8 research outputs found
Simplifying silicon burning: Application of quasi-equilibrium to (alpha) network nucleosynthesis
While the need for accurate calculation of nucleosynthesis and the resulting rate of thermonuclear energy release within hydrodynamic models of stars and supernovae is clear, the computational expense of these nucleosynthesis calculations often force a compromise in accuracy to reduce the computational cost. To redress this trade-off of accuracy for speed, the authors present an improved nuclear network which takes advantage of quasi- equilibrium in order to reduce the number of independent nuclei, and hence the computational cost of nucleosynthesis, without significant reduction in accuracy. In this paper they will discuss the first application of this method, the further reduction in size of the minimal alpha network. The resultant QSE- reduced alpha network is twice as fast as the conventional alpha network it replaces and requires the tracking of half as many abundance variables, while accurately estimating the rate of energy generation. Such reduction in cost is particularly necessary for future generation of multi-dimensional models for supernovae
Mass transfer dynamics in double degenerate binary systems
We present a numerical study of the mass transfer dynamics prior to the
gravitational wave-driven merger of a double white dwarf system. Recently,
there has been some discussion about the dynamics of these last stages,
different methods seemed to provide qualitatively different results. While
earlier SPH simulations indicated a very quick disruption of the binary on
roughly the orbital time scale, more recent grid-based calculations find
long-lived mass transfer for many orbital periods. Here we demonstrate how
sensitive the dynamics of this last stage is to the exact initial conditions.
We show that, after a careful preparation of the initial conditions, the
reportedly short-lived systems undergo mass transfer for many dozens of orbits.
The reported numbers of orbits are resolution-biased and therefore represent
only lower limits to what is realized in nature. Nevertheless, the study shows
convincingly the convergence of different methods to very similar results.Comment: 5 pages, 3 figures, for associated movie files, see
http://pandora.jacobs-university.de/~mdan/WD_coalescences.htm, to appear in
Journal of Physics Conference Proceedings for the 16th European White Dwarf
Worksho
The Role of Electron Captures in Chandrasekhar Mass Models for Type Ia Supernovae
The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received
increasing support from recent comparisons of observations with light curve
predictions and modeling of synthetic spectra. It explains SN Ia events via
thermonuclear explosions of accreting white dwarfs in binary stellar systems,
being caused by central carbon ignition when the white dwarf approaches the
Chandrasekhar mass. As the electron gas in white dwarfs is degenerate,
characterized by high Fermi energies for the high density regions in the
center, electron capture on intermediate mass and Fe-group nuclei plays an
important role in explosive burning. Electron capture affects the central
electron fraction Y_e, which determines the composition of the ejecta from such
explosions. Up to the present, astrophysical tabulations based on shell model
matrix elements were only available for light nuclei in the sd-shell. Recently
new Shell Model Monte Carlo (SMMC) and large-scale shell model diagonalization
calculations have also been performed for pf-shell nuclei. These lead in
general to a reduction of electron capture rates in comparison with previous,
more phenomenological, approaches. Making use of these new shell model based
rates, we present the first results for the composition of Fe-group nuclei
produced in the central regions of SNe Ia and possible changes in the
constraints on model parameters like ignition densities and burning front
speeds.Comment: 26 pages, 8 figures, submitted to Ap
Tidally-induced thermonuclear Supernovae
We discuss the results of 3D simulations of tidal disruptions of white dwarfs
by moderate-mass black holes as they may exist in the cores of globular
clusters or dwarf galaxies. Our simulations follow self-consistently the
hydrodynamic and nuclear evolution from the initial parabolic orbit over the
disruption to the build-up of an accretion disk around the black hole. For
strong enough encounters (pericentre distances smaller than about 1/3 of the
tidal radius) the tidal compression is reversed by a shock and finally results
in a thermonuclear explosion. These explosions are not restricted to progenitor
masses close to the Chandrasekhar limit, we find exploding examples throughout
the whole white dwarf mass range. There is, however, a restriction on the
masses of the involved black holes: black holes more massive than M swallow a typical 0.6 M dwarf before their tidal forces
can overwhelm the star's self-gravity. Therefore, this mechanism is
characteristic for black holes of moderate masses. The material that remains
bound to the black hole settles into an accretion disk and produces an X-ray
flare close to the Eddington limit of _\odot$), typically lasting for a few months. The combination
of a peculiar thermonuclear supernova together with an X-ray flare thus
whistle-blows the existence of such moderate-mass black holes. The next
generation of wide field space-based instruments should be able to detect such
events.Comment: 8 pages, 2 figures, EuroWD0
Nuclear astrophysics: the unfinished quest for the origin of the elements
Half a century has passed since the foundation of nuclear astrophysics. Since
then, this discipline has reached its maturity. Today, nuclear astrophysics
constitutes a multidisciplinary crucible of knowledge that combines the
achievements in theoretical astrophysics, observational astronomy,
cosmochemistry and nuclear physics. New tools and developments have
revolutionized our understanding of the origin of the elements: supercomputers
have provided astrophysicists with the required computational capabilities to
study the evolution of stars in a multidimensional framework; the emergence of
high-energy astrophysics with space-borne observatories has opened new windows
to observe the Universe, from a novel panchromatic perspective; cosmochemists
have isolated tiny pieces of stardust embedded in primitive meteorites, giving
clues on the processes operating in stars as well as on the way matter
condenses to form solids; and nuclear physicists have measured reactions near
stellar energies, through the combined efforts using stable and radioactive ion
beam facilities. This review provides comprehensive insight into the nuclear
history of the Universe and related topics: starting from the Big Bang, when
the ashes from the primordial explosion were transformed to hydrogen, helium,
and few trace elements, to the rich variety of nucleosynthesis mechanisms and
sites in the Universe. Particular attention is paid to the hydrostatic
processes governing the evolution of low-mass stars, red giants and asymptotic
giant-branch stars, as well as to the explosive nucleosynthesis occurring in
core-collapse and thermonuclear supernovae, gamma-ray bursts, classical novae,
X-ray bursts, superbursts, and stellar mergers.Comment: Invited Review. Accepted for publication in "Reports on Progress in
Physics" (version with low-resolution figures