Fracturing the optimal paths

Optimal paths play a fundamental role in numerous physical applications ranging from random polymers to brittle fracture, from the flow through porous media to information propagation. Here for the first time we explore the path that is activated once this optimal path fails and what happens when this new path also fails and so on, until the system is completely disconnected. In fact numerous applications can be found for this novel fracture problem. In the limit of strong disorder, our results show that all the cracks are located on a single self-similar connected line of fractal dimension $D_{b} \approx 1.22$. For weak disorder, the number of cracks spreads all over the entire network before global connectivity is lost. Strikingly, the disconnecting path (backbone) is, however, completely independent on the disorder.Comment: 4 pages,4 figure

Accurate Evolutions of Orbiting Binary Black Holes

We present a detailed analysis of binary black hole evolutions in the last orbit and demonstrate consistent and convergent results for the trajectories of the individual bodies. The gauge choice can significantly affect the overall accuracy of the evolution. It is possible to reconcile certain gauge-dependent discrepancies by examining the convergence limit. We illustrate these results using an initial data set recently evolved by Brügmann et al. [Phys. Rev. Lett. 92, 211101 (2004)]. For our highest resolution and most accurate gauge, we estimate the duration of this data set's last orbit to be approximately 59MADM

New Young Stars and Brown Dwarfs in the Upper Scorpius Association

To improve the census of the Upper Sco association (~11 Myr, ~145 pc), we have identified candidate members using parallaxes, proper motions, and color-magnitude diagrams from several wide-field imaging surveys and have obtained optical and infrared spectra of several hundred candidates to measure their spectral types and assess their membership. We also have performed spectroscopy on a smaller sample of previously known or suspected members to refine their spectral types and evidence of membership. We have classified 530 targets as members of Upper Sco, 377 of which lack previous spectroscopy. Our new compilation of all known members of the association contains 1631 objects. Although the census of Upper Sco has expanded significantly over the last decade, there remain hundreds of candidates that lack spectroscopy. The precise parallaxes and proper motions from the second data release of Gaia should extend down to substellar masses in Upper Sco, which will greatly facilitate the identification of the undiscovered members.Comment: Astronomical Journal, in press; machine readable tables and fits spectra available at http://personal.psu.edu/kll207/usco.ta

Alumni News: Winter 2003

Newsletter for Boston University School of Medicine alumni

Cluster counting: The Hoshen-Kopelman algorithm vs. spanning tree approaches

Two basic approaches to the cluster counting task in the percolation and related models are discussed. The Hoshen-Kopelman multiple labeling technique for cluster statistics is redescribed. Modifications for random and aperiodic lattices are sketched as well as some parallelised versions of the algorithm are mentioned. The graph-theoretical basis for the spanning tree approaches is given by describing the "breadth-first search" and "depth-first search" procedures. Examples are given for extracting the elastic and geometric "backbone" of a percolation cluster. An implementation of the "pebble game" algorithm using a depth-first search method is also described.Comment: LaTeX, uses ijmpc1.sty(included), 18 pages, 3 figures, submitted to Intern. J. of Modern Physics

Dynamics of Vibrated Granular Monolayers

We study statistical properties of vibrated granular monolayers using molecular dynamics simulations. We show that at high excitation strengths, the system is in a gas state, particle motion is isotropic, and the velocity distributions are Gaussian. As the vibration strength is lowered the system's dimensionality is reduced from three to two. Below a critical excitation strength, a gas-cluster phase occurs, and the velocity distribution becomes bimodal. In this phase, the system consists of clusters of immobile particles arranged in close-packed hexagonal arrays, and gas particles whose energy equals the first excited state of an isolated particle on a vibrated plate.Comment: 4 pages, 6 figs, revte

