5,949 research outputs found
The transportation depot: An orbiting vehicle support facility
This paper describes the details of an effort to produce conceptual designs for an orbiting platform, called a transportation depot, to handle assembly and processing of lunar, Martian, and related vehicles. High-level requirements for such a facility were established, and several concepts were developed to meet those requirements. By showing that the critical rigid-body momentum characteristics of each concept are similar to those of the dual-keel space station, some insight was gained about the controllability and utility of this type of facility. Finally, several general observations were made that highlight the advantages and disadvantages of particular design features
Cosmic-Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short-Wave Turbulence
The present paper is the last of a series studying the first-order Fermi
acceleration processes at relativistic shock waves with the method of Monte
Carlo simulations applied to shocks propagating in realistically modeled
turbulent magnetic fields. The model of the background magnetic field structure
of Niemiec & Ostrowski (2004, 2006) has been augmented here by a
large-amplitude short-wave downstream component, imitating that generated by
plasma instabilities at the shock front. Following Niemiec & Ostrowski (2006),
we have considered ultrarelativistic shocks with the mean magnetic field
oriented both oblique and parallel to the shock normal. For both cases
simulations have been performed for different choices of magnetic field
perturbations, represented by various wave power spectra within a wide
wavevector range. The results show that the introduction of the short-wave
component downstream of the shock is not sufficient to produce power-law
particle spectra with the "universal" spectral index 4.2. On the contrary,
concave spectra with cutoffs are preferentially formed, the curvature and
cutoff energy being dependent on the properties of turbulence. Our results
suggest that the electromagnetic emission observed from astrophysical sites
with relativistic jets, e.g. AGN and GRBs, is likely generated by particles
accelerated in processes other than the widely invoked first-order Fermi
mechanism.Comment: 9 pages, 8 figures, submitted to Ap
Distilling a Collection: Creating a Distilling, Fermenting, and Brewing Special Collection at the James E. Walker Library
This essay recounts the steps we took to launch the new Distilling, Fermenting, and Brewing Collection in our Special Collections department. It began with the routine task of weeding our general collection but quickly led to other activities which included a broader collection analysis; consideration of campus, local and state resources; consultation with development and fundraising staff; and research in the anti-quarian trade. Knowing what we had and what we could acquire in the book market, surmising the po-tential for private and industry support, and understanding the multiple scholarly possibilities embed-ded in our collection, we embarked on building a resource that can attract students, faculty, outside re-searchers, and private supporters
Connected component identification and cluster update on GPU
Cluster identification tasks occur in a multitude of contexts in physics and
engineering such as, for instance, cluster algorithms for simulating spin
models, percolation simulations, segmentation problems in image processing, or
network analysis. While it has been shown that graphics processing units (GPUs)
can result in speedups of two to three orders of magnitude as compared to
serial codes on CPUs for the case of local and thus naturally parallelized
problems such as single-spin flip update simulations of spin models, the
situation is considerably more complicated for the non-local problem of cluster
or connected component identification. I discuss the suitability of different
approaches of parallelization of cluster labeling and cluster update algorithms
for calculations on GPU and compare to the performance of serial
implementations.Comment: 15 pages, 14 figures, one table, submitted to PR
The Near-Infrared Broad Emission Line Region of Active Galactic Nuclei -- I. The Observations
We present high quality (high signal-to-noise ratio and moderate spectral
resolution) near-infrared (near-IR) spectroscopic observations of 23 well-known
broad-emission line active galactic nuclei (AGN). Additionally, we obtained
simultaneous (within two months) optical spectroscopy of similar quality. The
near-IR broad emission line spectrum of AGN is dominated by permitted
transitions of hydrogen, helium, oxygen, and calcium, and by the rich spectrum
of singly-ionized iron. In this paper we present the spectra, line
identifications and measurements, and address briefly some of the important
issues regarding the physics of AGN broad emission line regions. In particular,
we investigate the excitation mechanism of neutral oxygen and confront for the
first time theoretical predictions of the near-IR iron emission spectrum with
