2,620 research outputs found
Type Ia Supernova Explosion Models: Homogeneity versus Diversity
Type Ia supernovae (SN Ia) are generally believed to be the result of the
thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs,
mainly because such thermonuclear explosions can account for the right amount
of Ni-56, which is needed to explain the light curves and the late-time
spectra, and the abundances of intermediate-mass nuclei which dominate the
spectra near maximum light. Because of their enormous brightness and apparent
homogeneity SN Ia have become an important tool to measure cosmological
parameters. In this article the present understanding of the physics of
thermonuclear explosions is reviewed. In particular, we focus our attention on
subsonic (``deflagration'') fronts, i.e. we investigate fronts propagating by
heat diffusion and convection rather than by compression. Models based upon
this mode of nuclear burning have been applied very successfully to the SN Ia
problem, and are able to reproduce many of their observed features remarkably
well. However, the models also indicate that SN Ia may differ considerably from
each other, which is of importance if they are to be used as standard candles.Comment: 11 pages, 4 figures. To appear in Proc. 10th Ann. Astrophys. Conf.
"Cosmic Explosions", Univ. of Maryland 1999, eds. S.S. Holt and W.W. Zhan
Die konsep van biobeskikbaarheid en die neerlê van eenvormige standaarde vir toelaatbare chemiese kontaminante in die grond
The original publication is available at http://www.satnt.ac.za/Soils are very heterogeneous substrates providing an environmental matrix with varying spatial and temporal
gradients of pH, organic carbon, particle size distribution, moisture content as well as biological factors
associated with soil organisms. These chemical, physical as well as biological factors determine the
bioavailability of chemicals to soil-dwelling invertebrates. This review of recent literature on the use of the
bioavailability concept in soil ecotoxicology indicates that the concept is often used unqualified and
indiscriminately to mean different things to different authors. A clear understanding of the concept is crucial
for toxicity testing, environmental monitoring, risk assessment and the setting of soil quality criteria since
knowledge of the actual exposure of organisms, and not merely the total amount of the chemical, is required.
The aim of this paper is to contribute towards a clarification of the concept. Apart from defining or
describing bioavailability, the problems related to the comparability of toxicity data between soils and species, inter-soil and inter-species comparisons of toxicity data are discussed. The potential role that
biomarkers can play in assessing bioavailability, is touched upon. In an effort to prescribe uniform criteria or
standards for environmental quality, both biotic and abiotic characteristics, which determine the
bioavailablity of contaminants should be considered. This requires a dynamic approach which takes both
uptake processes as well as a variety of other biological factors into consideration. It is concluded that
bioavialiblity should be interpreted qualitatively and that the rate of uptake of a contaminant could possibly
serve as a measure of bioavailability. The development of standardised protocols for exposure of selected
species and the measurement of biological responses with the aid of biomarkers could serve to refine and
take risk assessment a step further.Publishers' Versio
The concept of bioavailability and establishing uniform standards for permissible chemical contamination of soil
The original publication is available at http://www.satnt.ac.zaSoils are very heterogeneous substrates providing an environmental matrix with varying spatial and temporal gradients of pH, organic carbon, particle size distribution, moisture content as well as biological factors associated with soil organisms. These chemical, physical as well as biological factors determine the bioavailability of chemicals to soil-dwelling invertebrates. This review of recent literature on the use of the bioavailability concept in soil ecotoxicology indicates that the concept is often used unqualified and indiscriminately to mean different things to different authors. A clear understanding of the concept is crucial for toxicity testing, environmental monitoring, risk assessment and the setting of soil quality criteria since knowledge of the actual exposure of organisms, and not merely the total amount of the chemical, is required. The aim of this paper is to contribute towards a clarification of the concept. Apart from defining or describing bioavailability, the problems related to the comparability of toxicity data between soils and species, inter-soil and inter-species comparisons of toxicity data are discussed. The potential role that biomarkers can play in assessing bioavailability, is touched upon. In an effort to prescribe uniform criteria or standards for environmental quality, both biotic and abiotic characteristics, which determine the bioavailablity of contaminants, should be