Elastic theory of icosahedral quasicrystals - application to straight dislocations

In quasicrystals, there are not only conventional, but also phason displacement fields and associated Burgers vectors. We have calculated approximate solutions for the elastic fields induced by two-, three- and fivefold straight screw- and edge-dislocations in infinite icosahedral quasicrystals by means of a generalized perturbation method. Starting from the solution for elastic isotropy in phonon and phason spaces, corrections of higher order reflect the two-, three- and fivefold symmetry of the elastic fields surrounding screw dislocations. The fields of special edge dislocations display characteristic symmetries also, which can be seen from the contributions of all orders.Comment: 13 pages, 11 figure

Study of liquid jet impingement on screens

A model is presented for an unconfined flow, such as a free jet, impinging on a screen which incorporates the influence of liquid deflection by the screen. The boundary layer blockage coefficient is introduced. This coefficient depends on the screen weave geometry and the jet impingement angle, and essentially accounts for the increase in fluid particle trajectory length through the screen resulting from the flow deflection. Comparisons were made with previous experimental studies to determine empirical values of the blockage coefficient. It is concluded that the new model reliably predicts the bulk flow and penetration characteristics of an impinging liquid jet interacting with a screen

A mercuric detector system for X-ray astronomy. 2. Results from flight tests of a balloon borne instrument

To establish the expected sensitivity of a new hard X-ray telescope design, an experiment was conducted to measure the background counting rate at balloon altitudes (40 km) of mercuric iodide, a room temperature solid state X-ray detector. The prototype detector consisted of two thin mercuric iodide (HgI2) detectors surrounded by a large bismuth germanate (Bi4Ge3O12) scintillator operated in anticoincidence. The bismuth germanate shield vetoed most of the background counting rate induced by atmospheric gamma-rays, neutrons and cosmic rays. A balloon-borne gondola containing a prototype detector assembly was designed, constructed and flown twice in the spring of 1982 from Palestine, Texas. The second flight of this instrument established a differential background counting rate of 4.2 O.7 x 10-5 counts/sec cm keV over the energy range of 40 to 80 keV. This measurement was within 50% of the predicted value. The measured rate is approx 5 times lower than previously achieved in shielded NaI/CsI or Ge systems operating in the same energy range. The prediction was based on a Monte Carlo simulation of the detector assembly in the radiation environment at float altitude

An orientable, stabilized balloon-borne gondola for around-the-world flights

A system capable of pointing a balloon-borne telescope at selected celestial objects to an accuracy of approximately 10 arc minutes for an extended period (weeks to months) without reliance on telemetry is described. A unique combination of a sun/star tracker, an on-board computer, and a gyrocompass is utilized for navigation, source acquisition and tracking, and data compression and recording. The possibilities for intelligent activities by the computer are also discussed

The Impact of Type Ia Supernova Ejecta on Binary Companions

We present adaptive mesh refinement (AMR) hydrodynamical simulations of the interaction between Type Ia supernovae and their companion stars within the context of the single-degenerate model. Results for 3D red-giant companions without binary evolution agree with previous 2D results by Marietta et al. We also consider evolved helium-star companions in 2D. For a range of helium-star masses and initial binary separations, we examine the mass unbound by the interaction and the kick velocity delivered to the companion star. We find that unbound mass versus separation obeys a power law with index between -3.1 and -4.0, consistent with previous results for hydrogen-rich companions. Kick velocity also obeys a power-law relationship with binary separation, but the slope differs from those found for hydrogen-rich companions. Assuming accretion via Roche-lobe overflow, we find that the unbound helium mass is consistent with observational limits. Ablation (shock heating) appears to be more important in removing gas from helium-star companions than from hydrogen-rich ones, though stripping (momentum transfer) dominates in both cases.Comment: 6 pages, 2 figures, to appear in the proceedings of the conference "Binary Star Evolution: Mass Loss, Accretion, and Mergers" at Mykonos, Greece, June 22-25, 201

A Direct Multigrid Poisson Solver for Oct-Tree Adaptive Meshes

We describe a finite-volume method for solving the Poisson equation on oct-tree adaptive meshes using direct solvers for individual mesh blocks. The method is a modified version of the method presented by Huang and Greengard (2000), which works with finite-difference meshes and does not allow for shared boundaries between refined patches. Our algorithm is implemented within the FLASH code framework and makes use of the PARAMESH library, permitting efficient use of parallel computers. We describe the algorithm and present test results that demonstrate its accuracy.Comment: 10 pages, 6 figures, accepted by the Astrophysical Journal; minor revisions in response to referee's comments; added char

Glassy behavior of molecular crystals: A comparison between results from MD-simulation and mode coupling theory

We have investigated the glassy behavior of a molecular crystal built up with chloroadamantane molecules. For a simple model of this molecule and a rigid fcc lattice a MD simulation was performed from which we obtained the dynamical orientational correlators $S_{\lambda \lambda '}({\bf{q}},t)$ and the ``self'' correlators $S_{\lambda \lambda '}^{(s)}(t)$, with $\lambda = (\ell, m)$, $\lambda' = (\ell', m')$. Our investigations are for the diagonal correlators $\lambda = \lambda'$. Since the lattice constant decreases with decreasing temperature which leads to an increase of the steric hindrance of the molecules, we find a strong slowing down of the relaxation. It has a high sensitivity on $\lambda$, $\lambda '$. For most $(\ell,m)$, there is a two-step relaxation process, but practically not for $(\ell,m) = (2,1)$, $(3,2)$, $(4,1)$ and $(4,3)$. Our results are consistent with the $\alpha$-relaxation scaling laws predicted by mode coupling theory from which we deduce the glass transition temperature $T_c^{MD} \cong 217K$. From a first principle solution of the mode coupling equations we find $T_c^{MCT} \cong 267K$. Furthermore mode coupling theory reproduces the absence of a two-step relaxation process for $(\ell,m)=(2,1)$, $(3,2)$, $(4,1)$ and $(4,3)$, but underestimates the critical nonergodicity parameters by about 50 per cent for all other $(\ell,m)$. It is suggested that this underestimation originates from the anisotropic crystal field which is not accounted for by mode coupling theory. Our results also imply that phonons have no essential influence on the long time relaxation

Robust Emergent Activity in Dynamical Networks

We study the evolution of a random weighted network with complex nonlinear dynamics at each node, whose activity may cease as a result of interactions with other nodes. Starting from a knowledge of the micro-level behaviour at each node, we develop a macroscopic description of the system in terms of the statistical features of the subnetwork of active nodes. We find the asymptotic characteristics of this subnetwork to be remarkably robust: the size of the active set is independent of the total number of nodes in the network, and the average degree of the active nodes is independent of both the network size and its connectivity. These results suggest that very different networks evolve to active subnetworks with the same characteristic features. This has strong implications for dynamical networks observed in the natural world, notably the existence of a characteristic range of links per species across ecological systems.Comment: 4 pages, 5 figure

Rapid fluctuations in the high-energy X-ray flux from a source in Crux

Balloonborne X ray telescopic observations of two point sources in Cru