19,815 research outputs found
Sustaining supercooled mixed phase via resonant oscillations of the order parameter
We investigate the dynamics of a first order transition when the order
parameter field undergoes resonant oscillations, driven by a periodically
varying parameter of the free energy. This parameter could be a background
oscillating field as in models of pre-heating after inflation. In the context
of condensed matter systems, it could be temperature , or pressure, external
electric/magnetic field etc. We show that with suitable driving frequency and
amplitude, the system remains in a type of mixed phase, without ever completing
transition to the stable phase, even when the oscillating parameter of the free
energy remains below the corresponding critical value (for example, with
oscillating temperature, always remains below the critical temperature
). This phenomenon may have important implications. In cosmology, it will
imply prolonged mixed phase in a first order transition due to coupling with
background oscillating fields. In condensed matter systems, it will imply that
using oscillating temperature (or, more appropriately, pressure waves) one may
be able to sustain liquids in a mixed phase indefinitely at low temperatures,
without making transition to the frozen phase.Comment: 17 pages, 7 figures, Expanded version with more detail
Density dependent strong coupling constant of QCD derived from compact star data
The present work is an endeavour to connect the properties of tiny nearly
massless objects with those of some of the most massive ones, the compact
stars.
Since 1996 there is major influx of X-ray and ray data from binary
stars, one or both of which are compact objects that are difficult to explain
as neutron stars since they contain a mass M in too small a radius R . The
suggestion has been put forward that these are strange quark stars (SS)
explainable in a simple model with chiral symmetry restoration (CSR) for the
quarks and the M, R and other properties like QPOs (quasi periodic
oscillations) in their X-ray power spectrum.
It would be nice if this astrophysical data could shed some light on
fundamental properties of quarks obeying QCD. One can relate the strong
coupling constant of QCD, to the quark mass through the
Dyson-Schwinger gap equation using the real time formalism of Dolan and Jackiw.
This enables us to obtain the density dependence of from the simple
CSR referred to above. This way fundamental physics, difficult to extract from
other models like for example lattice QCD, can be constrained from present-day
compact star data and may be put back to modelling the dense quark phase of
early universe.Comment: 7 pages, 4 figure
Preliminary development of an intelligent computer assistant for engine monitoring
As part of the F-18 high-angle-of-attack vehicle program, an AI method was developed for the real time monitoring of the propulsion system and for the identification of recovery procedures for the F404 engine. The aim of the development program is to provide enhanced flight safety and to reduce the duties of the propulsion engineers. As telemetry data is received, the results are continually displayed in a number of different color graphical formats. The system makes possible the monitoring of the engine state and the individual parameters. Anomaly information is immediately displayed to the engineer
Can only flavor-nonsinglet H dibaryons be stable against strong decays?
Using the QCD sum rule approach, we show that the flavor-nonsinglet
dibaryon states with J, J, I=1 (27plet) are nearly
degenerate with the J, I=0 singlet dibaryon, which has been
predicted to be stable against strong decay, but has not been observed. Our
calculation, which does not require an instanton correction, suggests that the
is slightly heavier than these flavor-nonsinglet s over a wide range
of the parameter space. If the singlet mass lies above the threshold (2231~MeV), then the strong interaction breakup to would produce a very broad resonance in the
invariant mass spectrum which would be very difficult to observe. On the other
hand, if these flavor-nonsinglet J=0 and 1 dibaryons are also above the
threshold, but below the breakup threshold (2254
MeV), then because the direct, strong interaction decay to the channel is forbidden, these flavor-nonsinglet states might be more
amenable to experimental observation. The present results allow a possible
reconciliation between the reported observation of
hypernuclei, which argue against a stable , and the possible existence of
dibaryons in general.Comment: 10 pages, 2 figure
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
On the Core of Dynamic Cooperative Games
We consider dynamic cooperative games, where the worth of coalitions varies
over time according to the history of allocations. When defining the core of a
dynamic game, we allow the possibility for coalitions to deviate at any time
and thereby to give rise to a new environment. A coalition that considers a
deviation needs to take the consequences into account because from the
deviation point on, the game is no longer played with the original set of
players. The deviating coalition becomes the new grand coalition which, in
turn, induces a new dynamic game. The stage games of the new dynamical game
depend on all previous allocation including those that have materialized from
the deviating time on.
We define three types of core solutions: fair core, stable core and credible
core. We characterize the first two in case where the instantaneous game
depends on the last allocation (rather than on the whole history of
allocations) and the third in the general case. The analysis and the results
resembles to a great extent the theory of non-cooperative dynamic games.Comment: 25 page
Simple geometric algorithms to aid in clearance management for robotic mechanisms
Global geometric shapes such as lines, planes, circles, spheres, cylinders, and the associated computational algorithms which provide relatively inexpensive estimates of minimum spatial clearance for safe operations were selected. The Space Shuttle, remote manipulator system, and the Power Extension Package are used as an example. Robotic mechanisms operate in quarters limited by external structures and the problem of clearance is often of considerable interest. Safe clearance management is simple and suited to real time calculation, whereas contact prediction requires more precision, sophistication, and computational overhead
Chromium (VI) Biosorption by Immobilized Biomass of Bacillus ceres M116
Biosorption is potentially an attractive technology for treatment of wastewater for retaining heavy metals from dilute solutions. This study investigated the feasibility of Bacillus cereus M116 immobilized in different carriers as a biosorbent for chromium removal from aqueous solutions in batch mode; optimum conditions were determined. Experimental results showed the bacterial strain immobilized in calcium alginate gel matrix was most effective in removing Cr(VI) ion from solution. The uptake of metal was very fast initially, and equilibrium was attained within 80 mins. The overall biosorption process was best described by the pseudo second-order kinetics. Intraparticle diffusion was not the only rate-determining step. The sorption data conformed well to the Fruendlich isotherm model. The highest value of Cr(VI) uptake by Bacillus cereus M116 (6.0g/L ,dry basis) immobilized in 3% calcium alginate was 92.5% at 25°C, when initial chromium concentration was 50 mg /L
- …