Silicon nanoparticles and interstellar extinction

To examine a recently proposed hypothesis that silicon nanoparticles are the source of extended red emission (ERE) in the interstellar medium, we performed a detailed modeling of the mean Galactic extinction in the presence of silicon nanoparticles. For this goal we used the appropriate optical constants of nanosized Si, essentially different from those of bulk Si due to quantum confinement. It was found that a dust mixture of silicon nanoparticles, bare graphite grains, silicate core-organic refractory mantle grains and three-layer silicate-water ice-organic refractory grains works well in explaining the extinction and, in addition, results in the acceptable fractions of UV/visible photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions barely agree with the fraction of UV/visible photons needed to excite the observed ERE, we conclude that the intrinsic photon conversion efficiency of the photoluminescence by silicon nanoparticles must be near 100%, if they are the source of the ERE.Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6 pages, 3 figures (8 Postscript files), accepted for publication in ApJ Letters, complete Postscript file is also available at http://physics.technion.ac.il/~zubko/eb.html#SNP

New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints

We present new interstellar dust models which have been derived by simultaneously fitting the far-ultraviolet to near-infrared extinction, the diffuse infrared (IR) emission and, unlike previous models, the elemental abundance constraints on the dust for different interstellar medium abundances, including solar, F and G star, and B star abundances. The fitting problem is a typical ill-posed inversion problem, in which the grain size distribution is the unknown, which we solve by using the method of regularization. The dust model contains various components: PAHs, bare silicate, graphite, and amorphous carbon particles, as well as composite particles containing silicate, organic refractory material, water ice, and voids. The optical properties of these components were calculated using physical optical constants. As a special case, we reproduce the Li & Draine (2001) results, however their model requires an excessive amount of silicon, magnesium, and iron to be locked up in dust: about 50 ppm (atoms per million of H atoms), significantly more than the upper limit imposed by solar abundances of these elements, about 34, 35, and 28 ppm, respectively. A major conclusion of this paper is that there is no unique interstellar dust model that simultaneously fits the observed extinction, diffuse IR emission, and abundances constraints.Comment: 70 pages, 23 figures, accepted for publication in the Astrophysical Journal Supplemen

Remote Sensing Data Visualization, Fusion and Analysis via Giovanni

We describe Giovanni, the NASA Goddard developed online visualization and analysis tool that allows users explore various phenomena without learning remote sensing data formats and downloading voluminous data. Using MODIS aerosol data as an example, we formulate an approach to the data fusion for Giovanni to further enrich online multi-sensor remote sensing data comparison and analysis

Effects of Grain Size on the Spectral Energy Distribution of Dusty Circumstellar Envelopes

We study the effects of dust grain size on the spectral energy distribution (SED) of spherical circumstellar envelopes. Based on the self-similarity relations of dusty SEDs derived by Ivezic & Elitzur (1997), we expect an approximate invariance of the IR SED for models with different grain sizes. Approximate invariance follows from the fact that differently sized grains have similar optical properties at long wavelengths where the dust reprocesses the starlight. In this paper, we discuss what are the physical requirements on the model parameters to maintain the approximate invariance of the IR SED. Single grain size models are studied for a wide range of grain sizes in three optical depth regimes. In this study, we find limits for the cases where the IR SED is and is not capable of conveying information about grain sizes, and to what extent it does so. We find that approximate invariance occurs for a much larger range of grain sizes than previously believed, and, when approximate invariance holds, the SED is controlled mainly by one parameter, the reprocessing optical depth, a quantity that measures the fraction of starlight that is absorbed by the dust grains. Models with a grain size distribution are studied as well. For these models, we find that, in many instances, the concept of approximate invariance may be extended from the IR SED to all wavelengths. This means that, for a wide range of optical depths, models with different grain size distributions will produce very similar SEDs and, hence, the reprocessing optical depth is the only quantity that can be unambiguously obtained from the SED. The observational consequences of this result are discussed in detail

Energy Extraction Resistors for the Main Dipole and Quadrupole Circuits of the LHC

