial reporter enzyme β-galactosidase) were generated to characterize the role of fibronectin in sequestration of tTG in the pericellular matrix. Enzyme-linked immunosorbent assay style procedures using extracts of transiently transfected COS-7 cells and immobilized fibronectin showed that the truncation abolished fibronectin binding. Similarly, the association of tTG with the pericellular matrix of cells in suspension or with the extracellular matrix deposited by cell monolayers was prevented by the truncation. These results demonstrate that tTG binds to the pericellular fibronectin coat of cells via its N-terminal β-sandwich domain and that this interaction is crucial for cell surface association of tTG
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