6,113 research outputs found
Memory difference control of unknown unstable fixed points: Drifting parameter conditions and delayed measurement
Difference control schemes for controlling unstable fixed points become
important if the exact position of the fixed point is unavailable or moving due
to drifting parameters. We propose a memory difference control method for
stabilization of a priori unknown unstable fixed points by introducing a memory
term. If the amplitude of the control applied in the previous time step is
added to the present control signal, fixed points with arbitrary Lyapunov
numbers can be controlled. This method is also extended to compensate arbitrary
time steps of measurement delay. We show that our method stabilizes orbits of
the Chua circuit where ordinary difference control fails.Comment: 5 pages, 8 figures. See also chao-dyn/9810029 (Phys. Rev. E 70,
056225) and nlin.CD/0204031 (Phys. Rev. E 70, 046205
Control of gradient-driven instabilities using shear Alfv\'en beat waves
A new technique for manipulation and control of gradient-driven instabilities
through nonlinear interaction with Alfv\'en waves in a laboratory plasma is
presented. A narrow field-aligned density depletion is created in the Large
Plasma Device (LAPD), resulting in coherent unstable fluctuations on the
periphery of the depletion. Two independent kinetic Alfv\'en waves are launched
along the depletion at separate frequencies, creating a nonlinear beat-wave
response at or near the frequency of the original instability. When the
beat-wave has sufficient amplitude, the original unstable mode is suppressed,
leaving only the beat-wave response at a different frequency, generally at
lower amplitude.Comment: Submitted for Publication in Physical Review Letters. Revision 2
reflects changes suggested by referees for PRL submission. One figure
removed, several major changes to another figure, and a number of major and
minor changes to the tex
Avalanches in a Bose-Einstein condensate
Collisional avalanches are identified to be responsible for an 8-fold
increase of the initial loss rate of a large 87-Rb condensate. We show that the
collisional opacity of an ultra-cold gas exhibits a critical value. When
exceeded, losses due to inelastic collisions are substantially enhanced. Under
these circumstances, reaching the hydrodynamic regime in conventional BEC
experiments is highly questionable.Comment: 4 pages, 2 figures, 1 tabl
Rubidium in Metal-Deficient Disk and Halo Stars
We report the first extensive study of stellar Rb abundances. High-resolution
spectra have been used to determine, or set upper limits on, the abundances of
this heavy element and the associated elements Y, Zr, and Ba in 44 dwarfs and
giants with metallicities spanning the range -2.0 <[Fe/H] < 0.0. In
metal-deficient stars Rb is systematically overabundant relative to Fe; we find
an average [Rb/Fe] of +0.21 for the 32 stars with [Fe/H] < -0.5 and measured
Rb. This behavior contrasts with that of Y, Zr, and Ba, which, with the
exception of three new CH stars (HD 23439A and B and BD +5 3640), are
consistently slightly deficient relative to Fe in the same stars; excluding the
three CH stars, we find the stars with [Fe/H] < -0.5 have average [Y/Fe],
[Zr/Fe], and [Ba/Fe] of --0.19 (24 stars), --0.12 (28 stars), and --0.06 (29
stars), respectively. The different behavior of Rb on the one hand and Y, Zr,
and Ba on the other can be attributed in part to the fact that in the Sun and
in these stars Rb has a large r-process component while Y, Zr, and Ba are
mostly s-process elements with only small r-process components. In addition,
the Rb s-process abundance is dependent on the neutron density at the
s-processing site. Published observations of Rb in s-process enriched red
giants indicate a higher neutron density in the metal-poor giants. These
observations imply a higher s-process abundance for Rb in metal-poor stars. The
calculated combination of the Rb r-process abundance, as estimated for the
stellar Eu abundances, and the s-process abundance as estimated for red giants
accounts satisfactorily for the observed run of [Rb/Fe] with [Fe/H].Comment: 23 pages, 5 tables, 7 figure
Stability borders of feedback control of delayed measured systems
When stabilization of unstable periodic orbits or fixed points by the method
given by Ott, Grebogi and Yorke (OGY) has to be based on a measurement delayed
