Supernova Fallback onto Magnetars and Propeller-powered Supernovae

We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields (~10^(15) G) and short spin periods (~1-10 ms) have an important influence on how the magnetar interacts with the infalling material. At long spin periods, weak magnetic fields, and high accretion rates, sufficient material is accreted to form a black hole, as is commonly found for massive progenitor stars. When B ≾ 5 × 10^(14) G, accretion causes the magnetar to spin sufficiently rapidly to deform triaxially and produces gravitational waves, but only for ≈50-200 s until it collapses to a black hole. Conversely, at short spin periods, strong magnetic fields, and low accretion rates, the magnetar is in the "propeller regime" and avoids becoming a black hole by expelling incoming material. This process spins down the magnetar, so that gravitational waves are only expected if the initial protoneutron star is spinning rapidly. Even when the magnetar survives, it accretes at least ≈0.3 M_☉, so we expect magnetars born within these types of environments to be more massive than the 1.4 M_☉ typically associated with neutron stars. The propeller mechanism converts the ~10^(52)erg of spin energy in the magnetar into the kinetic energy of an outflow, which shock heats the outgoing supernova ejecta during the first ~10-30 s. For a small ~5 M_☉ hydrogen-poor envelope, this energy creates a brighter, faster evolving supernova with high ejecta velocities ~(1-3) × 10^4 km s^(–1) and may appear as a broad-lined Type Ib/c supernova. For a large ≳ 10 M_☉ hydrogen-rich envelope, the result is a bright Type IIP supernova with a plateau luminosity of ≳ 10^(43)erg s^(–1) lasting for a timescale of ~60-80 days

Frustrated order on extrinsic geometries

We study, analytically and theoretically, defects in a nematically-ordered surface that couple to the extrinsic geometry of a surface. Though the intrinsic geometry tends to confine topological defects to regions of large Gaussian curvature, extrinsic couplings tend to orient the nematic in the local direction of maximum or minimum bending. This additional frustration is unavoidable and most important on surfaces of negative Gaussian curvature, where it leads to a complex ground state thermodynamics. We show, in contradistinction to the well-known effects of intrinsic geometry, that extrinsic curvature expels disclinations from the region of maximum curvature above a critical coupling threshold. On catenoids lacking an "inside-outside" symmetry, defects are expelled altogether.Comment: 4 pages, 3 figure

Numerical Modeling of the Early Light Curves of Type IIP Supernovae

The early rise of Type IIP supernovae (SN IIP) provides important information for constraining the properties of their progenitors. This can in turn be compared to pre-explosion imaging constraints and stellar models to develop a more complete picture of how massive stars evolve and end their lives. Using the SuperNova Explosion Code (SNEC), we model the first 40 days of SNe IIP to better understand what constraints can be derived from their early light curves. We use two sets of red supergiant progenitor models with zero-age main sequence masses in the range between 9 Msol and 20 Msol. We find that the early properties of the light curve depend most sensitively on the radius of the progenitor, and thus provide a relation between the g-band rise time and the radius at the time of explosion. This relation will be useful for deriving constraints on progenitors from future observations, especially in cases where detailed modeling of the entire rise is not practical. When comparing to observed rise times, the radii we find are a factor of a few larger than previous semi-analytic derivations and generally in better agreement with what is found with current stellar evolution calculations.Comment: 8 pages, 7 figure

Integrating Research Data Management into Geographical Information Systems

Ocean modelling requires the production of high-fidelity computational meshes upon which to solve the equations of motion. The production of such meshes by hand is often infeasible, considering the complexity of the bathymetry and coastlines. The use of Geographical Information Systems (GIS) is therefore a key component to discretising the region of interest and producing a mesh appropriate to resolve the dynamics. However, all data associated with the production of a mesh must be provided in order to contribute to the overall recomputability of the subsequent simulation. This work presents the integration of research data management in QMesh, a tool for generating meshes using GIS. The tool uses the PyRDM library to provide a quick and easy way for scientists to publish meshes, and all data required to regenerate them, to persistent online repositories. These repositories are assigned unique identifiers to enable proper citation of the meshes in journal articles.Comment: Accepted, camera-ready version. To appear in the Proceedings of the 5th International Workshop on Semantic Digital Archives (http://sda2015.dke-research.de/), held in Pozna\'n, Poland on 18 September 2015 as part of the 19th International Conference on Theory and Practice of Digital Libraries (http://tpdl2015.info/

Dirac fermion wave guide networks on topological insulator surfaces

Magnetic texturing on the surface of a topological insulator allows the design of wave guide networks and beam splitters for domain-wall Dirac fermions. Guided by simple analytic arguments we model a Dirac fermion interferometer consisting of two parallel pathways, whereby a newly developed staggered-grid leap-frog discretization scheme in 2+1 dimensions with absorbing boundary conditions is employed. The net transmission can be tuned between constructive to destructive interference, either by variation of the magnetization (path length) or an applied bias (wave length). Based on this principle, a Dirac fermion transistor is proposed. Extensions to more general networks are discussed.Comment: Submitted to PR

Effect of lattice mismatch-induced strains on coupled diffusive and displacive phase transformations

Materials which can undergo slow diffusive transformations as well as fast displacive transformations are studied using the phase-field method. The model captures the essential features of the time-temperature-transformation (TTT) diagrams, continuous cooling transformation (CCT) diagrams, and microstructure formation of these alloys. In some materials systems there can exist an intrinsic volume change associated with these transformations. We show that these coherency strains can stabilize mixed microstructures (such as retained austenite-martensite and pearlite-martensite mixtures) by an interplay between diffusive and displacive mechanisms, which can alter TTT and CCT diagrams. Depending on the conditions there can be competitive or cooperative nucleation of the two kinds of phases. The model also shows that small differences in volume changes can have noticeable effects on the early stages of martensite formation and on the resulting microstructures. -- Long version of cond-mat/0605577 -- Keywords: Ginzburg-Landau, martensite, pearlite, spinodal decomposition, shape memory, microstructures, TTT diagram, CCT diagram, elastic compatibilityComment: 10 pages, 13 figures, long version of cond-mat/0605577. Physical Review B, to appear in volume 75 (2007

A Particle-based Multiscale Solver for Compressible Liquid-Vapor Flow

To describe complex flow systems accurately, it is in many cases important to account for the properties of fluid flows on a microscopic scale. In this work, we focus on the description of liquid-vapor flow with a sharp interface between the phases. The local phase dynamics at the interface can be interpreted as a Riemann problem for which we develop a multiscale solver in the spirit of the heterogeneous multiscale method, using a particle-based microscale model to augment the macroscopic two-phase flow system. The application of a microscale model makes it possible to use the intrinsic properties of the fluid at the microscale, instead of formulating (ad-hoc) constitutive relations

Social Preferences and Voting: An Exploration Using a Novel Preference Revealing Mechanism

Public referenda are frequently used to determine the provision of public goods. As public programs have distributional consequences, a compelling question is what role if any social preferences have on voting behavior. This paper explores this issue using laboratory experiments wherein voting outcomes lead to a known distribution of net benefits across participants. Preferences are elicited using a novel Random Price Voting Mechanism (RPVM), which is a more parsimonious mechanism than dichotomous choice referenda, but gives consistent results. Results suggest that social preferences, in particular a social efficiency motive, lead to economically meaningful deviations from self-interested voting choices and increase the likelihood that welfare-enhancing programs are implemented.Institutional and Behavioral Economics, Research Methods/ Statistical Methods, C91, C92, D64, D72, H41,