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