considered. This requires a dynamic approach which takes both uptake processes as well as a variety of other biological factors into consideration. It is concluded that bioavialiblity should be interpreted qualitatively and that the rate of uptake of a contaminant could possibly serve as a measure of bioavailability. The development of standardised protocols for exposure of selected species and the measurement of biological responses with the aid of biomarkers could serve to refine and take risk assessment a step further.Grond is ’n besonder heterogene substraat en ’n matriks vir variërende ruimtelike en temporale gradiënte van pH, organiese koolstof, partikelgroottes, voginhoude asook biologiese faktore geassosieer met grondlewende organismes. Hierdie chemiese, fisiese sowel as biologiese faktore bepaal die biobeskikbaarheid van chemiese stowwe vir organismes wat in die grond lewe. Hierdie oorsig van onlangse literatuur oor die gebruik van die konsep van biobeskikbaarheid in die ekotoksikologie, lei tot die slotsom dat hierdie konsep dikwels ongekwalifiseerd en teenstrydig gebruik word omdat dit verskillende dinge vir verskillende outeurs beteken. ’n Heldere begrip van die konsep is van wesenlike belang vir die interpretasie van toksisiteitstoetse, omgewingsmonitering en risiko-assessering van omgewingskontaminante. Dit is ook nodig vir die stel van grondkwaliteitskriteria omdat kennis van die werklike blootstelling van organismes nodig is, en nie bloot van die totale hoeveelheid van die chemiese stof in die omgewing nie. Die doel van hierdie bydrae is om die konsep duidelik toe te lig ter wille van konsekwente en verantwoordbare aanwending daarvan. Afgesien van die definiëring of omskrywing van biobeskikbaarheid, word die probleme rondom die vergelykbaarheid van toksisiteitsgegewens tussen gronde en tussen spesies bespreek. Verder word die potensiële rol wat biomerkers in grondorganismes kan speel om biobeskikbaarheid te bepaal, aangeroer. In ’n poging om eenvormige kriteria of standaarde vir omgewingskwaliteit neer te lê, sal rekening gehou moet word met beide biotiese sowel as abiotiese eienskappe wat die biobeskikbaarheid van chemiese stowwe vir grondorganismes bepaal. Dit vereis dus ’n dinamiese benadering wat beide fisies-chemies gedrewe desorpsieprosesse en fisiologies gedrewe opnameprosesse, sowel as ’n verskeidenheid ander biologiese faktore, in ag moet neem. Die bevinding is dat biobeskikbaarheid kwalitatief geïnterpreteer moet word en dat die opnamekoers van ’n chemiese stof moontlik as maatstaf van biobeskikbaarheid kan dien. Die ontwikkeling van gestandaardiseerde protokolle vir blootstelling van geselekteerde spesies en meting van biologiese response met behulp van biomerkers kan risiko-assessering verfyn en ’n stap verder neem.Publishers' Versio
Correction of diffraction effects in confocal raman microspectroscopy
A mathematical approach developed to correct depth profiles of
wet-chemically modified polymer films obtained by confocal Raman
microscopy is presented which takes into account scattered contributions originated from a diffraction-limited laser focal volume. It is demonstrated that the problem can be described using a linear Fredholm integral equation of the first kind which correlates apparent and true Raman intensities with the depth resolution curve of the instrument.
The calculations of the corrected depth profiles show that considerable differences between apparent and corrected depth profiles exist at the surface, especially when profiles with strong concentration gradients are dealt with or an instrument with poor depth resolution is used. Degrees of modification at the surface obtained by calculation of the corrected depth profiles are compared with those measured by FTIR-ATR and show an excellent concordance.</p
Sellulêre biomerkerresponse as maatstaf van gevoeligheid van klipmossels (Mollusca) vir kadmiumbesoedeling
The original publication is available at http://www.satnt.ac.za/Die bioakkumulasie van kadmium in tussengetyspesies kan stres veroorsaak wat op sellulêre vlak
meetbaar is. Verskeie klipmosselspesies kom volop op rotse aan die Suid-Afrikaanse kuslyn voor
en kan moontlik vir ekotokiskologiese monitering gebruik word. Die oogmerk van die studie was
om sensitiwiteitsdata te verkry wat kan bydrae tot die kies van ’n geskikte spesie vir monitering en
die uiteindelike daarstelling van ’n model vir spesie sensitiwiteitsverspreiding (SSV) wat ’n
biomerkerrespons as eindpunt gebruik. Die klipmossels Cymbula oculus, Scutellastra longicosta,
Cymbula granatina en Scutellastra granularis en watermonsters is in Valsbaai versamel. Analises
van kadmium in water en biologiese monsters is met behulp van atoomabsorpsiespektrofotometrie
uitgevoer. Blootstellings van organismes aan subletale vlakke van kadmium is uitgevoer in statiese
vloeitenks oor ’n periode van drie dae. Daar was ’n matige toename in liggaamskonsentrasies van
kadmium oor tyd. Resultate wat by drie blootstellingskonsentrasies gekry is, het geen betekenisvolle
verskille in metaalkonsentrasies tussen die verskillende C. oculus monsters uitgewys nie.
