2,447 research outputs found
Multiple scattering of polarized radiation by non-spherical grains: first results
We present the first numerical radiative transfer simulation of multiple
light scattering in dust configurations containing aligned non-spherical
(spheroidal) dust grains. Such models are especially important if one wants to
explain the circular polarization of light, observed in a variety of
astronomical objects. The radiative transfer problem is solved on the basis of
the Monte Carlo method. Test simulations, confirming the correct numerical
implementation of the scattering mechanism, are presented. As a first
application, we investigate the linear and circular polarization of light
coming from a spherical circumstellar shell. This shell contains perfectly
aligned prolate or oblate spheroidal grains. The most remarkable features of
the simulated linear polarization maps are so-called polarization null points
where the reversal of polarization occurs. They appear in the case when the
grain alignment axis is perpendicular to the line of sight. The maps of
circular polarization have a sector-like structure with maxima at the ends of
lines inclined to the grain alignment axis by \pm 45\degr.Comment: 13 pages, 14 figures, accepted by A&
Using imprecise continuous time Markov chains for assessing the reliability of power networks with common cause failure and non-immediate repair.
We explore how imprecise continuous time Markov
chains can improve traditional reliability models based
on precise continuous time Markov chains. Specifically,
we analyse the reliability of power networks under very
weak statistical assumptions, explicitly accounting for
non-stationary failure and repair rates and the limited
accuracy by which common cause failure rates can be
estimated. Bounds on typical quantities of interest
are derived, namely the expected time spent in system
failure state, as well as the expected number of
transitions to that state. A worked numerical example
demonstrates the theoretical techniques described.
Interestingly, the number of iterations required for
convergence is observed to be much lower than current
theoretical bounds
The Signature of Primordial Grain Growth in the Polarized Light of the AU Mic Debris Disk
We have used the Hubble Space Telescope/ACS coronagraph to make polarization
maps of the AU Mic debris disk. The fractional linear polarization rises
monotonically from about 0.05 to 0.4 between 20 and 80 AU. The polarization is
perpendicular to the disk, indicating that the scattered light originates from
micron sized grains in an optically thin disk. Disk models, which
simultaneously fit the surface brightness and polarization, show that the inner
disk (< 40-50 AU) is depleted of micron-sized dust by a factor of more than
300, which means that the disk is collision dominated. The grains have high
maximum linear polarization and strong forward scattering. Spherical grains
composed of conventional materials cannot reproduce these optical properties. A
Mie/Maxwell-Garnett analysis implicates highly porous (91-94%) particles. In
the inner Solar System, porous particles form in cometary dust, where the
sublimation of ices leaves a "bird's nest" of refractory organic and silicate
material. In AU Mic, the grain porosity may be primordial, because the dust
"birth ring" lies beyond the ice sublimation point. The observed porosities
span the range of values implied by laboratory studies of particle coagulation
by ballistic cluster-cluster aggregation. To avoid compactification, the upper
size limit for the parent bodies is in the decimeter range, in agreement with
theoretical predictions based on collisional lifetime arguments. Consequently,
AU Mic may exhibit the signature of the primordial agglomeration process
whereby interstellar grains first assembled to form macroscopic objects.Comment: 12 pages, 8 figures, ApJ, in pres
One Sweetly Solemn Thought
https://digitalcommons.library.umaine.edu/mmb-ps/1867/thumbnail.jp
Zinc oxide films grown by galvanic deposition from 99% metals basis zinc nitrate electrolyte
The use of relatively low purity zinc nitrate for electrochemical deposition
of compact ZnO films is attractive for large scale production because of the
cost saving potential. ZnO films were grown on SnO2:F and magnetron sputtered
ZnO:Al templates using a three electrode potentiostatic system in galvanic
mode. The electrolyte consisted of a 0.1 M zinc nitrate solution (either
99.998% or 99% purity) and 1 mM aluminium nitrate for extrinsic doping, when
required. Moderate deposition rates of up to 0.9 nm sâ1 were achieved on
ZnO:Al templates with lower rates of up to 0.5 nm sâ1 on SnO2:F templates.
