2,230 research outputs found
Electropneumatic rheostat regulates high current
Electropneumatic rheostat maintains a constant direct current in each of several high-power parallel loads, of variable resistance, across a single source. It provides current regulation at any preset value by dissipating the proper amount of energy thermally, and uses a column of mercury to vary the effective length of a resistance element
Corotation Resonance and Diskoseismology Modes of Black Hole Accretion Disks
We demonstrate that the corotation resonance affects only some
non-axisymmetric g-mode oscillations of thin accretion disks, since it is
located within their capture zones. Using a more general (weaker radial WKB
approximation) formulation of the governing equations, such g-modes, treated as
perfect fluid perturbations, are shown to formally diverge at the position of
the corotation resonance. A small amount of viscosity adds a small imaginary
part to the eigenfrequency which has been shown to induce a secular instability
(mode growth) if it acts hydrodynamically. The g-mode corotation resonance
divergence disappears, but the mode magnitude can remain largest at the place
of the corotation resonance. For the known g-modes with moderate values of the
radial mode number and axial mode number (and any vertical mode number), the
corotation resonance lies well outside their trapping region (and inside the
innermost stable circular orbit), so the observationally relevant modes are
unaffected by the resonance. The axisymmetric g-mode has been seen by Reynolds
& Miller in a recent inviscid hydrodynamic accretion disk global numerical
simulation. We also point out that the g-mode eigenfrequencies are
approximately proportional to m for axial mode numbers |m|>0.Comment: 16 pages, no figures. Submitted to The Astrophysical Journa
Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity
We study the cosmological evolution of massless single-field scalar-tensor
theories of gravitation from the time before the onset of annihilation
and nucleosynthesis up to the present. The cosmological evolution together with
the observational bounds on the abundances of the lightest elements (those
mostly produced in the early universe) place constraints on the coefficients of
the Taylor series expansion of , which specifies the coupling of the
scalar field to matter and is the only free function in the theory. In the case
when has a minimum (i.e., when the theory evolves towards general
relativity) these constraints translate into a stronger limit on the
Post-Newtonian parameters and than any other observational
test. Moreover, our bounds imply that, even at the epoch of annihilation and
nucleosynthesis, the evolution of the universe must be very close to that
predicted by general relativity if we do not want to over- or underproduce
He. Thus the amount of scalar field contribution to gravity is very small
even at such an early epoch.Comment: 15 pages, 2 figures, ReVTeX 3.1, submitted to Phys. Rev. D1
Gravitational Waves in Generalised Brans-Dicke Theory
We have solved cosmological gravitational Wave(GW)equation in the frame work
of Generalised Brans-Dicke(GBD) theory for all epochs of the Universe.The
solutions are expressed in terms of the present value of the Brans-Dicke
coupling parameter .It is seen that the solutions represent
travelling growing modes for negative values of for all epochs of
the Universe.Comment: 7Pages,no figure
Anti-Proton Evolution in Little Bangs and Big Bang
The abundances of anti-protons and protons are considered within
momentum-integrated Boltzmann equations describing Little Bangs, i.e.,
fireballs created in relativistic heavy-ion collisions. Despite of a large
anti-proton annihilation cross section we find a small drop of the ratio of
anti-protons to protons from 170 MeV (chemical freeze-out temperature) till 100
MeV (kinetic freeze-out temperature) for CERN-SPS and BNL-RHIC energies thus
corroborating the solution of the previously exposed "ani-proton puzzle". In
contrast, the Big Bang evolves so slowly that the anti-baryons are kept for a
long time in equilibrium resulting in an exceedingly small fraction. The
adiabatic path of cosmic matter in the phase diagram of strongly interacting
matter is mapped out
Why people attend science festivals : interests, motivations and self-reported benefits of public engagement with research