Betekenisvolle verskille tussen die kontrole en die blootstellingsgroepe vir elke individuele
blootstellingstyd is vir die spesie verkry behalwe tussen die kontrole en die 1mg/L CdCl2
blootstellingsgroep na 24 en 72 uur van blootstelling. Cd liggaamskonsentrasies (sagteweefsel)
het tussen 4.56 en 21.41μg/g (nat massa) gevarieer.
Gemiddelde Cd konsentrasies in sagteweefsel van S. longicosta was aansienlik laer (variërend
tussen 1.18 en 19.58 μg/g Cd ) as in weefsel van C. oculus. Die kontrolegroep van hierdie spesie
het betekenisvol verskil van die 0.8 en 1 mg/L CdCl2 blootstellings na 48 en 72 uur.
Gemiddelde Cd liggaamskonsentrasies in S. granularis was die hoogste van al die blootgestelde
spesies en het ’n vlak van 148 μg/g Cd by die hoogste blootstellingskonsentrasie bereik en het
betekenisvol verskil van die ander monsters se gemiddeldes met die uitsondering van die 0.8 mg/
L CdCl2 blootstellingsgroep by 72 uur en die 1 mg/L CdCl2 groep by 24 uur. Betekenisvolle verskille
is ook verkry vir die liggaamskonsentrasies van Cd van C. granatina tussen die drie verskillende
blootstellingskonsentrasies en drie blootstellingstye.
Integriteit van lisosoommembrane is bepaal met behulp van die neutraalrooi retensiemetode.
Drie van die vier spesies het ’n betekenisvolle afname in retentsietye getoon met ’n toename in Cd
konsentrasie. Tussenspesieverskille in gevoelighede vir omgewingsrelevante kadmiumkonsentrasies
is deur die biomerkerresponse uitgewys. Gebaseer op die verlaging in NRR tye, is
die volgorde van relatiewe gevoeligheid vir kadmium as volg: S. granularis > C. oculus> S.
longicosta.> C. granatina.Publishers' Versio
Huldigingsrede gelewer by die begrafnis van prof. H.G. Stoker1
Prof. Stoker was vir byna sy hele lewe nou aan ons Universiteit verbonde. As student vanaf 1917-1921; as dosent vanaf 1924-1964; as emiritus-professor: 1965-1969; as ere-professor vanaf 1970; as ere-doktor, toegeken op 24-01-1971; as 'n toegewyde en lojale ondersteuner van die Puk tot en met sy afsterwe die afgelope Sondag
A new model for deflagration fronts in reactive fluids
We present a new way of modeling deflagration fronts in reactive fluids, the
main emphasis being on turbulent thermonuclear deflagration fronts in white
dwarfs undergoing a Type Ia supernova explosion. Our approach is based on a
level set method which treats the front as a mathematical discontinuity and
allows full coupling between the front geometry and the flow field. With only
minor modifications, this method can also be applied to describe contact
discontinuities. Two different implementations are described and their
physically correct behaviour for simple testcases is shown. First results of
the method applied to the concrete problems of Type Ia supernovae and chemical
hydrogen combustion are briefly discussed; a more extensive analysis of our
astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report
1122b).Comment: 11 pages, 13 figures, accepted by A&A, corrected and extended
according to referee's comment
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