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 (${\bf D} . ({\bf S}_i \times {\bf S}_j)$) 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 ‘lived to the full’. We call the resultant whole ‘living infrastructure’ to denote that it is both infrastructure for living and infrastructure that ‘lives’. 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 ‘live’. 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 ‘happens’ between multiple practices involved in medication therapy safety. We analyse this ‘happening’ 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 “meso-level” 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 $J'$ that interpolates between the pyrochlore lattice ($J'=0$) and the face-centered cubic lattice ($J'=J$). It is found that for $J'/J$ as low as $J'/J\ge 0.01$, the system is long range ordered : the disordered ground state of the pyrochlore antiferromagnet is unstable when introducing very small deviations from the pure $J'=0$ 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.