29,478 research outputs found
Kelvin-Helmholtz Instability in a Weakly Ionized Medium
Ambient interstellar material may become entrained in outflows from massive
stars as a result of shear flow instabilities. We study the linear theory of
the Kelvin - Helmholtz instability, the simplest example of shear flow
instability, in a partially ionized medium. We model the interaction as a two
fluid system (charged and neutral) in a planar geometry. Our principal result
is that for much of the relevant parameter space, neutrals and ions are
sufficiently decoupled that the neutrals are unstable while the ions are held
in place by the magnetic field. Thus, we predict that there should be a
detectably narrower line profile in ionized species tracing the outflow
compared with neutral species since ionized species are not participating in
the turbulent interface with the ambient ISM. Since the magnetic field is
frozen to the plasma, it is not tangled by the turbulence in the boundary
layer.Comment: 21 pages, 4 figure
The fish fauna of the Iwokrama Forest
Fishes were collected from the rivers in and around the Iwokrama Forest during January-February and November-December 1997. Four hundred species of fish were recorded from forty families in ten orders. Many of these fishes are newly recorded from Guyana and several are thought to be endemic. The number of species recorded for the area is surprising given the low level of effort and suggests that this area may be particularly important from a fish diversity perspective. This paper focuses on species of particular interest from a management perspective including those considered economically important, rare or endangered. The paper is also the basis for developing fisheries management systems in the Iwokrama Forest and Rupununi Wetlands
The soil conservation service : its basis of co-operation with landowners
The Soil Conservation Act of 1945 set up within the Department of Agriculture a Soil Conservation Service under the control of a Commissioner of Soil Conservation.
The Service aims to promote types of land use which will conserve the soil and prevent or overcome soil erosion. It also aims to educate landholders and the public generally in the aims and practice of soil conservation.
This article relates mainly to farm land activities, and in particular the co-operation of the Service with individual primary producer landholders for the prevention and control of erosion on their land
Rim curvature anomaly in thin conical sheets revisited
This paper revisits one of the puzzling behaviors in a developable cone
(d-cone), the shape obtained by pushing a thin sheet into a circular container
of radius by a distance [E. Cerda, S. Chaieb, F. Melo, and L.
Mahadevan, {\sl Nature} {\bf 401}, 46 (1999)]. The mean curvature was reported
to vanish at the rim where the d-cone is supported [T. Liang and T. A. Witten,
{\sl Phys. Rev. E} {\bf 73}, 046604 (2006)]. We investigate the ratio of the
two principal curvatures versus sheet thickness over a wider dynamic range
than was used previously, holding and fixed. Instead of tending
towards 1 as suggested by previous work, the ratio scales as .
Thus the mean curvature does not vanish for very thin sheets as previously
claimed. Moreover, we find that the normalized rim profile of radial curvature
in a d-cone is identical to that in a "c-cone" which is made by pushing a
regular cone into a circular container. In both c-cones and d-cones, the ratio
of the principal curvatures at the rim scales as ,
where is the pushing force and is the Young's modulus. Scaling
arguments and analytical solutions confirm the numerical results.Comment: 25 pages, 12 figures. Added references. Corrected typos. Results
unchange
Exact propagators on the lattice with applications to diffractive effects
The propagator of the discrete Schr\"odinger equation is computed and its
properties are revealed through a Feynman path summation in discrete space.
Initial data problems such as diffraction in discrete space and continuous time
are studied analytically by the application of the new propagator. In the
second part of this paper, the analogy between time propagation and 2D
scattering by 1D obstacles is explored. New results are given in the context of
diffraction by edges within a periodic medium. A connection with tight-binding
arrays and photonic crystals is indicated.Comment: Final version with two appendices. Published in J. Phys. A: Math.
