1,570 research outputs found
On the Explosion Mechanism of SNe Type Ia
In this article we discuss the first simulations of two- and
three-dimensional Type Ia supernovae with an improved hydrodynamics code. After
describing the various enhancements, the obtained results are compared to those
of earlier code versions, observational data and the findings of other
researchers in this field.Comment: 7 pages, 4 figure
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
SellulĂŞre 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 sellulĂŞre 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 21.41ÎĽg/g (nat massa) gevarieer.
Gemiddelde Cd konsentrasies in sagteweefsel van S. longicosta was aansienlik laer (variërend
tussen 1.18 en 19.58 ÎĽg/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
Full-star Type Ia supernova explosion models
We present full-star simulations of Type Ia supernova explosions on the basis
of the standard Chandrasekhar-mass deflagration model. Most simulations so far
considered only one spatial octant and assumed mirror symmetry to the other
octants. Two full-star models are evolved to homologous expansion and compared
with previous single-octant simulations. Therefrom we analyze the effect of
abolishing the artificial symmetry constraint on the evolution of the flame
surface. It turns out that the development of asymmetries depends on the chosen
initial flame configuration. Such asymmetries of the explosion process could
possibly contribute to the observed polarization of some Type Ia supernova
spectra.Comment: 11 pages, 10 figures, resolution of some figures reduced to meet
astro-ph file size restriction, submitted to A&
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
Nucleosynthesis in multi-dimensional SNIa explosions
We present the results of nucleosynthesis calculations based on
multidimensional (2D and 3D) hydrodynamical simulations of the thermonuclear
burning phase in SNIa. The detailed nucleosynthetic yields of our explosion
models are calculated by post-processing the ejecta, using passively advected
tracer particles. The nuclear reaction network employed in computing the
explosive nucleosynthesis contains 383 nuclear species. We analyzed two
different choices of ignition conditions (centrally ignited, in which the
spherical initial flame geometry is perturbated with toroidal rings, and
bubbles, in which multi-point ignition conditions are simulated). We show that
unburned C and O varies typically from ~40% to ~50% of the total ejected
material.The main differences between all our models and standard 1D
computations are, besides the higher mass fraction of unburned C and O, the C/O
ratio (in our case is typically a factor of 2.5 higher than in 1D
computations), and somewhat lower abundances of certain intermediate mass
nuclei such as S, Cl, Ar, K, and Ca, and of 56Ni. Because explosive C and O
burning may produce the iron-group elements and their isotopes in rather
different proportions one can get different 56Ni-fractions (and thus supernova
luminosities) without changing the kinetic energy of the explosion. Finally, we
show that we need the high resolution multi-point ignition (bubbles) model to
burn most of the material in the center (demonstrating that high resolution
coupled with a large number of ignition spots is crucial to get rid of unburned
material in a pure deflagration SNIa model).Comment: Accepted for A&A, 14 pages, 11 Figures, 2 Table
Three-dimensional simulations of type Ia supernovae
We present the results of three-dimensional hydrodynamical simulations of the
subsonic thermonuclear burning phase in type Ia supernovae. The burning front
model contains no adjustable parameters so that variations of the explosion
outcome can be linked directly to changes in the initial conditions. In
particular, we investigate the influence of the initial flame geometry on the
explosion energy and find that it appears to be weaker than in 2D. Most
importantly, our models predict global properties such as the produced nickel
masses and ejecta velocities within their observed ranges without any fine
tuning.Comment: 7 pages, 5 figures, accepted by A&
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