2,127 research outputs found
Does Policy Matter? On Governmentsâ Attempts to Control Unwanted Migration
Public policy making on asylum takes place in an environment of intense public scrutiny, strong institutional constraints and international collective action problems. By assessing the relative importance of key pull factors of international migration, this article explains why, even when controlling for their differences in size, some states receive a much larger number of asylum seekers than others. The analysis of 20 OECD countries for the period 1985-1999 further shows that some of the most high profile public policy measuresâsafe third country provisions, dispersal and voucher schemesâaimed, at least in part, at deterring unwanted migration and at addressing the highly unequal distribution of asylum burdens have often been ineffective. This is because the key determinants of an asylum seekerâs choice of host country are historical, economic and reputational factors that largely lie beyond the reach of asylum policy makers. Finally, the paper argues that the effectiveness of unilateral policy measures will be further undermined by multilateral attempts to harmonise restrictive policies and that current efforts such as those by the European Union will consolidate, rather than effectively address, existing disparities in the distribution of asylum burdens.public policy effectiveness, asylum, migration pull factors, deterrence, burden sharing, collective action problems, international co-operation, European Union
Tables of Nuclear Cross Sections and Reaction Rates: an Addendum to the Paper "Astrophysical Reaction Rates from Statistical Model Calculations"
In a previous publication [Atomic Data and Nuclear Data Tables 75, 1 (2000)],
we had given seven parameter analytical fits to theoretical reaction rates
derived from nuclear cross sections calculated in the statistical model
(Hauser-Feshbach formalism) for targets with 9<Z<84 (Ne to Bi) and for a mass
range reaching the neutron and proton driplines. Reactions considered were
(n,gamma), (n,p), (n,alpha), (p,gamma), (p,alpha), (alpha,gamma), and their
inverse reactions. On public demand, we present here the theoretical nuclear
cross sections and astrophysical reaction rates from which those rate fits were
derived, and we provide these data as on-line electronic files. Corresponding
to the fitted rates, two complete data sets are provided, one of which includes
a phenomenological treatment of shell quenching for neutron-rich nuclei.Comment: 23 pages, 3 tables; scheduled to appear in Atomic Data Nuclear Data
Tables 79 (2001) (September issue); preprint and data also available at
http://quasar.physik.unibas.ch/~tommy/adndt.htm
Global statistical model calculations and the role of isospin
An improved code for the calculation of astrophysical reaction rates in the
statistical model is presented. It includes the possibility to study isospin
effects. Such effects heavily affect capture rates involving self-conjugate
nuclei and may also be found in reactions on other intermediate and heavy
targets.Comment: 5 pages LaTeX, uses iopconf.sty, to appear in the proceedings of the
2nd Oak Ridge Symposium on Atomic and Nuclear Astrophysics, ed. A.
Mezzacappa, IOP, in pres
Silicon Burning II: Quasi-Equilibrium and Explosive Burning
Having examined the application of quasi-equilibrium to hydrostatic silicon
burning in Paper I of this series, Hix & Thielemann (1996), we now turn our
attention to explosive silicon burning. Previous authors have shown that for
material which is heated to high temperature by a passing shock and then cooled
by adiabatic expansion, the results can be divided into three broad categories;
\emph{incomplete burning}, \emph{normal freezeout} and \emph{-rich
freezeout}, with the outcome depending on the temperature, density and cooling
timescale. In all three cases, we find that the important abundances obey
quasi-equilibrium for temperatures greater than approximately 3 GK, with
relatively little nucleosynthesis occurring following the breakdown of
quasi-equilibrium. We will show that quasi-equilibrium provides better
abundance estimates than global nuclear statistical equilibrium, even for
normal freezeout and particularly for -rich freezeout. We will also
examine the accuracy with which the final nuclear abundances can be estimated
from quasi-equilibrium.Comment: 27 pages, including 15 inline figures. LaTeX 2e with aaspp4 and
graphicx packages. Accepted to Ap
Computational Methods for Nucleosynthesis and Nuclear Energy Generation
This review concentrates on the two principle methods used to evolve nuclear
abundances within astrophysical simulations, evolution via rate equations and
via equilibria. Because in general the rate equations in nucleosynthetic
applications form an extraordinarily stiff system, implicit methods have proven
mandatory, leading to the need to solve moderately sized matrix equations.
Efforts to improve the performance of such rate equation methods are focused on
efficient solution of these matrix equations, by making best use of the
sparseness of these matrices. 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 with complete
equilibrium fail.Comment: LaTeX2e with graphicx, 40 Pages with 5 embedded figures. To be
published in Computational Astrophysics, The Journal of Computational and
Applied Mathematics, eds. H. Riffert, K. Werne
Silicon Burning I: Neutronization and the Physics of Quasi-Equilibrium
As the ultimate stage of stellar nucleosynthesis, and the source of the iron
peak nuclei, silicon burning is important to our understanding of the evolution
of massive stars and supernovae. Our reexamination of silicon burning, using
results gleaned from simulation work done with a large nuclear network (299
nuclei and more than 3000 reactions) and from independent calculations of
equilibrium abundance distributions, offers new insights into the
quasi-equilibrium mechanism and the approach to nuclear statistical
equilibrium. We find that the degree to which the matter has been neutronized
is of great importance, not only to the final products but also to the rate of
energy generation and the membership of the quasi-equilibrium groups. A small
increase in the global neutronization results in much larger free neutron
fluences, increasing the abundances of more neutron-rich nuclei. As a result,
incomplete silicon burning results in neutron richness among the isotopes of
the iron peak much larger than the global neutronization would indicate.
Finally, we briefly discuss the limitations and pitfalls of models for silicon
burning currently employed within hydrodynamic models. In a forthcoming paper
we will present a new approximation to the full nuclear network which preserves
the most important features of the large nuclear network calculations at a
significant improvement in computational speed. Such improved methods are
ideally suited for hydrodynamic calculations which involve the production of
iron peak nuclei, where the larger network calculation proves unmanageable.Comment: 44 pages of TeX with 25 Postscript figures, uses psfig.sty, To appear
in the The Astrophysical Journal, April 1 1996. Complete PostScript version
of the paper is also available from
http://tycho.as.utexas.edu/~raph/Publications.htm
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