When the LHC will be operating at its maximum beam energy, its superconducting dipole chains store a total magnetic energy of more than 11 GJ. At the same time, the QF and QD quadrupole circuits store a total energy of 400 MJ. Even with the sectorisation of each of the three principal power circuits into eight individually powered segments, the stored energy of a single circuit is considerable. During normal operation the energy in the dipole circuits is safely returned to the mains grid, using the thyristor-based, 'booster' unit of the power converters, operating in inversion. For the quadrupole chains, where the converter is of a mono-polar topology, the stored energy is dissipated into the resistive part of the warm d.c. power lines (busbars and cables) in a slow, controlled run-down. When a magnet quenches, however, such a slow energy transfer, taking 20 minutes from the rated LHC current, will not be possible. The 'cold' diode, taking over the magnet current in case of a quench, will not survive this slow current decay. For this reason, energy extraction facilities will be inserted into the power circuits. These systems are being designed to absorb the total circuit energy and de-excite the chains with a current decay time constant of 104 s for the dipoles and 40 s for the quadrupoles. The resulting maximum decay rates (-125 A/s and -325 A/s respectively) are comfortably below the levels where quench-back will occur. The energy extraction systems are based on an array of special, mechanical d.c. circuit breakers and absorber resistors, which are switched into the circuit by opening of the breakers. The design and construction of these large power resistors of a unique concept are the topics of this paper. The project is being realised as collaboration between, IHEP-Protvino, CERN and European Industry

Dust Grain-Size Distributions From MRN to MEM

Employing the Maximum Entropy Method algorithm, we fit interstellar extinction measurements which span the wavelength range 0.125-3 micron. We present a uniform set of MEM model fits, all using the same grain materials, optical constants and abundance constraints. In addition, we are taking advantage of improved UV and IR data and better estimates of the gas-to-dust ratio. The model fits cover the entire range of extinction properties that have been seen in the Galaxy and the Magellanic Clouds. The grain models employed for this presentation are the simplistic homogeneous spheres models (i.e., Mathis, Rumpl, & Nordsieck 1977) with two (graphite, silicate) or three (graphite, silicate, amorphous carbon) components. Though such usage is only a first step, the results do provide interesting insight into the use of grain size as a diagnostic of dust environment. We find that the SMC Bar extinction curve cannot be fit using carbon grains alone. This is a challenge to the recent observational result indicating little silicon depletion in the SMC.Comment: 24 pages, 5 figures, accepted for publication in the Astrophysical Journa

Strain gradient induced polarization in SrTiO3 single crystals

Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, affects the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO$_3$ as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO$_3$ may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO$_3$ may be polar.Comment: 4 pages, 3 figures, 1 tabl

Диэлькометрический метод контроля и диагностики электрических свойств полимерных материалов электротехнического назначения

A dielcometric methods and techniques for monitoring and diagnosis of the electrical properties of polymer composites are developed. The electrical properties of the composites according to the content and type of filler is conducted in the frequency range of the electric field of 50 Hz-1 MHz.Целью данной статьи является разработка диэлькометрического метода и методики для контроля и диагностики электрических свойств полимерных композитов. Исследование электрических свойств композитов в зависимости от содержания и типа наполнителя проведено в диапазоне частот электрического поля 50 Гц - 1 МГц

ИМПЕДАНСНЫЙ МЕТОД КОНТРОЛЯ И ДИАГНОСТИКИ КАЧЕСТВА ЭЛЕКТРОИЗОЛЯЦИОННЫХ МАТЕРИАЛОВ, ИСПОЛЬЗУЕМЫХ В ЭЛЕКТРОТЕХНИЧЕСКОЙ ПРОМЫШЛЕННОСТИ

A method of impedance control and diagnostics of quality of insulating materials, used in electrotechnical industry, is proposed. Electric-capacitive transmitters are created. Formulas to make monitoring and diagnostics of dielectric parameters of insulating materials, such as relative permittivity, dielectric loss tangent, and relevant standards are developed.Разработан импедансный метод контроля и диагностики качества электроизоляционных материалов, используемых в электротехнической промышленности. Созданы электроемкостные измерительные преобразователи, разработана методика, получены формулы, проведен контроль и диагностика диэлектрических показателей электроизоляционных материалов, таких как относительная диэлектрическая проницаемость, тангенс угла диэлектрических потерь, соответствующих нормам, регламентированным стандартам

The Vega Debris Disk -- A Surprise from Spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved and its angular size is much larger than found previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and 105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an r^-1 power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger than 180 um and at radii of 86-200 AU from the star, can reproduce the observed 850 um flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters amongthese large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances where we detect its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS results. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and subsequent collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2, 3a, 3b and 4 have been degraded to lower resolutions.