Proper Motions of PSRs B1757-24 and B1951+32: Implications for Ages and Associations

Over the last decade, considerable effort has been made to measure the proper motions of the pulsars B1757-24 and B1951+32 in order to establish or refute associations with nearby supernova remnants and to understand better the complicated geometries of their surrounding nebulae. We present proper motion measurements of both pulsars with the Very Large Array, increasing the time baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5 yr, respectively, compared to previous observations. We confirm the non-detection of proper motion of PSR B1757-24, and our measurement of (mu_a, mu_d) = (-11 +/- 9, -1 +/- 15) mas yr^{-1} confirms that the association of PSR B1757-24 with SNR G5.4-1.2 is unlikely for the pulsar characteristic age of 15.5 kyr, although an association can not be excluded for a significantly larger age. For PSR B1951+32, we measure a proper motion of (mu_a, mu_d) = (-28.8 +/- 0.9, -14.7 +/- 0.9) mas yr^{-1}, reducing the uncertainty in the proper motion by a factor of two compared to previous results. After correcting to the local standard of rest, the proper motion indicates a kinetic age of ~51 kyr for the pulsar, assuming it was born near the geometric center of the supernova remnant. The radio-bright arc of emission along the pulsar proper motion vector shows time-variable structure, but moves with the pulsar at an approximately constant separation ~2.5", lending weight to its interpretation as a shock structure driven by the pulsar.Comment: LaTeX file uses emulateapj.cls; 7 pages, 4 figures, to be published ApJ February 10, 2008, v674 p271-278. Revision reflects journal formatting; there are no substantial revision

The Role of Seed Bank and Germination Dynamics in the Restoration of a Tidal Freshwater Marsh in the Sacramento–San Joaquin Delta

Liberty Island, California, is a historical freshwater tidal wetland that was converted to agricultural fields in the early 1900s. Liberty Island functioned as farmland until an accidental levee break flooded the area in 1997, inadvertently restoring tidal marsh hydrology. Since then, wetland vegetation has naturally recolonized part of the site. We conducted a seed bank assay at the site and found that despite a lack of germination or seedling recruitment at the site, the seed bank contained a diverse plant community, indicating that the site’s continuous flooding was likely suppressing germination. Additionally, the frequency of germinating seeds in the seed bank did not represent the dominant adult plant community. We conducted a cold stratification study to determine if this observed disparity could be explained by seed germination dynamics, and whether germination could be enhanced using a pre-germination cold exposure, particularly for species of concern for wetland restoration. The cold stratification study showed that longer durations of pre-germination cold enhanced germination in Schoenoplectus acutus, but reduced germination in Schoenoplectus californicus, and had no effect on Typha latifolia. Overall, germination of S. californicus and S. acutus was much lower than T. latifolia. Our findings suggest that seeding may not be an effective restoration technique for Schoenoplectus spp., and, to improve restoration techniques, further study is needed to more comprehensively understand the reproduction ecology of important marsh species

Orthogonal signed-distance coordinates and vector calculus near evolving curves and surfaces

We provide an elementary derivation of an orthogonal coordinate system for boundary layers around evolving smooth surfaces and curves based on the signed-distance function. We go beyond previous works on the signed-distance function and collate useful vector calculus identities for these coordinates. These results and provided code enable consistent accounting of geometric effects in the derivation of boundary layer asymptotics for a wide range of physical systems.Comment: 24 pages, 3 figures, Mathematica code available at https://github.com/ericwhester/signed-distance-cod

On the stability of isothermal shocks in black hole accretion disks

Most black holes possess accretion disks. Models of such disks inform observations and constrain the properties of the black holes and their surrounding medium. Here, we study isothermal shocks in a thin black hole accretion flow. Modelling infinitesimal molecular viscosity allows the use of multiple-scales matched asymptotic methods. We thus derive the first explicit calculations of isothermal shock stability. We find that the inner shock is always unstable, and the outer shock is always stable. The growth/decay rates of perturbations depend only on an effective potential and the incoming--outgoing flow difference at the shock location. We give a prescription of accretion regimes in terms of angular momentum and black hole radius. Accounting for angular momentum dissipation implies unstable outer shocks in much of parameter space, even for realistic viscous Reynolds numbers of the order $\approx 10^{20}$.Comment: 26 page

Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars

Evidence from pulsar wind nebula symmetry axes and radio polarization observations suggests that pulsar motions correlate with the spin directions. We assemble this evidence for young isolated pulsars and show how it can be used to quantitatively constrain birth kick scenarios. We illustrate by computing several plausible, but idealized, models where the momentum thrust is proportional to the neutrino cooling luminosity of the proto-neutron star. Our kick simulations include the effects of pulsar acceleration and spin-up and our maximum likelihood comparison with the data constrains the model parameters. The fit to the pulsar spin and velocity measurements suggests that: i) the anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a pre-kick spin of the star is required, the preferred magnitude is small 10-20rad/s, so that for the best-fit models iii) the bulk of the spin is kick-induced with $\bar \Omega$ ~120rad/s and iv) the models suggest that the anisotropy emerges on a timescale $\tau$ ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte

Magneto-Stokes Flow in a Shallow Free-Surface Annulus

We analyse a magnetohydrodynamic flow inspired by the kinematic reversibility of viscous Taylor-Couette flows. The system considered here shares the cylindrical-annular geometry of the Taylor-Couette cell, but uses applied electromagnetic forces to drive "magneto-Stokes" flow in a shallow, free-surface layer of electrolyte. An analytical solution is presented and validated with coupled laboratory and numerical experiments. The dominant balance of Lorentz forcing and basal viscous drag reproduces the kinematic reversibility observed by G.I. Taylor with precise electromagnetic control. Induced fluid deformation may be undone by simply reversing the polarity of electric current through the system. We illustrate this analogy with theory and experiment, and we draw a further connection to potential flow using the Hele-Shaw approximation. The stability and controllability of the magneto-Stokes system make it an attractive tool for investigating shear flows in a variety of settings from industrial to astrophysical. In addition, the set-up's simplicity and robustness make magneto-Stokes flow a good candidate for PIV calibration and for educational demonstrations of magnetohydrodynamics, boundary layers, and flow transition

Early Intervention with Children at Risk of Emotional/Behavioral Disorders: A Critical Examination of Research Methodology and Practices

Children\u27s behavior problems pose challenges to families, schools, and society. The research literature argues that early detection/intervention is the most powerful course of action in ameliorating these problems in children at risk of emotional/behavioral disorders. However, specifying precisely what constitutes a quality program of early intervention is not a simple task. Current conceptualizations suggest that successful early intervention cannot be unidimensional in nature, but must consist of a complex series of interactions and transactions that synergistically serve to nurture and enhance both the development of the child and family. In this paper, we reviewed the accumulated research to learn more about the critical elements of early intervention. Specifically, we examined three major areas addressed in the literature. First, we describe literature search procedures and criteria for study inclusion, along with methods for analyzing these early intervention studies. Second, we examine the conduct of the intervention, including characteristics of study participants, types of interventions, types of measures, age of onset and length of intervention, treatment fidelity, and social validity measures. Finally, we draw upon that review to offer recommendations for future research

The Spectral Signature of Dust Scattering and Polarization in the Near IR to Far UV. I. Optical Depth and Geometry Effects

Spectropolarimetry from the near IR to the far UV of light scattered by dust provides a valuable diagnostic of the dust composition, grain size distribution and spatial distribution. To facilitate the use of this diagnostic, we present detailed calculations of the intensity and polarization spectral signature of light scattered by optically thin and optically thick dust in various geometries. The polarized light radiative transfer calculations are carried out using the adding-doubling method for a plane-parallel slab, and are extended to an optically thick sphere by integrating over its surface. The calculations are for the Mathis, Rumple & Nordsieck Galactic dust model, and cover the range from 1 $\mu m$ to 500 \AA. We find that the wavelength dependence of the scattered light intensity provides a sensitive probe of the optical depth of the scattering medium, while the polarization wavelength dependence provides a probe of the grain scattering properties, which is practically independent of optical depth. We provide a detailed set of predictions, including polarization maps, which can be used to probe the properties of dust through imaging spectropolarimetry in the near IR to far UV of various Galactic and extragalactic objects. In a following paper we use the codes developed here to provide predictions for the dependence of the intensity and polarization on grain size distribution and composition.Comment: 29 pages + 21 figures, accepted for the Astrophysical Journal Supplement February 2000 issue. Some revision, mostly in the introduction and the conclusions, and a couple of correction

Discovery of an optical bow-shock around pulsar B0740-28

We report the discovery of a faint H-alpha pulsar wind nebula (PWN) powered by the radio pulsar B0740-28. The characteristic bow-shock morphology of the PWN implies a direction of motion consistent with the previously measured velocity vector for the pulsar. The PWN has a flux density more than an order of magnitude lower than for the PWNe seen around other pulsars, but, for a distance 2 kpc, it is consistent with propagation through a medium of atomic density n_H ~ 0.25 cm^{-3}, and neutral fraction of 1%. The morphology of the PWN in the area close to the pulsar is distinct from that in downstream regions, as is also seen for the PWN powered by PSR B2224+65. In particular, the PWN associated with PSR B0740-28 appears to close at its rear, suggesting that the pulsar has recently passed through a transition from low density to high density ambient gas. The faintness of this source underscores that deep searches are needed to find further examples of optical pulsar nebulae.Comment: 5 pages, 1 figure, to appear in Astronomy & Astrophysics Letter