Theo
Defining the gap between research and practice in public relations programme evaluation - towards a new research agenda
The current situation in public relations programme evaluation is neatly summarized by McCoy who commented that 'probably the most common buzzwords in public relations in the last ten years have been evaluation and accountability' (McCoy 2005, 3). This paper examines the academic and practitioner-based literature and research on programme evaluation and it detects different priorities and approaches that may partly explain why the debate on acceptable and agreed evaluation methods continues. It analyses those differences and proposes a research agenda to bridge the gap and move the debate forward
Surface polaritons on left-handed cylinders: A complex angular momentum analysis
We consider the scattering of electromagnetic waves by a left-handed cylinder
-- i.e., by a cylinder fabricated from a left-handed material -- in the
framework of complex angular momentum techniques. We discuss both the TE and TM
theories. We emphasize more particularly the resonant aspects of the problem
linked to the existence of surface polaritons. We prove that the long-lived
resonant modes can be classified into distinct families, each family being
generated by one surface polariton propagating close to the cylinder surface
and we physically describe all the surface polaritons by providing, for each
one, its dispersion relation and its damping. This can be realized by noting
that each surface polariton corresponds to a particular Regge pole of the
matrix of the cylinder. Moreover, for both polarizations, we find that there
exists a particular surface polariton which corresponds, in the large-radius
limit, to the surface polariton which is supported by the plane interface.
There exists also an infinite family of surface polaritons of
whispering-gallery type which have no analogs in the plane interface case and
which are specific to left-handed materials.Comment: published version. v3: reference list correcte
Robin conditions on the Euclidean ball
Techniques are presented for calculating directly the scalar functional
determinant on the Euclidean d-ball. General formulae are given for Dirichlet
and Robin boundary conditions. The method involves a large mass asymptotic
limit which is carried out in detail for d=2 and d=4 incidentally producing
some specific summations and identities. Extensive use is made of the
Watson-Kober summation formula.Comment: 36p,JyTex, misprints corrected and a section on the massive case
adde
Time-resolved optical photometry of the ultra-compact binary 4U0614+091
We present a detailed optical study of the ultra-compact X-ray binary
4U0614+091. We have used 63 hrs of time-resolved optical photometry taken with
three different telescopes (IAC80, NOT and SPM) to search for optical
modulations. The power spectra of each dataset reveals sinusoidal modulations
with different periods, which are not always present. The strongest modulation
has a period of 51.3 mins, a semi-amplitude of 4.6 mmags, and is present in the
IAC80 data. The SPM and NOT data show periods of 42 mins and 64 mins
respectively, but with much weaker amplitudes, 2.6 mags and 1.3 mmags
respectively. These modulations arise from either X-ray irradiation of the
inner face of the secondary star and/or a superhump modulation from the
accretion disc, or quasi-periodic modulations in the accretion disc. It is
unclear whether these periods/quasi-periodic modulations are related to the
orbital period, however, the strongest period of 51.3 mins is close to earlier
tentative orbital periods. Further observations taken over a long base-line are
encouraged.Comment: Accepted for publication in PAS
Scattering Mechanisms in a High Mobility Low Density Carbon-Doped (100) GaAs Two-Dimensional Hole System
We report on a systematic study of the density dependence of mobility in a
low-density Carbon-doped (100) GaAs two-dimensional hole system (2DHS). At T=
50 mK, a mobility of 2.6 x 10^6 cm^2/Vs at a density p=6.2 x 10^10 cm^- was
measured. This is the highest mobility reported for a 2DHS to date. Using a
back-gated sample geometry, the density dependence of mobility was studied from
2.8 x 10^10 cm^-2 to 1 x 10^11 cm^-2. The mobility vs. density cannot be fit to
a power law dependence of the form mu ~ p^alpha using a single exponent alpha.
Our data indicate a continuous evolution of the power law with alpha ranging
from ~ 0.7 at high density and increasing to ~ 1.7 at the lowest densities
measured. Calculations specific to our structure indicate a crossover of the
dominant scattering mechanism from uniform background impurity scattering at
high density to remote ionized impurity scattering at low densities. This is
the first observation of a carrier density-induced transition from background
impurity dominated to remote dopant dominated transport in a single sample.Comment: 4 pages, 5 figures, prepared with LaTex2
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