293 research outputs found
Coulomb-oscillator duality in spaces of constant curvature
In this paper we construct generalizations to spheres of the well known
Levi-Civita, Kustaanheimo-Steifel and Hurwitz regularizing transformations in
Euclidean spaces of dimensions 2, 3 and 5. The corresponding classical and
quantum mechanical analogues of the Kepler-Coulomb problem on these spheres are
discussed.Comment: 33 pages, LaTeX fil
Studying Turbulence from Doppler-broadened Absorption Lines: Statistics of Logarithms of Intensity
We continue our work on developing techniques for studying turbulence with
spectroscopic data. We show that Doppler-broadened absorption spectral lines,
in particularly, saturated absorption lines, can be used within the framework
of the earlier-introduced technique termed the Velocity Coordinate spectrum
(VCS). The VCS relates the statistics of fluctuations along the velocity
coordinate to the statistics of turbulence, thus it does not require spatial
coverage by sampling directions in the plane of the sky. We consider lines with
different degree of absorption and show that for lines of optical depth less
than one, our earlier treatment of the VCS developed for spectral emission
lines is applicable, if the optical depth is used instead of intensity. This
amounts to correlating the logarithms of absorbed intensities. For larger
optical depths and saturated absorption lines, we show, that the amount of
information that one can use is, inevitably, limited by noise. In practical
terms, this means that only wings of the line are available for the analysis.
In terms of the VCS formalism, this results in introducing an additional
window, which size decreases with the increase of the optical depth. As a
result, strongly saturated absorption lines carry the information only about
the small scale turbulence. Nevertheless, the contrast of the fluctuations
corresponding to the small scale turbulence increases with the increase of the
optical depth, which provides advantages for studying turbulence combining
lines with different optical depths. Combining different absorption lines one
can tomography turbulence in the interstellar gas in all its complexity.Comment: 12 pages, 2 figure
Gravitational instability in the strongly nonlinear regime: A study of various approximations
We study the development of gravitational instability in the strongly
non-linear regime. For this purpose we use a number of statistical indicators
such as filamentary statistics, spectrum of overdense/underdense regions and
the void probability function, each of which probes a particular aspect of
gravitational clustering. We use these statistical indicators to discriminate
between different approximations to gravitational instability which we test
against N-body simulations. The approximations which we test are, the truncated
Zel'dovich approximation (TZ), the adhesion model (AM), and the frozen flow
(FF) and linear potential (LP) approximations. Of these we find that FF and LP
break down relatively early, soon after the non-linear length scale exceeds
-- the mean distance between peaks of the gravitational potential. The
reason for this break down is easy to understand, particles in FF are
constrained to follow the streamlines of the initial velocity field. Shell
crossing is absent in this case and structure gradually freezes as particles
begin to collect near minima of the gravitational potential. In LP particles
follow the lines of force of the primordial potential, oscillating about its
minima at late times when the non-linear length scale . Unlike FF and LP the adhesion model (and to some extent TZ) continues to
give accurate results even at late times when . This
is because both AM and TZ useComment: mn.sty (Latex), 20 pages + 11 figures (not included: hardcopy
available on request from [email protected]), Submitted to MNRAS,
IUCAA-24/9
Velocity Modification of Power Spectrum from Absorbing Medium
Quantitative description of the statistics of intensity fluctuations within
spectral line data cubes introduced in our earlier work is extended to the
absorbing media. A possibility of extracting 3D velocity and density statistics
from both integrated line intensity as well as from the individual channel maps
is analyzed. We find that absorption enables the velocity effects to be seen
even if the spectral line is integrated over frequencies. This regime that is
frequently employed in observations is characterized by a non-trivial relation
between the spectral index of velocities and the spectral index of intensity
fluctuations. For instance when density is dominated by fluctuations at large
scales, i.e. when correlations scale as r^{-\gamma}, \gamma<0, the intensity
fluctuations exhibit a universal spectrum of fluctuations ~K^{-3} over a range
of scales. When small scale fluctuations of density contain most of the energy,