observations.Comment: 45 pages, 17 figures, accepted by ApJ
Gene-edited meat: Disentangling consumers’ attitudes and potential purchase behaviour
Novel gene-editing (GE) technologies provide promising opportunities to increase livestock productivity and to tackle several global livestock production sustainability and food security challenges. However, these technologies, as with previous genetic modification technologies in food production, are very likely to generate social controversy and opposition toward their use in the meat industry. Here, we explored public attitudes and consumption predisposition toward gene-edited meat products and their potential added benefits to livestock farming. Our results show that societal perception currently comes as a package, where the use of gene-editing technology acts as an extrinsic cue of meat products quality, and is used to make a range of inferences about all quality facets at once. Although consumers with anti-GE attitudinal positions generally were not sensitive to price discounts or added benefits, added benefits increased the consumption predisposition of most moderate and pro-GE consumers, where benefits related to animal welfare had larger effects than those relating to the environment or human health issues.Publishe
Transport of Cosmic Rays in Chaotic Magnetic Fields
The transport of charged particles in disorganised magnetic fields is an
important issue which concerns the propagation of cosmic rays of all energies
in a variety of astrophysical environments, such as the interplanetary,
interstellar and even extra-galactic media, as well as the efficiency of Fermi
acceleration processes. We have performed detailed numerical experiments using
Monte-Carlo simulations of particle propagation in stochastic magnetic fields
in order to measure the parallel and transverse spatial diffusion coefficients
and the pitch angle scattering time as a function of rigidity and strength of
the turbulent magnetic component. We confirm the extrapolation to high
turbulence levels of the scaling predicted by the quasi-linear approximation
for the scattering frequency and parallel diffusion coefficient at low
rigidity. We show that the widely used Bohm diffusion coefficient does not
provide a satisfactory approximation to diffusion even in the extreme case
where the mean field vanishes. We find that diffusion also takes place for
particles with Larmor radii larger than the coherence length of the turbulence.
We argue that transverse diffusion is much more effective than predicted by the
quasi-linear approximation, and appears compatible with chaotic magnetic
diffusion of the field lines. We provide numerical estimates of the Kolmogorov
length and magnetic line diffusion coefficient as a function of the level of
turbulence. Finally we comment on applications of our results to astrophysical
turbulence and the acceleration of high energy cosmic rays in supernovae
remnants, in super-bubbles, and in jets and hot spots of powerful
radio-galaxies.Comment: To be published in Physical Review D, 20 pages 9 figure
A Study of Mo(4+)Quinoxalyl-Dithiolenes as Models for the Non-Innocent Pyranopterin in the Molybdenum Cofactor
A model system for the molybdenum cofactor has been developed that illustrates the noninnocent behavior of an N-heterocycle appended to a dithiolene chelate on molybdenum. The pyranopterin of the molybdenum cofactor is modeled by a quinoxalyldithiolene ligand (S(2)BMOQO) formed from the reaction of molybdenum tetrasulfide and quinoxalylalkyne. The resulting complexes TEA[Tp*MoX(S(2)BMOQO)] [1, X = S; 3, X = O; TEA = tetraethylammonium; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] undergo a dehydration-driven intramolecular cyclization within quinoxalyldithiolene, forming Tp*MoX(pyrrolo-S(2)BMOQO) (2, X = S; 4, X = O). 4 can be oxidized by one electron to produce the molybdenum(5+) complex 5. In a preliminary report of this work, evidence from X-ray crystallography, electronic absorption and resonance Raman spectroscopies, and density functional theory (DFT) bonding calculations revealed that 4 possesses an unusual asymmetric dithiolene chelate with significant thione-thiolate character. The results described here provide a detailed description of the reaction conditions that lead to the formation of 4. Data from cyclic voltammetry, additional DFT calculations, and several spectroscopic methods (IR, electronic absorption, resonance Raman, and electron paramagnetic resonance) have been used to characterize the properties of members in this suite of five Mo(S(2)BMOQO) complexes and further substantiate the highly electron-withdrawing character of the pyrrolo-S(2)BMOQO ligand in 2, 4, and 5. This study of the unique noninnocent ligand S(2)BMOQO provides examples of the roles that the N-heterocycle pterin can play as an essential part of the molybdenum cofactor. The versatile nature of a dithiolene appended by heterocycles may aid in modulating the redox processes of the molybdenum center during the course of enzyme catalysis
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