Path integral Monte Carlo simulations of silicates

We investigate the thermal expansion of crystalline SiO$_2$ in the $\beta$-- cristobalite and the $\beta$-quartz structure with path integral Monte Carlo (PIMC) techniques. This simulation method allows to treat low-temperature quantum effects properly. At temperatures below the Debye temperature, thermal properties obtained with PIMC agree better with experimental results than those obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press

The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles

By solving the time-dependent radiation transfer problem of stellar radiation through evolving superbubbles within a smoothly varying HI distribution, we estimate the fraction of ionizing photons emitted by OB associations that escapes the HI disk of our Galaxy into the halo and intergalactic medium (IGM). We consider both coeval star-formation and a Gaussian star-formation history with a time spread sigma_t = 2 Myr. We consider both a uniform H I distribution and a two-phase (cloud/intercloud) model, with a negligible filling factor of hot gas. We find that the shells of the expanding superbubbles quickly trap or attenuate the ionizing flux, so that most of the escaping radiation escapes shortly after the formation of the superbubble. For the coeval star-formation history, the total fraction of Lyman Continuum photons that escape both sides of the disk in the solar vicinity is f_esc approx 0.15 +/- 0.05. For the Gaussian star formation history, f_esc approx 0.06 +/- 0.03, a value roughly a factor of two lower than the results of Dove & Shull (1994), where superbubbles were not considered. For a local production rate of ionizing photons Psi_LyC = 4.95 X 10^7 cm^{-2} s^{-1}, the flux escaping the disk is Phi_LyC approx (1.5-3.0) X 10^6 cm^{-2} s^{-1} for coeval and Gaussian star formation, comparable to the flux required to sustain the Reynolds layer.Comment: Revised version (expanded), accepted for publication by ApJ, 38 pages, 8 figures, aasms4.sty and aabib.sty files include

ASCA and Radio/RXTE Observations of GX 339-4

We have analyzed three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) observations and eight separate Rossi X-ray Timing Explorer (RXTE) observations of the black hole candidate GX 339-4 in its low luminosity, spectrally hard state. Three of the RXTE observations were strictly simultaneous with 843 MHz and 8.3- 9.1 GHz radio observations. All data sets show evidence for an approximately 6.4 keV Fe line with equivalent widths approximately 20-100 eV. 'Reflection models' show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. None of the models fit to the X-ray data, however, simultaneously explain the observed radio properties. We argue that the spatial extent of the observed radio emission is at least 0(10(exp 7 GM/c squared). Timing analysis reveals that all observations save one show evidence of a persistent f(qpo approximately equals 0.3 Hz quasi-periodic oscillations(quasi-periodic oscillations)). The broad band (10-3-102 Hz) power appears to be dominated by two independent processes that can be modeled as very broad Lorentzians with Q approximately less than 1. Similar to Cyg X-1, the hard photon variability is seen to lag the soft photon vaxiability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time lag is seen to be positively correlated with the flux of GX 339-4

Dependence of copolymer sequencing based on lactone ring size and ε-substitution

The copolymerization of an ε-substituted ε-lactone, menthide (MI), and a range of nonsubstituted lactones (6-, 7-, 8-, and 9-membered rings) was investigated in order to determine the factors that affect the sequencing of the MI copolymers. Analysis by quantitative 13C NMR spectroscopy showed the copolymerization of MI with a nonsubstituted lactone of ring size 7 or less produced a randomly sequenced copolymer, as a consequence of the smaller lactone polymerizing first and undergoing rapid transesterification as MI was incorporated. Conversely, copolymerization with larger ring lactones (ring size 8 and above) produced block-like copolymers as a consequence of MI polymerizing initially, which does not undergo rapid transesterification side reactions during the incorporation of the second monomer. Terpolymerizations of a small ring lactone, macrolactone, and menthide demonstrated methods of producing lactone terpolymers with different final sequences, depending on when the small ring lactone was injected into the reaction mixture

Grid tool integration within the eMinerals Project

In this article we describe the eMinerals mini grid, which is now running in production mode. Thisis an integration of both compute and data components, the former build upon Condor, PBS and thefunctionality of Globus v2, and the latter being based on the combined use of the Storage ResourceBroker and the CCLRC data portal. We describe how we have integrated the middleware components,and the different facilities provided to the users for submitting jobs within such an environment. We willalso describe additional functionality we found it necessary to provide ourselves

Lessons in scientific data interoperability: XML and the eMinerals project

A collaborative environmental eScience project produces a broad range of data, notable as much for its diversity, in source and format, as its quantity. We find that extensible markup language (XML) and associated technologies are invaluable in managing this deluge of data. We describe FX, a toolkit for allowing Fortran codes to read and write XML, thus allowing existing scientific tools to be easily re-used in an XML-centric workflow

Ring-opening polymerisation of alkyl-substituted ε-caprolactones:kinetic effects of substitution position†

Ring-opening polymerisation (ROP) of lactones has been proven as a powerful technique to generate polyesters with high levels of control over molar mass and polymer dispersity. However, the introduction of functional groups on the monomer ring structure can dramatically influence the ability of a monomer to undergo ROP. Therefore, understanding the structure–reactivity relationship of functional monomers is essential to gain access to materials with chemical functionality via direct polymerisation. Herein, we report how structural modifications of alkyl-substituted ε-caprolactones affected their reactivity towards the ring-opening of the functional monomer. We observed that the reactivity was strongly influenced by the substituent position, wherein the δ-substituted monomer exhibited the fastest polymerisation kinetics. In contrast, a substituent placement in the ε-position significantly reduced polymerisation time compared to other substituent positions. Moreover, the thermal properties of the resultant functional ε-polycaprolactones were investigated and showed no significant change in the thermal transitions. This demonstrates that functional caprolactone monomers with sterically demanding functional groups can still undergo direct ring-opening polymerisation and that careful positioning of these functional groups enables control of the rate of polymerisation, a crucial parameter to be considered for the design of new prospective functional monomers and their industrial applications