39,403 research outputs found

    Scintillation Caustics in Planetary Occultation Light Curves

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    We revisit the GSC5249-01240 light curve obtained during its occultation by Saturn's North polar region. In addition to refractive scintillations, the power spectrum of intensity fluctuations shows an enhancement of power between refractive and diffractive regimes. We identify this excess power as due to high amplitude spikes in the light curve and suggest that these spikes are due to caustics associated with ray crossing situations. The flux variation in individual spikes follows the expected caustic behavior, including diffraction fringes which we have observed for the first time in a planetary occultation light curve. The presence of caustics in scintillation light curves require an inner scale cut off to the power spectrum of underlying density fluctuations associated with turbulence. Another possibility is the presence of gravity waves in the atmosphere. While occultation light curves previously showed the existence of refractive scintillations, a combination of small projected stellar size and a low relative velocity during the event have allowed us to identify caustics in this occultation. This has led us to re-examine previous data sets, in which we have also found likely examples of caustics.Comment: 4 pages, 3 figures; ApJL submitte

    Stability of metal-rich very massive stars

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    We revisit the stability of very massive nonrotating main-sequence stars at solar metallicity, with the goal of understanding whether radial pulsations set a physical upper limit to stellar mass. Models of up to 938 solar masses are constructed with the Mesa code, and their linear stability in the fundamental mode, assumed to be the most dangerous, is analysed with a fully nonadiabatic method. Models above 100 MSun have extended tenuous atmospheres ("shelves") that affect the stability of the fundamental. Even when positive, this growth rate is small, in agreement with previous results. We argue that small growth rates lead to saturation at small amplitudes that are not dangerous to the star. A mechanism for saturation is demonstrated involving nonlinear parametric coupling to short-wavelength g modes and the damping of the latter by radiative diffusion. The shelves are subject to much more rapidly growing strange modes. This also agrees with previous results but is extended here to higher masses. The strange modes probably saturate via shocks rather than mode coupling but have very small amplitudes in the core, where almost all of the stellar mass resides. Although our stellar models are hydrostatic, the structure of their outer parts suggests that optically thick winds, driven by some combination of radiation pressure, transsonic convection, and strange modes, are more likely than pulsation in the fundamental mode to limit the main-sequence lifetime.Comment: 14 pages, 8 figures, 1 appendix; this version to be published in MNRA

    Secular Instability and Planetesimal Formation in the Dust Layer

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    Late in the gaseous phase of a protostellar disk, centimeter-sized bodies probably settle into a thin ``dust layer'' at the midplane. A velocity difference between the dust layer and the gas gives rise to turbulence, which prevents further settling and direct gravitational instability of the layer. The associated drag on the surface of the layer causes orbital decay in a few thousand years---as opposed to a few hundred years for an isolated meter-sized body. Within this widely-accepted theoretical framework, we show that the turbulent drag causes radial instabilities even if the selfgravity of the layer is negligible. We formulate axisymmetric, height-integrated dynamical equations for the layer that incorporate turbulent diffusion of mass and momentum in radius and height, vertical settling, selfgravity, and resistance to compression due to gas entrained within the dust layer. In steady-state, the equations describe the inward radial drift of a uniform dust layer. In perturbation, overdense rings form on an orbital timescale with widths comparable to the dust-layer thickness. Selfgravity is almost irrelevant to the linear growth rate but will eventually fragment and collapse the rings into planetesimals larger than a kilometer. We estimate that the drag instability is most efficient at 1 AU when most of the ``dust'' mass lies in the size range 0.1-10 meters.Comment: 25 pp., 2 figures. Uses aastex version 5.0

    Lightning Observations above and below clouds

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    The quantitative optical characteristics of cloud to ground (CG) and intracloud (IC) lightning above clouds were studied. A data base of a number of pulse paramaters such as energy, rise times, pulse widths and pulse intervals was complied and categorized for first return strokes, subsequent strokes, the intracloud part of CG flashes and IC flashes. It is found that: (1) single stroke CG's are more readily distinguishable from IC flashes than multiple stroke CG's; (2) there is no significant difference between the energy of first and subsequent return stroke pulses; and (3) the pulse rise times and pulse widths are time broadened. Lightning activity in a mesoscale convective weather system (MCS) was examined. The CG flash rates average almost 50 per minute for 7 hours. It is shown that lightning above storms embedded within the MCS IC lightning activity can be much greater than CG activity at certain times in the MCS lifecycle

    A hard coat, a tough choice? The effects of host seed morphology and mechanics on the egg laying behaviour of the bruchid beetle, Callosobruchus maculatus.