Riemann solvers and undercompressive shocks of convex FPU chains

We consider FPU-type atomic chains with general convex potentials. The naive continuum limit in the hyperbolic space-time scaling is the p-system of mass and momentum conservation. We systematically compare Riemann solutions to the p-system with numerical solutions to discrete Riemann problems in FPU chains, and argue that the latter can be described by modified p-system Riemann solvers. We allow the flux to have a turning point, and observe a third type of elementary wave (conservative shocks) in the atomistic simulations. These waves are heteroclinic travelling waves and correspond to non-classical, undercompressive shocks of the p-system. We analyse such shocks for fluxes with one or more turning points. Depending on the convexity properties of the flux we propose FPU-Riemann solvers. Our numerical simulations confirm that Lax-shocks are replaced by so called dispersive shocks. For convex-concave flux we provide numerical evidence that convex FPU chains follow the p-system in generating conservative shocks that are supersonic. For concave-convex flux, however, the conservative shocks of the p-system are subsonic and do not appear in FPU-Riemann solutions

Scaling for the Percolation Backbone

We study the backbone connecting two given sites of a two-dimensional lattice separated by an arbitrary distance $r$ in a system of size $L$. We find a scaling form for the average backbone mass: $\sim L^{d_B}G(r/L)$, where $G$ can be well approximated by a power law for $0\le x\le 1$: $G(x)\sim x^{\psi}$ with $\psi=0.37\pm 0.02$. This result implies that $\sim L^{d_B-\psi}r^{\psi}$ for the entire range $0<r<L$. We also propose a scaling form for the probability distribution $P(M_B)$ of backbone mass for a given $r$. For $r\approx L, P(M_B)$ is peaked around $L^{d_B}$, whereas for $r\ll L, P(M_B)$ decreases as a power law, $M_B^{-\tau_B}$, with $\tau_B\simeq 1.20\pm 0.03$. The exponents $\psi$ and $\tau_B$ satisfy the relation $\psi=d_B(\tau_B-1)$, and $\psi$ is the codimension of the backbone, $\psi=d-d_B$.Comment: 3 pages, 5 postscript figures, Latex/Revtex/multicols/eps

Desarrollo gonadico invernal y desove primaveral, de la almeja amarilla Mesodesma mactroides (Bivalvia: Mesodesmatidae) en las playas de Santa Teresita

La almeja amarilla Mesodesma mactroides (Reeve, 1854) es un bivalvo de vida libre habitante de la zona intermareal de las playas del Atlántico sur. Su distribución se extiende desde San Pablo (S23°31, W46°37) en Brasil hasta Monte Hermoso (S38°59, W61°15) en Argentina. El ciclo reproductivo de la almeja amarilla fue determinado en las playas de Santa Teresita (S36°32, W56°41) desde diciembre de 2004 hasta diciembre de 2006. El desarrollo gonadal fue determinado utilizando cortes histológicos (N=576). Se determino el índice gonadosomatico como: CI=100WV/(Wt-WV), donde WV es el peso húmedo de la gónada (pie inclusive), Wt es el peso húmedo de las viseras total, también el numero y diámetro de los ovocitos. Los mismos están maduros cuando su diámetro supera los 37 µm (SE 0.87) con un máximo de 64.86 en el mes de septiembre del 2006. M. mactroides alcanza la primera madurez 16.36 mm de longitud (anterior-posterior). La proporción de sexos es 1:1, no se han registrado hermafroditas ni parasitismo. Durante todos los meses muestreados se encontraron individuos tanto activos como inactivos. Los estadios de maduración más avanzada se registraron en los meses de agosto-octubre, seguidos por los eventos de evacuación total y parcial en los meses de noviembre-diciembre. Sin embargo no hubo un periodo en el ciclo invierno-primavera donde se haya registrado inactividad total. La temperatura de superficie del agua es un factor importante en el inicio del periodo de evacuación. El índice (CI) soporta el resultado del estudio histológico, mostrando una correlación positiva entre la temperatura y la actividad en la reproducción, mas especialmente en los eventos de evacuación. Sin embargo, las pruebas que proporcionan los estudios histológicos son necesarias, ya que condiciones ambientales desfavorables como la falta de alimento y por ende desnutrición, producen de la misma forma un impacto en las condiciones de las almejas. El póster discute si el reclutamiento futuro se acopla con el evento reproductivo, y puede entonces ser predicho por monitoreo gonadal