14 research outputs found
An Approximation for the rp-Process
Hot (explosive) hydrogen burning or the Rapid Proton Capture Process
(rp-process) occurs in a number of astrophysical environments. Novae and X-ray
bursts are the most prominent ones, but accretion disks around black holes and
other sites are candidates as well. The expensive and often multidimensional
hydro calculations for such events require an accurate prediction of the
thermonuclear energy generation, while avoiding full nucleosynthesis network
calculations. In the present investigation we present an approximation scheme
applicable in a temperature range which covers the whole range of all presently
known astrophysical sites. It is based on the concept of slowly varying
hydrogen and helium abundances and assumes a kind of local steady flow by
requiring that all reactions entering and leaving a nucleus add up to a zero
flux. This scheme can adapt itself automatically and covers situations at low
temperatures, characterized by a steady flow of reactions, as well as high
temperature regimes where a -equilibrium is established.
In addition to a gain of a factor of 15 in computational speed over a full
network calculation, and an energy generation accurate to more than 15 %, this
scheme also allows to predict correctly individual isotopic abundances. Thus,
it delivers all features of a full network at a highly reduced cost and can
easily be implemented in hydro calculations.Comment: 18 pages, LaTeX using astrobib and aas2pp4, includes PostScript
figures; Astrophysical Journal, in press. PostScript source also available at
http://quasar.physik.unibas.ch/preps.htm
Thermonuclear Kinetics in Astrophysics
Over the billions of years since the Big Bang, the lives, deaths and
afterlives of stars have enriched the Universe in the heavy elements that make
up so much of ourselves and our world. This review summarizes the methods used
to evolve these nuclear abundances within astrophysical simulations. These
methods fall into 2 categories; evolution via rate equations and via
equilibria. Because the rate equations in nucleosynthetic applications involve
a wide range of timescales, implicit methods have proven mandatory, leading to
the need to solve matrix equations. Efforts to improve the performance of such
rate equation methods are focused on efficient solution of these matrix
equations, in particular by making best use of the sparseness of these
matrices, and finding methods that require less frequent matrix solutions.
Recent work to produce hybrid schemes which use local equilibria to reduce the
computational cost of the rate equations is also discussed. Such schemes offer
significant improvements in the speed of reaction networks and are accurate
under circumstances where calculations which assume complete equilibrium fail.Comment: 27 pages, 2 figures, a review for a special issue of Nuclear Physics
Entwicklung einer prozessrechnergesteuerten Mehrkammer-Ionitrieranlage mit Drehschiebeeinrichtung fuer das Ionitrieren von Serienteilen im Durchlaufverfahren
TIB: RN 8592(152) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Constraining the astrophysical origin of the p-nuclei through nuclear physics and meteoritic data
A small number of naturally occurring, proton-rich nuclides (the p-nuclei) cannot be made in the s- and r-processes. Their origin is not well understood. Massive stars can produce p-nuclei through photodisintegration of pre-existing intermediate and heavy nuclei. This so-called γ-process requires high stellar plasma temperatures and occurs mainly in explosive O/Ne burning during a core-collapse supernova. Although the γ-process in massive stars has been successful in producing a large range of p-nuclei, significant deficiencies remain. An increasing number of processes and sites has been studied in recent years in search of viable alternatives replacing or supplementing the massive star models. A large number of unstable nuclei, however, with only theoretically predicted reaction rates are included in the reaction network and thus the nuclear input may also bear considerable uncertainties. The current status of astrophysical models, nuclear input and observational constraints is reviewed. After an overview of currently discussed models, the focus is on the possibility to better constrain those models through different means. Meteoritic data not only provide the actual isotopic abundances of the p-nuclei but can also put constraints on the possible contribution of proton-rich nucleosynthesis. The main part of the review focuses on the nuclear uncertainties involved in the determination of the astrophysical reaction rates required for the extended reaction networks used in nucleosynthesis studies. Experimental approaches are discussed together with their necessary connection to theory, which is especially pronounced for reactions with intermediate and heavy nuclei in explosive nuclear burning, even close to stability.Peer reviewe
Element synthesis in stars
Except for H-1, H-2, He-3, He-4, and Li-7, originating from the Big Bang, all heavier elements are made in stellar evolution and stellar explosions. Nuclear physics, and in many cases nuclear structure far from stability, enters in a crucial way. Therefore, we examine in this review the role of nuclear physics in astrophysics in general and in particular how it affects stellar events and the resulting nucleosynthesis. Stellar modeling addresses four major aspects: 1. energy generation and nucleosynthesis, 2. energy transport via conduction, radiation or possibly convection, 3. hydrodynamics/hydrostatics, and finally 4. thermodynamic properties of the matter involved. Nuclear Physics enters via nuclear reaction cross sections and nuclear structure (affecting the composition changes and nuclear energy generation), neutrino-nucleon and neutrino-nucleus cross sections (affecting neutrino opacities and transport), and e.g. the equation of state at and beyond nuclear densities which creates a relation between the nuclear many body problem and and hydrodynamic response like pressure and entropy. In the following we review these four topics by highlighting the role and impact of nuclear physics in each of these aspects of stellar modeling. The main emphasis is put on the connection to element synthesis