Electron Population Aging Models for Wide-Angle Tails

Color-color diagrams have been useful in studying the spectral shapes in radio galaxies. At the workshop we presented color-color diagrams for two wide-angle tails, 1231+674 and 1433+553, and found that the standard aging models do not adequately represent the observed data. Although the JP and KP models can explain some of the observed points in the color-color diagram, they do not account for those found near the power-law line. This difficulty may be attributable to several causes. Spectral tomography has been previously used to discern two separate electron populations in these sources. The combination spectra from two such overlying components can easily resemble a range of power-laws. In addition, any non-uniformity in the magnetic field strength can also create a power-law-like spectrum. We will also discuss the effects that angular resolution has on the shape of the spectrum.Comment: 4 pages, 1 figure, proceedings from 1999 'Life Cycles of Radio Galaxies' workshop at STScI in Baltimore, M

Relaxation and breakup of an initially extended drop in an otherwise quiescent fluid

In this paper we examine some general features of the time-dependent dynamics of drop deformation and breakup at low Reynolds numbers. The first aspect of our study is a detailed numerical investigation of the ‘end-pinching’ behaviour reported in a previous experimental study. The numerics illustrate the effects of viscosity ratio and initial drop shape on the relaxation and/or breakup of highly elongated droplets in an otherwise quiescent fluid. In addition, the numerical procedure is used to study the simultaneous development of capillary-wave instabilities at the fluid-fluid interface of a very long, cylindrically shaped droplet with bulbous ends. Initially small disturbances evolve to finite amplitude and produce very regular drop breakup. The formation of satellite droplets, a nonlinear phenomenon, is also observed

Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer

We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ~2000. Our observations span the K-band, and include the Br gamma transition of Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several targets we also present data from Keck/NIRSPEC that provide higher spectral resolution, but a seeing-limited spatial resolution, of the same spectral features. We analyze the Br gamma emission in the context of both disk and infall/outflow models, and conclude that the Br gamma emission traces gas at very small stellocentric radii, consistent with the magnetospheric scale. However some Br gamma-emitting gas also seems to be located at radii of >0.1 AU, perhaps tracing the inner regions of magnetically launched outflows. CO emission is detected from several objects, and we generate disk models that reproduce both the KI and NIRSPEC data well. We infer the CO spatial distribution to be coincident with the distribution of continuum emission in most cases. Furthermore the Br gamma emission in these objects is roughly coincident with both the CO and continuum emission. We present potential explanations for the spatial coincidence of continuum, Br gamma, and CO overtone emission, and explore the implications for the low occurrence rate of CO overtone emission in young stars. Finally, we provide additional discussion of V1685 Cyg, which is unusual among our sample in showing large differences in emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA

Use of quantum quenches to probe the equilibrium current patterns of ultracold atoms in an optical lattice

Atomic bosons and fermions in an optical lattice can realize a variety of interesting condensed matter states that support equilibrium current patterns in the presence of synthetic magnetic fields or non-abelian gauge fields. As a route to probing such mass currents, we propose a nonequilibrium quantum quench of the Hamiltonian that dynamically converts the current patterns into experimentally measurable real-space density patterns. We illustrate how a specific such "unidirectional" quench of the optical lattice can be used to uncover checkerboard and stripe current orders in lattice Bose superfluids and Fermi gases, as well as chiral edge currents in an integer quantum Hall state.Comment: 5 pages, 4 figures, Slightly revised, to appear in Phys. Rev. A (Rapid Communication

Quasi-Chemical and Structural Analysis of Polarizable Anion Hydration

Quasi-chemical theory is utilized to analyze the roles of solute polarization and size in determining the structure and thermodynamics of bulk anion hydration for the Hofmeister series Cl$^-$, Br$^-$, and I$^-$. Excellent agreement with experiment is obtained for whole salt hydration free energies using the polarizable AMOEBA force field. The quasi-chemical approach exactly partitions the solvation free energy into inner-shell, outer-shell packing, and outer-shell long-ranged contributions by means of a hard-sphere condition. Small conditioning radii, even well inside the first maximum of the ion-water(oxygen) radial distribution function, result in Gaussian behavior for the long-ranged contribution that dominates the ion hydration free energy. The spatial partitioning allows for a mean-field treatment of the long-ranged contribution, leading to a natural division into first-order electrostatic, induction, and van der Waals terms. The induction piece exhibits the strongest ion polarizability dependence, while the larger-magnitude first-order electrostatic piece yields an opposing but weaker polarizability dependence. In addition, a structural analysis is performed to examine the solvation anisotropy around the anions. As opposed to the hydration free energies, the solvation anisotropy depends more on ion polarizability than on ion size: increased polarizability leads to increased anisotropy. The water dipole moments near the ion are similar in magnitude to bulk water, while the ion dipole moments are found to be significantly larger than those observed in quantum mechanical studies. Possible impacts of the observed over-polarization of the ions on simulated anion surface segregation are discussed.Comment: slight revision, in press at J. Chem. Phy

Relaxation time of the topological T1 process in a two-dimensional foam

The elementary topological T1 process in a two-dimensional foam corresponds to the "flip" of one soap film with respect to the geometrical constraints. From a mechanical point of view, this T1 process is an elementary relaxation process through which the entire structure of an out-of-equilibrium foam evolves. The dynamics of this elementary relaxation process has been poorly investigated and is generally neglected during simulations of foams. We study both experimentally and theoretically the T1 dynamics in a dry two-dimensional foam. We show that the dynamics is controlled by the surface viscoelastic properties of the soap films (surface shear plus dilatational viscosity, ms+k, and Gibbs elasticity e), and is independent of the shear viscosity of the bulk liquid. Moreover, our approach illustrates that the dynamics of T1 relaxation process provides a convenient tool for measuring the surface rheological properties: we obtained e = 32+/-8 mN/m and ms+k = 1.3+/-0.7 mPa.m.s for SDS, and e = 65+/-12 mN/m and ms+k = 31+/-12 mPa.m.s for BSA, in good agreement with values reported in the literature

Effect of contrast on the perception of direction of a moving pattern

A series of experiments examining the effect of contrast on the perception of moving plaids was performed to test the hypothesis that the human visual system determines the direction of a moving plaid in a two-staged process: decomposition into component motion followed by application of the intersection-of-contraints rule. Although there is recent evidence that the first tenet of the hypothesis is correct, i.e., that plaid motion is initially decomposed into the motion of the individual grating components, the nature of the second-stage combination rule has not yet been established. It was found that when the gratings within the plaid are of different contrast the preceived direction is not predicted by the intersection-of-constraints rule. There is a strong (up to 20 deg) bias in the direction of the higher-constrast grating. A revised model, which incorporates a contrast-dependent weighting of perceived grating speed as observed for one-dimensional patterns, can quantitatively predict most of the results. The results are then discussed in the context of various models of human visual motion processing and of physiological responses of neurons in the primate visual system