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    This study investigated whether the egg laying behaviour of Callosobruchus maculatus, an important storage pest of leguminous seed crops, was related to the mechanical properties of the host seed. The hypothesis was that females avoid laying eggs on seeds with particularly tough seed coats and hard cotyledons to reduce the resistance their larvae are subjected to when they bore into the seed to complete their development. Females were presented with seeds from three leguminous species: Vigna unguiculata, V. angularis and V. radiata. The distribution of oviposition sites on the seed's surface was related to the morphology and mechanical properties of the seed. Vickers microhardness and fracture tests were used to investigate regional variation and compare the properties of seeds with and without eggs adhered to their surface. There were no significant regional differences in the hardness of the cotyledon material along the longitudinal axis of the seed (P > 0.05). However, there were significant differences between the mechanical properties of the seed coat and the cotyledons; in V. unguiculata the seed coat was tougher, 1249 ± 80.8 J m−2, than the cotyledons, 402 ± 30.0 J m−2 (P 0.05). Mechanical data are discussed in relation to the egg laying behaviour of C. maculatus

    Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

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    Photoevaporation is an important dispersal mechanism for protoplanetary disks. We conduct hydrodynamic simulations coupled with ray-tracing radiative transfer and consistent thermochemistry to study photoevaporative winds driven by ultraviolet and X-ray radiation from the host star. Most models have a three-layer structure: a cold midplane, warm intermediate layer, and hot wind, the last having typical speeds 30 km s1\sim 30~\mathrm{km\ s}^{-1} and mass-loss rates 109 M yr1\sim 10^{-9}~M_\odot~\mathrm{yr}^{-1} when driven primarily by ionizing UV radiation. Observable molecules including CO, OH and H2O re-form in the intermediate layer and survive at relatively high wind temperatures due to reactions being out of equilibrium. Mass-loss rates are sensitive to the intensity of radiation in energy bands that interact directly with hydrogen. Comparison with previous works shows that mass loss rates are also sensitive to the treatment of both the hydrodynamics and the thermochemistry. Divergent results concerning the efficiency of X-ray photoevaporation are traced in part to differing assumptions about dust and other coolants.Comment: 15 pages, 9 figures, submitted to Ap

    Phase linear interferometer experiment maintenance and calibration manual

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    This manual describes the necessary procedures for assuring continuous lightning sferics data collection at Marshall Space Flight Center. The phase linear interferometer experiment is under evaluation as a candidate RF sensor to support the space based optical lightning mapper system

    Chaotic scattering in solitary wave interactions: A singular iterated-map description

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    We derive a family of singular iterated maps--closely related to Poincare maps--that describe chaotic interactions between colliding solitary waves. The chaotic behavior of such solitary wave collisions depends on the transfer of energy to a secondary mode of oscillation, often an internal mode of the pulse. Unlike previous analyses, this map allows one to understand the interactions in the case when this mode is excited prior to the first collision. The map is derived using Melnikov integrals and matched asymptotic expansions and generalizes a ``multi-pulse'' Melnikov integral and allows one to find not only multipulse heteroclinic orbits, but exotic periodic orbits. The family of maps derived exhibits singular behavior, including regions of infinite winding. This problem is shown to be a singular version of the conservative Ikeda map from laser physics and connections are made with problems from celestial mechanics and fluid mechanics.Comment: 29 pages, 17 figures, submitted to Chaos, higher-resolution figures available at author's website: http://m.njit.edu/goodman/publication

    A Study of Vitrified Nuclear Wasteforms by Molecular Dynamics, Electron Microscopy and Raman Spectroscopy

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    In this study an attempt is made to create molecular dynamics (MD) models of borate glass, alkali borosilicate glasses, and UK vitreous High Level Radioactive Wasteforms. The study also includes experimental studies of vitrified wasteforms by helium pycnometry, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray florescence spectroscopy (XRF) and Raman spectroscopy. Molecular dynamics models of alkali borosilicate glasses were created using Buckingham and BHM potentials in the constant pressure and temperature ensemble. The models using BHM potentials showed more realistic boron coordination numbers than those using Buckingham potentials. However structural features such as Si-O, Li-O and Na-O nearest neighbour distances and O Si O and O B O bond angles were considered satisfactory using Buckingham potentials. SEM images showing phase separation in four different vitrified wasteforms are presented. The chemical composition of the phases were determined using SEM EDX. XRF spectroscopy was obtained from the wasteforms in powder form and show qualitative agreement with nominal compositions. Raman spectroscopy also revealed the presence of MoO4 tetrahedra in a glass environment and in phases such as CaMoO4 and Na(Gd,Nd)(MoO4)2. The presence of ruthenium, cerium and zirconium phases were also found in the Raman spectra of wasteforms. MD models of three simplified vitrified wasteforms were created using Buckingham potentials. Two models of each wasteform were created. The first models used only two-body potentials and showed MoO6 octahedra connected to borosilicate network formers. In the second model of each wasteform, an additional O Mo O three-body potential was applied. The results of the second models showed MoO4 tetrahedra detached from the borosilicate network which is a realistic feature in comparison to the experimental observations
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