Black hole neutron star coalescence as a source of gamma-ray bursts

We present the results of hydrodynamic (SPH) simulations showing the coalescence of a black hole with a neutron star to be a promising theoretical source of short duration gamma-ray bursts. The favorable features of the process include rapid onset, millisecond variability, a duration much longer than the dynamical timescale, and a range of outcomes sufficient to allow variety in the properties of individual gamma-ray bursts. Interestingly, the process of coalescence differs rather markedly from past predictions.Comment: 6 pages, 4 Postscript figures, uses aipproc.sty . To appear in 4th Gamma Ray Bursts Symposium, C.A. Meegan and P. Cushman, ed

Compensated Half-metallicity in the Trigonally Distorted Perovskite-type NiCrO3_3

Using first principles calculations, we investigate the electronic and magnetic properties of the trigonally distorted (R-3c) perovskite-derived NiCrO$_3$. Within the local spin density approximation (LSDA), our calculations show that this system is an exactly compensated half-metal (CHM). The local spin moments of Cr 2.04, and antialigned Ni -1.41 and three oxygens -0.63 (in the units of $\mu_B$), indicate high spin S=3/2 Cr$^{3+}$ and S=3/2 (NiO$_3$)$^{3-}$ units. Considering reasonable values of the on-site Coulomb repulsion U on both Ni and Cr ions with LDA+U approach, this system becomes an insulator (as reported by Chamberland and Cloud) having a narrow gap in the spin-up channel, whereas the other channel has a large gap of ~3 eV. Although inclusion of U seemingly leads to the transition Ni$^{2+} --> high spin S=3/2 Ni$^{3+}$, consistent with the experimentally observed effective moment, the zero net moment remains unchanged due to either reduction of oxygen local moments or enhancement of Cr local moment. Compression of volume by 10to CHM even when correlation effects are included.These results suggest the possibility of a CHM state in NiCrO$_3$ and provide another route to search for CHM, which is a property sought by many.Comment: 5 pages, 5 embedded figures, (To be published in PRB rapid Commun.

Charge and Spin Ordering in Insulating Na0.5_{0.5}CoO2_2: Effects of Correlation and Symmetry

Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge- and spin-ordering in insulating Na$_{0.5}$CoO$_2$. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co$^{4+}$ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90 degree angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge transfer" type within the Co 3d system.Comment: 5 pages, 3 figure

Evaluating foam heterogeneity

New analytical tool is available to calculate the degree of foam heterogeneity based on the measurement of gas diffusivity values. Diffusion characteristics of plastic foam are described by a system of differential equations based on conventional diffusion theory. This approach saves research and computation time in studying mass or heat diffusion problems

Reduction of porosity in aluminum weldments

Method is described for elimination of porosity of aluminum weldments by replacing polyvinyl chloride tubing (used to connect welder to gas source, and is permeable to moisture at high humidity) with copper tubing. In addition liquid argon gas is used at weld stations

Gauge Theory of Gravity Requires Massive Torsion Field

One of the greatest unsolved issues of the physics of this century is to find a quantum field theory of gravity. According to a vast amount of literature unification of quantum field theory and gravitation requires a gauge theory of gravity which includes torsion and an associated spin field. Various models including either massive or massless torsion fields have been suggested. We present arguments for a massive torsion field, where the probable rest mass of the corresponding spin three gauge boson is the Planck mass.Comment: 3 pages, Revte

Sweeping the cd-Index and the Toric h-Vector

We derive formulas for the cd-index and the toric h-vector of a convex polytope P from a sweeping by a hyperplane. These arise from interpreting the corresponding S-shelling of the dual of P. We describe a partition of the faces of the complete truncation of P to reflect explicitly the nonnegativity of its cd-index and what its components are counting. One corollary is a quick way to compute the toric h-vector directly from the cd-index. We also propose an "extended toric" h-vector that fully captures the information in the flag h-vector.Comment: 23 page