The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star

The propeller regime of disk accretion to a rapidly rotating magnetized star is investigated here for the first time by axisymmetric 2.5D magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in which the magnetic field is predominantly toroidal. A smaller fraction of the star's poloidal magnetic flux inflates vertically, forming a magnetically dominated tower. Matter accumulates in the equatorial region outside magnetosphere and accretes to the star quasi-periodically through elongated funnel streams which cause the magnetic field to reconnect. The star spins-down owing to the interaction of the closed magnetosphere with the disk. For the considered conditions, the spin-down torque varies with the angular velocity of the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller stage may be important in the evolution of X-ray pulsars, cataclysmic variables and young stars. In particular, it may explain the present slow rotation of the classical T Tauri stars.Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are available at http://www.astro.cornell.edu/us-russia/disk_prop.ht

Statistics of Neutron Stars at the Stage of Supersonic Propeller

We analyze the statistical distribution of neutron stars at the stage of a supersonic propeller. An important point of our analysis is allowance for the evolution of the angle of inclination of the magnetic axis to the spin axis of the neutron star for the boundary of the transition to the supersonic propeller stage for two models: the model with hindered particle escape from the stellar surface and the model with free particle escape. As a result, we have shown that a consistent allowance for the evolution of the inclination angle in the region of extinct radio pulsars for the two models leads to an increase in the total number of neutron stars at the supersonic propeller stage. This increase stems from he fact that when allowing for the evolution of the inclination angle $\chi$ for neutron stars in the region of extinct radio pulsars and, hence, for the boundary of the transition to the propeller stage, this transition is possible at shorter spin periods (P~5-10 s) than assumed in the standard model.Comment: 15 pages, 6 figures; scale corrected for figures 3-

Propagation of the First Flames in Type Ia Supernovae

We consider the competition of the different physical processes that can affect the evolution of a flame bubble in a Type Ia supernovae -- burning, turbulence and buoyancy. Even in the vigorously turbulent conditions of a convecting white dwarf, thermonuclear burning that begins at a point near the center (within 100 km) of the star is dominated by the spherical laminar expansion of the flame, until the burning region reaches kilometers in size. Consequently flames that ignite in the inner ~20 km promptly burn through the center, and flame bubbles anywhere must grow quite large--indeed, resolvable by large-scale simulations of the global system--for significant motion or deformation occur. As a result, any hot-spot that successfully ignites into a flame can burn a significant amount of white dwarf material. This potentially increases the stochastic nature of the explosion compared to a scenario where a simmering progenitor can have small early hot-spots float harmlessly away. Further, the size where the laminar flame speed dominates other relevant velocities sets a characteristic scale for fragmentation of larger flame structures, as nothing--by definition--can easily break the burning region into smaller volumes. This makes possible the development of semi-analytic descriptions of the earliest phase of the propagation of burning in a Type Ia supernovae, which we present here. Our analysis is supported by fully resolved numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap

A semantic web service-based architecture for the interoperability of e-government services

We propose a semantically-enhanced architecture to address the issues of interoperability and service integration in e-government web information systems. An architecture for a life event portal based on Semantic Web Services (SWS) is described. The architecture includes loosely-coupled modules organized in three distinct layers: User Interaction, Middleware and Web Services. The Middleware provides the semantic infrastructure for ontologies and SWS. In particular a conceptual model for integrating domain knowledge (Life Event Ontology), application knowledge (E-government Ontology) and service description (Service Ontology) is defined. The model has been applied to a use case scenario in e-government and the results of a system prototype have been reported to demonstrate some relevant features of the proposed approach

The 24-Cell and Calabi-Yau Threefolds with Hodge Numbers (1,1)

Calabi-Yau threefolds with h^11(X)=h^21(X)=1 are constructed as free quotients of a hypersurface in the ambient toric variety defined by the 24-cell. Their fundamental groups are SL(2,3), a semidirect product of Z_3 and Z_8, and Z_3 x Q_8.Comment: 22 pages, 3 figures, 3 table

Spherical Bondi accretion onto a magnetic dipole

Quasi-spherical supersonic (Bondi-type) accretion to a star with a dipole magnetic field is investigated using resistive magnetohydrodynamic simulations. A systematic study is made of accretion to a non-rotating star, while sample results for a rotating star are also presented. A new stationary subsonic accretion flow is found with a steady rate of accretion to the magnetized star smaller than the Bondi accretion rate. Dependences of the accretion rate and the flow pattern on the magnetic momentum of the star and the magnetic diffusivity are presented. For slow star's rotation the accretion flow is similar to that in non-rotating case, but in the case of fast rotation the structure of the subsonic accretion flow is fundamentally different and includes a region of ``propeller'' outflow. The methods and results described here are of general interest and can be applied to systems where matter accretes with low angular momentum.Comment: 15 pages, 15 figures, used emulapj.st

Microscopic sub-barrier fusion calculations for the neutron star crust

Fusion of very neutron rich nuclei may be important to determine the composition and heating of the crust of accreting neutron stars. Fusion cross sections are calculated using time-dependent Hartree-Fock theory coupled with density-constrained Hartree-Fock calculations to deduce an effective potential. Systems studied include 16O+16O, 16O+24O, 24O+24O, 12C+16O, and 12C+24O. We find remarkable agreement with experimental cross sections for the fusion of stable nuclei. Our simulations use the SLy4 Skyrme force that has been previously fit to the properties of stable nuclei, and no parameters have been fit to fusion data. We compare our results to the simple S\~{a}o Paulo static barrier penetration model. For the asymmetric systems 12C+24O or 16O+24O we predict an order of magnitude larger cross section than those predicted by the S\~{a}o Paulo model. This is likely due to the transfer of neutrons from the very neutron rich nucleus to the stable nucleus and dynamical rearrangements of the nuclear densities during the collision process. These effects are not included in potential models. This enhancement of fusion cross sections, for very neutron rich nuclei, can be tested in the laboratory with radioactive beams.Comment: 9 pages, 11 figures, corrected small errors in Figs 10, 11, Phys. Rev. C in pres