4,979 research outputs found
Reionization of Hydrogen and Helium by Early Stars and Quasars
We compute the reionization histories of hydrogen and helium due to the
ionizing radiation fields produced by stars and quasars. For the quasars we use
a model based on halo-merger rates that reproduces all known properties of the
quasar luminosity function at high redshifts. The less constrained properties
of the ionizing radiation produced by stars are modeled with two free
parameters: (i) a transition redshift, z_tran, above which the stellar
population is dominated by massive, zero-metallicity stars and below which it
is dominated by a Scalo mass function; (ii) the product of the escape fraction
of stellar ionizing photons from their host galaxies and the star-formation
efficiency, f_esc f_*. We constrain the allowed range of these free parameters
at high redshifts based on the lack of the HI Gunn-Peterson trough at z<6 and
the upper limit on the total intergalactic optical depth for electron
scattering, tau_es<0.18, from recent cosmic microwave background (CMB)
experiments. We find that quasars ionize helium by a redshift z~4, but cannot
reionize hydrogen by themselves before z~6. A major fraction of the allowed
combinations of f_esc f_* and z_tran lead to an early peak in the ionized
fraction due to metal-free stars at high redshifts. This sometimes results in
two reionization epochs, namely an early HII or HeIII overlap phase followed by
recombination and a second overlap phase. Even if early overlap is not
achieved, the peak in the visibility function for scattering of the CMB often
coincides with the early ionization phase rather than with the actual
reionization epoch. Consequently, tau_es does not correspond directly to the
reionization redshift. We generically find values of tau_es>7%, that should be
detectable by the MAP satellite.Comment: 33 pages, 10 figures, Accepted for publication in Ap
Representations of p-brane topological charge algebras
The known extended algebras associated with p-branes are shown to be
generated as topological charge algebras of the standard p-brane actions. A
representation of the charges in terms of superspace forms is constructed. The
charges are shown to be the same in standard/extended superspace formulations
of the action.Comment: 22 pages. Typos fixed, refs added. Minor additions to comments
sectio
Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7
Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a
spin-freezing transition and magnetic irreversibilities below a temperature Tf
similar to what is observed nonlinear magnetization measurements on Y2Mo2O7
that provide strong evidence that there is an underlying thermodynamic phase
transition at Tf, which is characterized by critical exponents \gamma \approx
2.8 and \beta \approx 0.8. These values are typical of those found in random
spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is
immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included).
Submitted to Phys. Rev. Letters
Analytic treatment of geodesics in five-dimensional Myers-Perry space--times
We present the complete set of analytical solutions of the geodesic equation
in the five-dimensional Myers-Perry space-time with equal rotation parameter in
terms of the Weierstra{\ss}' elliptic and Weierstra{\ss}' zeta and sigma
functions. We study the underlying polynomials in the polar and radial
equations which depend on the parameters of the metric and conserved quantities
of a test particle and characterize the motion by their zeros. We exemplify the
efficiency of the analytical method on the orbits of test particles.Comment: 15 pages, 7 figures, to be published in PRD. Version with improved
reference
Dzyaloshinski-Moriya interactions in the kagome lattice
The kagom\'e lattice exhibits peculiar magnetic properties due to its
strongly frustated cristallographic structure, based on corner sharing
triangles. For nearest neighbour antiferromagnetic Heisenberg interactions
there is no Neel ordering at zero temperature both for quantum and classical s
pins. We show that, due to the peculiar structure, antisymmetric
Dzyaloshinsky-Moriya interactions ()
are present in this latt ice. In order to derive microscopically this
interaction we consider a set of localized d-electronic states. For classical
spins systems, we then study the phase diagram (T, D/J) through mean field
approximation and Monte-Carlo simulations and show that the antisymmetric
interaction drives this system to ordered states as soon as this interaction is
non zero. This mechanism could be involved to explain the magnetic structure of
Fe-jarosites.Comment: 4 pages, 2 figures. Presented at SCES 200
Living Infrastructure
Infrastructure is widely regarded merely as a material (lifeless) system that brings together the activities of diverse practices. In contrast to this view, we propose that when infrastructure provides a site where practices are held at once both near and apart, life under the influence of these practices is âlived to the fullâ. We call the resultant whole âliving infrastructureâ to denote that it is both infrastructure for living and infrastructure that âlivesâ. The key idea is that a living infrastructure becomes the site where an opening between certain regions of life, that share some concern, happens. We will argue that such infrastructure is an on-going achievement of becoming, which requires nurturing and vigilance to produce and its continued productivity: otherwise it will cease to âliveâ. We present an empirical case from the German healthcare environment - the Federal Unified Medication Plan for medication therapy safety. We argue in detail that this is a nascent living infrastructure providing a site where a productive opening âhappensâ between multiple practices involved in medication therapy safety. We analyse this âhappeningâ to establish how this opening took hold, how it was kept open, and how it was kept productive
The Use of an Explicitly Theory-Driven Data Coding Method for High-Level Theory Testing in IOIS
As part of an international empirical study we have developed a high-level theory of the structure and evolution of inter-organizational information systems (IOIS), but face two issues in testing this theory; the very large set of factors possibly influencing IOIS and need to bound a complex research object. We describe a novel solution involving an explicitly theory-driven coding of our raw empirical data to produce a new interpreted âmeso-levelâ ground upon which to derive and test predictions. To justify this approach, we first give a pragmatic analysis which traces the problem to the traditional choice between top-down (deductive) and bottom-up (inductive) methods for linking theory and data. We then assess the validity of our approach against standard philosophical positions. We find that a hard Empiricist view commonly espoused in information systems would not admit our approach but that Critical Realism does and suggests a conceptual interpretation of our meso-level
Classical heisenberg antiferromagnet away from the pyrochlore lattice limit: entropic versus energetic selection
The stability of the disordered ground state of the classical Heisenberg
pyrochlore antiferromagnet is studied within extensive Monte Carlo simulations
by introducing an additional exchange interaction that interpolates
between the pyrochlore lattice () and the face-centered cubic lattice
(). It is found that for as low as , the system is
long range ordered : the disordered ground state of the pyrochlore
antiferromagnet is unstable when introducing very small deviations from the
pure limit. Furthermore, it is found that the selected phase is a
collinear state energetically greater than the incommensurate phase suggested
by a mean field analysis. To our knowledge this is the first example where
entropic selection prevails over the energetic one.Comment: 5 (two-column revtex4) pages, 1 table, 7 ps/eps figures. Submitted to
Phys. Rev.
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