by orbit lengths, the performance of unmodified OGY method is expected
to decline. For experimental considerations, it is desired to know the range of
stability with minimal knowledge of the system. We find that unmodified OGY
control fails beyond a maximal Ljapunov number of
. In this paper the area of stability is
investigated both for OGY control of known fixed points and for difference
control of unknown or inaccurately known fixed points. An estimated value of
the control gain is given. Finally we outline what extensions have to be
considered if one wants to stabilize fixed points with Ljapunov numbers above
.Comment: 5 pages LaTeX using revtex and epsfig (4 figs included). Revised
versio
Solid state television camera system Patent
Solid state television camera system consisting of monolithic semiconductor mosaic sensor and molecular digital readout system
Triggering up states in all-to-all coupled neurons
Slow-wave sleep in mammalians is characterized by a change of large-scale
cortical activity currently paraphrased as cortical Up/Down states. A recent
experiment demonstrated a bistable collective behaviour in ferret slices, with
the remarkable property that the Up states can be switched on and off with
pulses, or excitations, of same polarity; whereby the effect of the second
pulse significantly depends on the time interval between the pulses. Here we
present a simple time discrete model of a neural network that exhibits this
type of behaviour, as well as quantitatively reproduces the time-dependence
found in the experiments.Comment: epl Europhysics Letters, accepted (2010
Extremely Metal-Poor Stars. VII. The Most Metal-Poor Dwarf, CS 22876-032
We report high-resolution, high-signal-to-noise, observations of the
extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system
has a long period : P = 424.7 0.6 days. It comprises two main sequence
stars having effective temperatures 6300 K and 5600 K, with a ratio of
secondary to primary mass of 0.89 0.04. The metallicity of the system is
[Fe/H] = -3.71 0.11 0.12 (random and systematic errors) -- somewhat
higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of
less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have
significantly lower values, ~ 0.0-0.1, suggesting that the heavier elements
might have been underproduced relative to Mg in the material from which this
object formed. In the context of the hypothesis that the abundance patterns of
extremely metal-poor stars are driven by individual enrichment events and the
models of Woosley and Weaver (1995), the data for CS 22876-032 are consistent
with its having been enriched by a zero-metallicity supernova of mass 30
M. As the most metal-poor near-main-sequence-turnoff star currently
known, the primary of the system has the potential to strongly constrain the
primordial lithium abundance. We find A(Li) (= log(N(Li)/N(H)) + 12.00) = 2.03
0.07, which is consistent with the finding of Ryan et al. (1999) that for
stars of extremely low metallicity A(Li) is a function of [Fe/H].Comment: 27 pages, 9 figures, accepted for publication in The Astrophysical
Journal, Sept. 1, 2000 issu
New Lithium Measurements in Metal-Poor Stars
We provide *lambda*6708 Li 1 measurements in 37 metal-poor stars, most of
which are poorly-studied or have no previous measurements, from high-resolution
and high-S/N spectroscopy obtained with the McDonald Observatory 2.1m and 2.7m
telescopes. The typical line strength and abundance uncertainties, confirmed by
the thinness of the Spite plateau manifested by our data and by comparison with
previous measurements, are <=4 mAng and <=0.07-0.10 dex respectively. Two rare
moderately metal-poor solar-Teff dwarfs, HIP 36491 and 40613, with
significantly depleted but still detectable Li are identified; future light
element determinations in the more heavily depeleted HIP 40613 may provide
constraints on the Li depletion mechanism acting in this star. We note two
moderately metal-poor and slightly evolved stars, HIP 105888 and G265-39, that
appear to be analogs of the low-Li moderately metal-poor subgiant HD 201889.
Preliminary abundance analysis of G 265-39 finds no abnormalities that suggest
the low Li content is associated with AGB mass-transfer or deep mixing and
p-capture. We also detect line doubling in HIP 4754, heretofore classified as
SB1.Comment: Accepted for publication in PASP, volume 912 (Feb 2012) 15 pages, 3
figures, 2 table
- …