As a form of public engagement, science festivals have rapidly expanded in size and number over recent years. However, as with other domains of informal public engagement that are not linked to policy outcomes, existing research does not fully address science festivals’ impacts and popularity.This study adduces evidence from surveys and focus groups to elucidate the perspectives of visitors at a large UK science festival. Results show that visitors value the opportunities science festivals afford to interact with scientific researchers and to encounter different types of science engagement aimed at adults, children and families. The most significant self-reported impact of attending a science festival was the development of increased interest and curiosity about new areas of scientific knowledge within a socially stimulating and enjoyable setting
Representation theory of some infinite-dimensional algebras arising in continuously controlled algebra and topology
In this paper we determine the representation type of some algebras of
infinite matrices continuously controlled at infinity by a compact metrizable
space. We explicitly classify their finitely presented modules in the finite
and tame cases. The algebra of row-column-finite (or locally finite) matrices
over an arbitrary field is one of the algebras considered in this paper, its
representation type is shown to be finite.Comment: 33 page
Contrasting carbonate depositional systems for Pliocene cool-water limestones cropping out in central Hawke's Bay, New Zealand
Pliocene limestone formations in central Hawke's Bay (eastern North Island, New Zealand) accumulated on and near the margins of a narrow forearc basin seaway within the convergent Australia/Pacific plate boundary zone. The active tectonic setting and varied paleogeographic features of the limestone units investigated, in association with probable glacioeustatic sea-level fluctuations, resulted in complex stratigraphic architectures and contrasting types of carbonate accumulation on either side of the seaway. Here, we recognise recurring patterns of sedimentary facies, and sequences and systems tracts bounded by key physical surfaces within the limestone sheets. The facies types range from Bioclastic (B) to Siliciclastic (S) end-members via Mixed (M) carbonate-siliciclastic deposits. Skeletal components are typical cool-water associations dominated by epifaunal calcitic bivalves, bryozoans, and especially barnacles. Siliciclastic contents vary from one formation to another, and highlight siliciclastic-rich limestone units in the western ranges versus siliciclastic-poor limestone units in the eastern coastal hills. Heterogeneities in facies types, stratal patterns, and also in diagenetic pathways between eastern and western limestone units are considered to originate in the coeval occurrence in different parts of the forearc basin of two main morphodynamic carbonate systems over time
Precision Prediction for the Big-Bang Abundance of Primordial Helium
Within the standard models of particle physics and cosmology we have
calculated the big-bang prediction for the primordial abundance of \he to a
theoretical uncertainty of less than 0.1 \pct ,
improving the current theoretical precision by a factor of 10. At this accuracy
the uncertainty in the abundance is dominated by the experimental uncertainty
in the neutron mean lifetime, . The following
physical effects were included in the calculation: the zero and
finite-temperature radiative, Coulomb and finite-nucleon-mass corrections to
the weak rates; order- quantum-electrodynamic correction to the plasma
density, electron mass, and neutrino temperature; and incomplete neutrino
decoupling. New results for the finite-temperature radiative correction and the
QED plasma correction were used. In addition, we wrote a new and independent
nucleosynthesis code designed to control numerical errors to be less than
0.1\pct. Our predictions for the \EL[4]{He} abundance are presented in the form
of an accurate fitting formula. Summarizing our work in one number, . Further,
the baryon density inferred from the Burles-Tytler determination of the
primordial D abundance, , leads to the
prediction: . This ``prediction'' and an accurate measurement of the primeval \he
abundance will allow an important consistency test of primordial
nucleosynthesis.Comment: Replaced fitting formulas - new versions differ by small but
significant amount. Other minor changes. 30 pages, 17 figures, 5 table
Gravitational Radiation from Nonaxisymmetric Instability in a Rotating Star
We present the first calculations of the gravitational radiation produced by
nonaxisymmetric dynamical instability in a rapidly rotating compact star. The
star deforms into a bar shape, shedding of its mass and
of its angular momentum. The gravitational radiation is calculated in the
quadrupole approximation. For a mass M and radius km, the gravitational waves have frequency kHz and amplitude
at the distance of the Virgo Cluster. They carry off
energy and radiate angular momentum .Comment: 16 pages, LaTeX with REVTEX macros, reprints available - send mailing
address to [email protected]. Published: PRL 72, 1314 (1994
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