Observation of SEM images of the films revealed a wall-like morphology whose
lateral thickness (parallel to the substrate) reduced as aluminium was added
to the system either in the electrolyte or from the substrate. However, pre-
deposition activation of the template by applying a negative voltage
(approximately â2 V) allowed the growth of compact films even for the low
purity electrolyte. The optical band gap energy of intrinsically doped films
was lower than that of the Al doped films. The composite electrical
conductivity of all the films studied, as inferred from sheet resistance and
Hall effect measurements of the ZnO/template stacks was much less than that of
the uncoated templates. A strong E2 (high) mode at around 437 cmâ1 was visible
in the Raman spectra for most films confirming the formation of ZnO. However,
both the Raman modes and XRD reflections associated with wurtzite ZnO
diminished for the Al doped films indicating a high level of mainly oxygen
related defects. Based on these data, further studies are underway to improve
the doping efficiency of aluminium, the crystalline structure and thus the
conductivity of such films
The steady state load of five firefighting tasks
Purpose. Physiologic demands of five common tasks in firefighting have been examined. Methods. Eight male volunteers, being dressed up as smoke divers (+21 kg extra load), carried out the following tasks at constant pace for 5 min: Walking at 1.4 m·sâ1, walking (all walks at the same speed) while carrying a 10 kg ladder, walking carrying two hose packs of 16 kg together, walking carrying a 32 kg spreader tool, finally climbing up and down a ladder at preset pace. A 5 min break separated each exercise. Heart rate, O2-uptake and ventilation were measured continuously, and blood lactate con-centration was recorded after each task. Results. The end-exercise heart rate rose from 108 to 180 bpm from first to last task, blood lactate concentration rose from 1 to 7 mmol·Lâ1, O2-uptake rose from 19 to 48 ml·kgâ1·minâ1, and ventilation rose from 38 to 124 L·minâ1. Discussion. Walking was an easy task even when dressed up as a smoke diver. Adding loads increased demands; ladder climbing taxed >90% of the subjectsâ aerobic power. Conclusions. The physiologic demands varied considera-bly between different tasks.acceptedVersio
Mechanistic study of the gas phase chemistry during the spray deposition of Zn O,S films by mass spectrometry
The mass spectrometer, is a powerful tool to identify species and investigate reactions in the gas phase. In this work, the mechanism of aerosol assisted chemical vapor deposition AACVD of Zn O,S films prepared from H2S and zinc acetylacetonate Zn acac 2 precursor solutions is elucidated by mass spectrometry. The thermochemical behavior of Zn acac 2 is investigated by characterizing the influence of the solvent H2O or ethanol , the pH value of the precursor solution and the effect of the reactant H2S, and by tracking gaseous intermediate products using mass spectrometry. Based on these results, a proton promoted thermolysis mechanism for the AACVD Zn O,S film formation is then proposed, which is initiated by the hydrolysis with H2O as the first stage, followed either by the rearrangement with an intramolecular proton or by the reaction with an extramolecular proton to produce ZnO or Zn O,S . A real time mass tracking of the AACVD process reveals that only an adequate amount of H2S promotes the chemical gas phase decomposition and sulfurization process, while an excess of H2S depletes the gaseous Zn acac 2 concentration and consequently reduces the film growth rate. The knowledge of the thermal decomposition process helps to optimize synthesis conditions and to adjust film properties to meet the requirement of the application in chalcopyrite or kesterite thin film solar cell
Using imprecise continuous time Markov chains for assessing the reliability of power networks with common cause failure and non-immediate repair
We explore how imprecise continuous time Markov chains can improve traditional reliability models based on precise continuous time Markov chains. Specifically, we analyse the reliability of power networks under very weak statistical assumptions, explicitly accounting for non-stationary failure and repair rates and the limited accuracy by which common cause failure rates can be estimated. Bounds on typical quantities of interest are derived, namely the expected time spent in system failure state, as well as the expected number of transitions to that state. A worked numerical example demonstrates the theoretical techniques described. Interestingly, the number of iterations required for convergence is observed to be much lower than current theoretical bounds
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