Perspectives of Nuclear Physics in Europe - European Science ...

Currently, all stellar models studying nucleosynthesis
are 1-D. Multi-dimensional effects such as convection
or the impact of rotation are implemented phenomenologically.
Nevertheless, coupling full reaction networks
to these calculations is already computationally
expensive and often reduced networks combined with
postprocessing are used. Future improved models and
advanced computing will lead to more detailed astrophysical
predictions requiring more accurate nuclear
input. Concerning ccSN, it has been shown that a multi-
D treatment is essential for obtaining an explosion and
to describe the inner, turbulent layers of the exploding
star. Due to the computationally expensive, but necessary,
treatment of neutrino transport, large networks
still cannot be coupled to the hydrodynamics even for
the 1-D cases.
For Nova and X-ray Bursts the most accurate knowledge
of the nucleosynthesis accompanying such
cataclysmic events relies on hydrodynamic models in
spherical symmetry (1-D). This includes the state-of-theart
codes KEPLER (Santa Cruz), NOVA (Arizona), SHIVA
(Barcelona), AGILE (Basel), and the stellar evolution
code from Tel Aviv (Israel). The first successful multidimensional
calculations of nova outbursts have recently
been performed. These include 2-D simulations with the
codes VULCAN (Tel Aviv), PROMETHEUS (Garching),
and FLASH (Chicago, Barcelona), although because of
computational limitations, only simplified networks to
handle approximately the energetics of the explosion are
used. Moreover, the multidimensional nova simulations
performed have been limited to about 1000 s around
the peak of the explosion (while the overall phenomenon
lasts for 100,000 yr, from the onset of accretion). Hence,
the adopted strategy requires mapping of 1-D models
into 2-D. In contrast, no realistic multidimensional calculation
of type I XRBs has been performed to date.
Successful multidimensional nova simulations,
intended to check whether Kelvin-Helmholtz instabilities
can naturally lead to self-enrichment of the solar-like
accreted envelopes with material from the outermost
layers of the underlying white dwarf core, at levels in
agreement with observations, would require 3-D calculations,
since the way in which turbulence develops
is completely different in 3-D than in 2-D. Arbitrarily
Lagrangian-Eulerian (ALE) schemes, like that implanted
in the VULCAN code, seem the best way to tackle the
nature of such explosions. Moreover, implicit, parallelized
hydro codes seem to be better suited to describe
the complete nuclear history of XRBs and nova explosions,
all the way from the onset of the early hydrostatic
stages through a full cycle. It is likely that the continuous
improvement in computational capabilities will allow the
use of more extended nucleosynthesis networks directly
coupled to the multidimensional hydro codes.
4.4.4 Conclusions
Future science developments
The pace and direction of development of the field will
depend on many things, making prediction difficult. In
part this arises because of the highly interdisciplinary
nature of the research, since advances in astronomical
observations, new satellite capabilities, advances in
astrophysical modelling etc. can produce new insights
and new needs for nuclear physics information. In part
it is the technological challenges of developing new
radioactive beams or neutron and photon capabilities,
new experimental techniques and new theoretical understanding.
That said, certain topics seem certain to be
at the forefront of the evolution of the field over the next
decade and these are summarised below.
In terms of the nucleosynthesis classes, the situation
is as follows.
Big Bang nucleosynthesis: The nuclear physics aspects
in this area are mostly understood with the remaining
uncertainty related to the lithium problem, which will
hopefully be clarified within the next few years.
Stellar nucleosynthesis: Although much work has been
done in the study of the nuclear reactions in the early
stages of a star’s life, our understanding is still plagued
by numerous critical gaps in our knowledge. Detailed
measurements are still needed on some key reactions
(e.g. 12 C(α,γ) 16 O) and will need continuing work with direct
and indirect methods at stable beam facilities over the
next five years. By contrast our understanding of the
advanced stages of burning is at a rudimentary stage and
will require much work on stable and radioactive beam
facilities over the next decade. Work also still remains to
be done on the s-process, both in terms of the neutron
capture rates themselves and the key reactions which
are believed to provide the source of neutrons. Our ability
to determine reaction rates is still at times bedevilled by
a lack of accuracy in our reaction model calculations,
and theoretical work on this is required.
Explosive nucleosynthesis: Our ability to describe
these processes in a quantitative way is limited and much
work is still needed which will require new radioactive
beam facilities and new experimental techniques to be
developed. This is unlikely to be completed in the next
decade, but major advances can be expected. Our current
understanding of the reaction networks in X-ray
Bursters is based on modelling which, to a large extend,
relies on reaction rate estimates from theoretical models.
There will be increasing refinement of these models,
cross checked by direct (or indirect) measurements of
key reactions. This will be linked to increasingly testing
observational data coming from a wide range of new
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 147