i.e. when correlations scale as r^{-\gamma}, \gamma>0, the resulting spectrum
of the integrated lines depends on the scaling of the underlying density and
scales as K^{-3+\gamma}. We show that if we take the spectral line slices that
are sufficiently thin we recover our earlier results for thin slice data
without absorption. As the result we extend the Velocity Channel Analysis (VCA)
technique to optically thick lines enabling studies of turbulence in molecular
clouds. In addition, the developed mathematical machinery enables a
quantitative approach to solving other problems that involved statistical
description of turbulence within emitting and absorbing gas.Comment: 51 page, 3 figures. Accepted to Astrophysical Journa
The Cosmic Microwave Background & Inflation, Then & Now
Boomerang, Maxima, DASI, CBI and VSA significantly increase the case for
accelerated expansion in the early universe (the inflationary paradigm) and at
the current epoch (dark energy dominance), especially when combined with data
on high redshift supernovae (SN1) and large scale structure (LSS). There are
``7 pillars of Inflation'' that can be shown with the CMB probe, and at least
5, and possibly 6, of these have already been demonstrated in the CMB data: (1)
a large scale gravitational potential; (2) acoustic peaks/dips; (3) damping due
to shear viscosity; (4) a Gaussian (maximally random) distribution; (5)
secondary anisotropies; (6) polarization. A 7th pillar, anisotropies induced by
gravity wave quantum noise, could be too small. A minimal inflation parameter
set, \omega_b,\omega_{cdm}, \Omega_{tot}, \Omega_Q,w_Q,n_s,\tau_C, \sigma_8},
is used to illustrate the power of the current data. We find the CMB+LSS+SN1
data give \Omega_{tot} =1.00^{+.07}_{-.03}, consistent with (non-baroque)
inflation theory. Restricting to \Omega_{tot}=1, we find a nearly scale
invariant spectrum, n_s =0.97^{+.08}_{-.05}. The CDM density, \Omega_{cdm}{\rm
h}^2 =.12^{+.01}_{-.01}, and baryon density, \Omega_b {\rm h}^2 =
>.022^{+.003}_{-.002}, are in the expected range. (The Big Bang nucleosynthesis
estimate is 0.019\pm 0.002.) Substantial dark (unclustered) energy is inferred,
\Omega_Q \approx 0.68 \pm 0.05, and CMB+LSS \Omega_Q values are compatible with
the independent SN1 estimates. The dark energy equation of state, crudely
parameterized by a quintessence-field pressure-to-density ratio w_Q, is not
well determined by CMB+LSS (w_Q < -0.4 at 95% CL), but when combined with SN1
the resulting w_Q < -0.7 limit is quite consistent with the w_Q=-1 cosmological
constant case.Comment: 20 pages, 8 figures, in Theoretical Physics, MRST 2002: A Tribute to
George Libbrandt (AIP), eds. V. Elias, R. Epp, R. Myer
Quantum Systems with Hidden Symmetry. Interbasis Expansions
This monograph is the English version of the book "Quantum systems with
hidden symmetry. Interbasis expansions" published in 2006 by the publishing
house FIZMATLIT (Moscow) in Russian. When compiling this version of the book,
typos and inaccuracies noted since the release of the Russian edition have been
corrected
Families of classical subgroup separable superintegrable systems
We describe a method for determining a complete set of integrals for a
classical Hamiltonian that separates in orthogonal subgroup coordinates. As
examples, we use it to determine complete sets of integrals, polynomial in the
momenta, for some families of generalized oscillator and Kepler-Coulomb
systems, hence demonstrating their superintegrability. The latter generalizes
recent results of Verrier and Evans, and Rodriguez, Tempesta and Winternitz.
Another example is given of a superintegrable system on a non-conformally flat
space.Comment: 9 page
Deep brain stimulation can suppress pathological synchronisation in parkinsonian patients
Background Although deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a highly effective therapeutic intervention in severe Parkinson's disease, its mechanism of action remains unclear. One possibility is that DBS suppresses local pathologically synchronised oscillatory activity.Methods To explore this, the authors recorded from DBS electrodes implanted in the STN of 16 patients with Parkinson's disease during simultaneous stimulation (pulse width 60 mu s; frequency 130 Hz) of the same target using a specially designed amplifier. The authors analysed data from 25 sides.Results The authors found that DBS progressively suppressed peaks in local field potential activity at frequencies between 11 and 30 Hz as voltage was increased beyond a stimulation threshold of 1.5 V. Median peak power had fallen to 54% of baseline values by a stimulation intensity of 3.0 V.Conclusion The findings suggest that DBS can suppress pathological 11-30 Hz activity in the vicinity of stimulation in patients with Parkinson's disease. This suppression occurs at stimulation voltages that are clinically effective
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