14,010 research outputs found

    Development of \u3ci\u3eOrius Insidiosus\u3c/i\u3e (Hemiptera: Anthocoridae) in Relation to Temperature

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    A developmental threshold of lO.3¬įC and a thermal constant of 307 day-degrees C were estimated for a Wisconsin population of Orius insidiosus (Say) (Hemiptera: Anthocoridae) by rearing eggs and nymphs at various constant temperatures

    Silicon and magnesium in planetary nebulae

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    The IUE satellite spectra of some planetary nebulae show features due to silicon and magnesium: Si III wavelengths 1883, 1892; Si IV wavelengths 1394, 1403; Mg II wavelengths 2796, 2804 and Mg V wavelengths 2784, 2929. With the aid of modeling techniques, the corresponding elemental abundances are found. In addition to previous observations of NGC 7662 and IC 418, data were found for NGC 2440, Hu 1-2, IC 2003 and IC 2165. Silicon appears depleted by up to an order of magnitude relative to the sun. Large variations of magnesium abundance are found, which are likely to reflect differing degrees of depletion due to grain formation

    Elemental abundances in high-excitation planetary nebulae

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    The IUE satellite was used to obtain low dispersion spectra of the high excitation planetary nebulae IC 351, IC 2003, NGC 2022, IC 2165, NGC 2440, Hu 1-2, and IC 5217. Numerical modeling was undertaken to determine the chemical composition of these objects with particular emphasis on obtaining elemental carbon and nitrogen abundances. Large variations in the C/N ratio from object to object are suggested

    Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations

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    We present results obtained with a 3D, Ly alpha radiative transfer code, applied to a fully cosmological galaxy formation simulation. The developed Monte Carlo code is capable of treating an arbitrary distribution of source Ly alpha emission, neutral hydrogen density, temperature, and peculiar velocity of the interstellar medium. We investigate the influence of resonant scattering on the appearance and properties of young galaxies by applying the code to a simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate 22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that resonant scattering of Ly alpha radiation can explain that young galaxies frequently are observed to be more extended on the sky in Ly alpha than in the optical. Moreover, it is shown that, for the system investigated, due to the anisotropic escape of the photons, the appearent maximum surface brightness can differ by a factor of ~15, and the total derived luminosity by a factor of ~4, depending on the orientation of the system relative to the observer.Comment: Letter updated to match version published in Ap

    Monte Carlo Simulation of Lyman Alpha Scattering and Application to Damped Lyman Alpha Systems

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    A Monte Carlo code to solve the transfer of Lyman alpha (Lya) photons is developed, which can predict the Lya image and two-dimensional Lya spectra of a hydrogen cloud with any given geometry, Lya emissivity, neutral hydrogen density distribution, and bulk velocity field. We apply the code to several simple cases of a uniform cloud to show how the Lya image and emitted line spectrum are affected by the column density, internal velocity gradients, and emissivity distribution. We then apply the code to two models for damped Lya absorption systems: a spherical, static, isothermal cloud, and a flattened, axially symmetric, rotating cloud. If the emission is due to fluorescence of the external background radiation, the Lya image should have a core corresponding to the region where hydrogen is self-shielded. The emission line profile has the characteristic double peak with a deep central trough. We show how rotation of the cloud causes the two peaks to shift in wavelength as the slit is perpendicular to the rotation axis, and how the relative amplitude of the two peaks is changed. In reality, damped Lya systems are likely to have a clumpy gas distribution with turbulent velocity fields, which should smooth the line emission profile, but should still leave the rotation signature of the wavelength shift across the system.Comment: 19 pages, 17 eps figures. One panel is added in Fig.1 to show the recoil effect. Revisions are made in response to the referee's comments. Accepted for publication in Ap

    The Formation and Role of Vortices in Protoplanetary Disks

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    We carry out a two-dimensional, compressible, simulation of a disk, including dust particles, to study the formation and role of vortices in protoplanetary disks. We find that anticyclonic vortices can form out of an initial random perturbation of the vorticity field. Vortices have a typical decay time of the order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk aspect ratio of H/r = 0.15). If vorticity is continuously generated at a constant rate in the flow (e.g. by convection), then a large vortex can form and be sustained (due to the merger of vortices). We find that dust concentrates in the cores of vortices within a few orbital periods, when the drag parameter is of the order of the orbital frequency. Also, the radial drift of the dust induces a significant increase in the surface density of dust particles in the inner region of the disk. Thus, vortices may represent the preferred location for planetesimal formation in protoplanetary disks. We show that it is very difficult for vortex mergers to sustain a relatively coherent outward flux of angular momentum.Comment: Sumitted to the Astrophysical Journal, October 20, 199

    Electron paramagnetic resonance studies of slowly tumbling vanadyl spin probes in nematic liquid crystals

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    An analysis of EPR line shapes by the method of Polnaszek, Bruno, and Freed is made for slowly tumbling vanadyl spin probes in viscous nematic liquid crystals. The use of typical vanadyl complexes as spin probes for nematic liquid crystals is shown to simplify the theoretical analysis and the subsequent interpretation. Rotational correlation times tau and orientational ordering parameters S sub Z where slow tumbling effects are expected to be observed in vanadyl EPR spectra are indicated in a plot. Analysis of the inertial effects on the probe reorientation, which are induced by slowly fluctuating torque components of the local solvent structure, yield quantitative values for tau and S sub Z. The weakly ordered probe VOAA is in the slow tumbling region and displays these inertial effects throughout the nematic range of BEPC and Phase V. VOAA exhibits different reorientation behavior near the isotropic-nematic transition temperature than that displayed far below this transition temperature

    Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization

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    The spectra of the first galaxies and quasars in the Universe should be strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyman-alpha line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyman-alpha line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyman-alpha photons emitted by a source at z=10 scatter over a characteristic angular radius of order 15 arcseconds around the source and compose a line which is broadened and redshifted by about a thousand km/s relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyman-alpha halos around high redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales where the Hubble flow is smooth and the gas is neutral, the Lyman-alpha brightness distribution can be used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3 corrected; new section added on the detectability of Lyman alpha halos; conclusions update

    The relative efficacy of price announcements and express communication for collusion: experimental findings

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    This study conducts experiments to determine the modes of communication that are able to produce and sustain collusion and how the efficacy of communication de- pends on market structure. Two communication treatments are considered: non-binding price announcements and unrestricted written communication. We find that price an- nouncements are conducive to coordinating on a high price but only under duopoly and when firms are symmetric. The standard experimental finding that collusion without com- munication is rare when there are more than two firms is shown to be robust to allowing firms to make price announcements. When firms are asymmetric, price announcements do result in higher prices but there is little evidence that firms are coordinating their behavior. When firms are allowed to engage in unrestricted written communication, co- ordination on high prices occurs for all market structures. We find that the incremental value to express communication (compared to price announcements) is greater when firms are asymmetric and there are more firms
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