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4.4 Nuclear Astrophysics
The development of new radioactive ion beam facilities
like FAIR at GSI and SPIRAL2 at GANIL, will allow
experimental studies of very exotic nuclei which will
have a direct impact on the modelling of compact stars.
For instance new mass measurements of neutron-rich
isotopes with mass numbers between 80 and 160 would
reduce the uncertainties in the composition of the outer
crust of neutron stars. This would also put constraints on
theoretical models of the inner crust by shedding light
on the evolution of nuclear shell structure with isospin
asymmetry. This is of paramount importance for determining
the properties of the neutron star crust which in
turn are needed as inputs for modelling various astrophysical
observations. Analysis of multifragmentation
in heavy-ion collisions, as well as experimental studies
of shape co-existence in drip-line nuclei and Coulomb
frustration in heavy nuclei, could help elucidate the physics
of nuclear pastas that might exist at the crust-core
boundary. Two-particle transfer reactions in exotic halo
nuclei may provide insight into the pairing mechanism.
Likewise experiments in neutron-rich nuclei are needed
in order to pin down the equation of state of asymmetric
nuclear matter. The density dependence of the symmetry
energy could be probed by analyzing various isospin
effects in heavy-ion collisions (for instance isospin diffusion,
isoscaling, neutron-proton differential flow or pion
production) or by accurately measuring the neutron-skin
radius in heavy nuclei. Experiments using parity violating
weak neutral interaction, like PREX at Jefferson Lab,
seem very promising. The knowledge of the symmetry
energy is not only central for calculating the structure
of neutron stars but it is also crucial for simulating their
cooling. The new facilities mentioned above will also
allow efficient production of hypernuclei, like PANDA at
FAIR, thus offering new perspectives for studying hyperonic
matter. This is necessary for determining the interior
composition of neutron stars at densities above 2-3ρ 0 .
The presence of hyperons can strongly influence the
cooling of neutron stars and the damping of pulsations
(hence the emission of gravitational waves). In addition
they usually soften the equation of state thus lowering
the maximum mass dramatically. However this conclusion
could be changed by including hyperon three-body
forces which remain largely unknown. The properties of
matter at very high densities are very uncertain. Hopefully
future experiments like CBM at FAIR will provide valuable
constraints and will help us to understand the phase
transition between hadronic and quark matter.
4.4.3 Future Requirements
for Experiment and Theory
Europe has a long and distinguished leadership in
nuclear astrophysics. For long ISOLDE has provided
radioactive beams for study, Louvain-la-Neuve provided
the first facility with reaccelerated beams for nuclear
astrophysics measurements and LUNA at Gran Sasso
the world’s first underground accelerator facility for
nuclear astrophysics measurements. Forefront work is
now being carried out at a range of facilities (http://www.
nupecc.org/pub/hb04/hb2008.pdf) and there are world
leading activities in astrophysics modelling and theory.
Moreover the collaborative efforts of the community
(aided by NuPECC and the Integrated Activities in FP6
and FP7) have resulted in a new generation of facilities
which will be coming online during the present decade
(FAIR, SPIRAL2, HIE-ISOLDE and SPES) and which will
ensure that Europe can retain its preeminent position.
Stable Beam facilities
The figure of merit of a nuclear experiment from the
accelerator side requires different technical parameters
such as energy, intensity, stability, beam purity and this
is complemented by sophisticated target and detection
facilities. Only in a very few favourable cases the direct
astrophysical factor has been measured in the relevant
energy region, otherwise data at higher energies have
been extrapolated down to the Gamow region. Such an
extrapolation procedure can introduce additional uncertainties
in the result. Moreover, even in those few cases
for which direct measurements are able to reach the
Gamow region, the measured cross section is affected
by the electron screening potential which produces an
enhancement with respect to the bare-nucleus cross
section. Such an enhancement is different in the stellar
environment, thus, even in these favourable cases,
an extrapolation procedure using higher energy data
is required in order to extract the bare-nucleus cross
section, which is the needed rate calculations input
parameter.
In the last decade indirect methods, like the Asymptotic
Normalization Coefficient (ANC) and Trojan Horse Method
(THM), were often used in order to avoid the extrapolation
procedure. The two methods have been well tested
and successfully applied in many reactions involved in
different stellar scenarios, and have been supported by
relevant theoretical developments. More experimental
and theoretical efforts on the application of the two
methods are needed. In particular the THM requires
further theoretical work to improve the knowledge on
the specific features and limitations of the approach.
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