646,408 research outputs found
HOMFLY for twist knots and exclusive Racah matrices in representation [333]
Next step is reported in the program of Racah matrices extraction from the
differential expansion of HOMFLY polynomials for twist knots: from the
double-column rectangular representations R=[rr] to a triple-column and
triple-hook R=[333]. The main new phenomenon is the deviation of the particular
coefficient from the corresponding skew dimension, what
opens a way to further generalizations.Comment: 10 page
Studying the small scale ISM structure with supernovae
AIMS. In this work we explore the possibility of using the fast expansion of
a Type Ia supernova photosphere to detect extra-galactic ISM column density
variations on spatial scales of ~100 AU on time scales of a few months.
METHODS. We constructed a simple model which describes the expansion of the
photodisk and the effects of a patchy interstellar cloud on the observed
equivalent width of Na I D lines. Using this model we derived the behavior of
the equivalent width as a function of time, spatial scale and amplitude of the
column density fluctuations.
RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na
I column densities exceeding a few 10^11 cm^-2 would be easily detected. In
contrast, the effects of a more realistic, patchy ISM become measurable in a
fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on
a scale of 1000 AU.
CONCLUSIONS. The proposed technique provides a unique way to probe the
extra-galactic small scale structure, which is out of reach for any of the
methods used so far. The same tool can also be applied to study the sub-AU
Galactic ISM structure.Comment: 6 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
Tracking ground state Ba+ ions in an expanding laserâplasma plume using time-resolved vacuum ultraviolet photoionization imaging
We report results from a study of the integrated column density and expansion dynamics of ground-state-selected Ba+ ions in a laserâplasma plume using a new experimental systemâVPIF (vacuum-ultraviolet photoabsorption imaging facility). The ions are tracked by recording the attenuation of a pulsed and collimated vacuum ultraviolet beam, tuned to the 5pâ6d inner-shell resonance of singly ionized barium, as the expanding plasma plume moves across it. The attenuated beam is allowed to fall on a CCD array where the spatial distribution of the absorption is recorded. Time-resolved ion velocity and integrated column density maps are readily extracted from the photoionization images
Finite Temperature Depinning of a Flux Line from a Nonuniform Columnar Defect
A flux line in a Type-II superconductor with a single nonuniform columnar
defect is studied by a perturbative diagrammatic expansion around an annealed
approximation. The system undergoes a finite temperature depinning transition
for the (rather unphysical) on-the-average repulsive columnar defect, provided
that the fluctuations along the axis are sufficiently large to cause some
portions of the column to become attractive. The perturbative expansion is
convergent throughout the weak pinning regime and becomes exact as the
depinning transition is approached, providing an exact determination of the
depinning temperature and the divergence of the localization length.Comment: RevTeX, 4 pages, 3 EPS figures embedded with epsf.st
Identification of the Sequence of Steps Intrinsic to Spheromak Formation
A planar coaxial electrostatic helicity source is used for studying the relaxation process intrinsic to spheromak formation Experimental observations reveal that spheromak formation involves: (1) breakdown and creation of a number of distinct, arched, filamentary, plasma-filled flux loops that span from cathode to anode gas nozzles, (2) merging of these loops to form a central column, (3) jet-like expansion of the central column, (4) kink instability of the central column, (5) conversion of toroidal flux to poloidal flux by the kink instability. Steps 1 and 3 indicate that spheromak formation involves an MHD pumping of plasma from the gas nozzles into the magnetic flux tube linking the nozzles. In order to measure this pumping, the gas puffing system has been modified to permit simultaneous injection of different gas species into the two ends of the flux tube linking the wall. Gated CCD cameras with narrow-band optical filters are used to track the pumped flows
Spectral properties of zero temperature dynamics in a model of a compacting granular column
The compacting of a column of grains has been studied using a one-dimensional
Ising model with long range directed interactions in which down and up spins
represent orientations of the grain having or not having an associated void.
When the column is not shaken (zero 'temperature') the motion becomes highly
constrained and under most circumstances we find that the generator of the
stochastic dynamics assumes an unusual form: many eigenvalues become
degenerate, but the associated multi-dimensional invariant spaces have but a
single eigenvector. There is no spectral expansion and a Jordan form must be
used. Many properties of the dynamics are established here analytically; some
are not. General issues associated with the Jordan form are also taken up.Comment: 34 pages, 4 figures, 3 table
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