35 research outputs found
A Model of Habitability Within the Milky Way Galaxy
We present a model of the Galactic Habitable Zone (GHZ), described in terms
of the spatial and temporal dimensions of the Galaxy that may favour the
development of complex life. The Milky Way galaxy is modelled using a
computational approach by populating stars and their planetary systems on an
individual basis using Monte-Carlo methods. We begin with well-established
properties of the disk of the Milky Way, such as the stellar number density
distribution, the initial mass function, the star formation history, and the
metallicity gradient as a function of radial position and time. We vary some of
these properties, creating four models to test the sensitivity of our
assumptions. To assess habitability on the Galactic scale, we model supernova
rates, planet formation, and the time required for complex life to evolve. Our
study improves on other literature on the GHZ by populating stars on an
individual basis and by modelling SNII and SNIa sterilizations by selecting
their progenitors from within this preexisting stellar population. Furthermore,
we consider habitability on tidally locked and non-tidally locked planets
separately, and study habitability as a function of height above and below the
Galactic midplane. In the model that most accurately reproduces the properties
of the Galaxy, the results indicate that an individual SNIa is ~5.6 \times more
lethal than an individual SNII on average. In addition, we predict that ~1.2%
of all stars host a planet that may have been capable of supporting complex
life at some point in the history of the Galaxy. Of those stars with a
habitable planet, ~75% of planets are predicted to be in a tidally locked
configuration with their host star. The majority of these planets that may
support complex life are found towards the inner Galaxy, distributed within,
and significantly above and below, the Galactic midplane.Comment: Accepted for publication in Astrobiology. 40 pages, 12 figures, 3
table
The Pervasiveness of Global Data in Evolving Software Systems
Abstract. In this research, we investigate the role of common coupling in evolving software systems. It can be argued that most software de-velopers understand that the use of global data has many harmful side-effects, and thus should be avoided. We are therefore interested in the answer to the following question: if global data does exist within a soft-ware project, how does global data usage evolve over a software project’s lifetime? Perhaps the constant refactoring and perfective maintenance eliminates global data usage, or conversely, perhaps the constant addi-tion of features and rapid development introduce an increasing reliance on global data? We are also interested in identifying if global data usage patterns are useful as a software metric that is indicative of an interesting or significant event in the software’s lifetime. The focus of this research is twofold: first to develop an effective and automatic technique for studying global data usage over the lifetime of large software systems and secondly, to leverage this technique in a case-study of global data usage for several large and evolving software systems in an effort to reach answers to these questions.
Adsorption of mono- and multivalent cat- and anions on DNA molecules
Adsorption of monovalent and multivalent cat- and anions on a deoxyribose
nucleic acid (DNA) molecule from a salt solution is investigated by computer
simulation. The ions are modelled as charged hard spheres, the DNA molecule as
a point charge pattern following the double-helical phosphate strands. The
geometrical shape of the DNA molecules is modelled on different levels ranging
from a simple cylindrical shape to structured models which include the major
and minor grooves between the phosphate strands. The densities of the ions
adsorbed on the phosphate strands, in the major and in the minor grooves are
calculated. First, we find that the adsorption pattern on the DNA surface
depends strongly on its geometrical shape: counterions adsorb preferentially
along the phosphate strands for a cylindrical model shape, but in the minor
groove for a geometrically structured model. Second, we find that an addition
of monovalent salt ions results in an increase of the charge density in the
minor groove while the total charge density of ions adsorbed in the major
groove stays unchanged. The adsorbed ion densities are highly structured along
the minor groove while they are almost smeared along the major groove.
Furthermore, for a fixed amount of added salt, the major groove cationic charge
is independent on the counterion valency. For increasing salt concentration the
major groove is neutralized while the total charge adsorbed in the minor groove
is constant. DNA overcharging is detected for multivalent salt. Simulations for
a larger ion radii, which mimic the effect of the ion hydration, indicate an
increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure
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Status of superconducting power transformer development
Development of the superconducting transformer is arguably the most difficult of the ac power applications of superconductivity - this is because of the need for very low ac losses, adequate fault and surge performance, and the rigors of the application environment. This paper briefly summarizes the history of superconducting transformer projects, reviews the key issues for superconducting transformers, and examines the status of HTS transformer development. Both 630-kVA, three-phase and 1-MVA single phase demonstration units are expected to operate in late 1996. Both efforts will further progress toward the development of economical and performance competitive superconducting transformers
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Impact of quasi-dc currents on three-phase distribution transformer installations
This report summarizes a series of tests designed to determine the response of quasi-dc currents on three-phase power distribution transformers for electric power systems. In general, if the dc injection is limited to the primary side of a step-down transformer, significant harmonic distortion is noted and an increase in the reactive power demand results. For dc injection on the secondary (load) side of the step-down transformer the harmonic content at the secondary side is quite high and saturation occurs with a relatively low level of dc injection; however, the reactive power demand is significantly lower. These tests produced no apparent damage to the transformers. Transformer damage is dependent on the duration of the dc excitation, the level of the excitation, and on thermal characteristics of the transfer. The transformer response time is found to be much shorter than seen in power transformer tests at lower dc injection levels. This shorter response time suggests that the response time is strongly dependent on the injected current levels, and that higher levels of dc injection for shorter durations could produce very high reactive power demands and harmonic distortion within a few tenths of a second. The added reactive power load could result in the blowing of fuses on the primary side of the transformer for even moderate dc injection levels, and neutral currents are quite large under even low-level dc injection. This smoking neutral'' results in high-level harmonic injection into equipment via the neutral and in possible equipment failure
Bureaucratic Failure and the UK's lack of Preparedness for Foot and Mouth Disease
This article explores the UK's lack of preparedness for Foot and Mouth Disease (FMD) and the consequences for the 2001 outbreak. First, it utilises literature on crisis management and policy failure in order explore the problems caused by, and explanations for, lack of crisis preparedness. Second, it examines in detail a series of overlapping bureaucratic failures, particularly on the part of the Ministry for Agriculture, Fisheries and Food (MAFF), which not only reduced the UK's capacity to cope with the epidemic, but also exacerbated it. These failures included cultural complacency, resource weaknesses, lack of contingency planning, delays in implementing EU legislation, confused priorities and a failure to learn from the previous major outbreak in 1967-68. Finally, it suggests that explanations for the UK's inadequate preparedness for FMD can be found at the individual, organisational and societal levels