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4.4 Nuclear Astrophysics
to the modern picture of the s-process two components,
connected to different stellar sites, contribute. The main
component produces nuclei with masses between 90
and 209 and refers to the He shell burning phase in
evolved AGB stars, whereas the weak component contributes
only to the mass region below mass number 90
and takes place during helium-core and carbon-shell
burning in massive stars.
Since the s process involves mainly neutron capture
rates of stable nuclei, a solid data base has been
established in the last decades. This has allowed testing
and refining stellar models with the result that the
main s process is certainly one of the best understood
nucleosynthesis processes from the nuclear physics
point of view. However, there are still many questions
waiting for answers. Due to the major improvement in
astrophysical models being able to describe the details
of He-shell flashes in AGB stars (the site of the main
s-process), there is an unprecedented demand for highly
precise and accurate neutron capture cross sections at
s-process energies.
One experimental challenge in the future will be
the measurement of neutron capture cross sections
on radioactive nuclei. This is required for the analysis
of branchings along the s-process path, which occur
whenever the neutron capture times of nuclei are comparable
to their stellar β-decay half-lives. The determination
of the branching ratio gives insights into the physical
conditions in the interior of the star and provides the
tools to effectively constrain modern stellar models.
Neutron capture measurements of branch point nuclei
are especially demanding, since they require the elaborate
production and preparation of radioactive samples
and neutron production facilities with extremely high
fluxes in the keV region to cope with the small available
sample masses.
More investigation is also required in the region
between Fe and Ba. Current s-process models cannot
explain the observed high abundances of the typical s
elements Sr, Y, and Zr in halo, thick disk and thin disk
stars with low metallicity. New processes such as for
example the lighter element primary process (LEPP) or
the νp process have been suggested. Several processes
obviously contribute to the nucleosynthesis of
medium-heavy nuclei. In order to disentangle the various
processes and to identify possible astrophysical sites
the individual contributions have to be identified and the
determination of the weak s-component is a natural first
step. However the neutron capture cross sections for
lighter nuclei are very small and most measurements in
this region show large uncertainties. Therefore these rates
should be re-measured with improved techniques. This
leads, together with the existing stellar model uncertainties
of massive stars, to large variations in the prediction
of the s-abundances of the weak component.
Besides neutron capture cross sections, (α,n)-
reactions on 13 C and 22 Ne also play a vital role, since
they act as the neutron sources. In particular, the weak s
process in massive stars and the subsequent stellar evolution
depends critically on the rate of the 22 Ne(α,n) 25 Mg
reaction. The cross sections are very small in the astrophysically
interesting energy region and despite all the
progress made in the past, further efforts to reduce the
uncertainties are required. Light nuclides can act as neutron
poisons, removing neutrons from the s-processing,
even when they have tiny neutron capture cross sections
but are abundant in the stellar plasma, e.g., 12 C and 16 O
with cross sections of few tens of µb. This poses a challenge
to experiment due to the small reaction rates, but
these cross sections must be measured.
Cataclysmic events and explosive
thermonuclear burning
Classical novae (CNe), type Ia supernovae (SNIa), and
type I X-ray bursts (XRBs) are explosive stellar events
that take place in close binary systems. They consist
of a compact object (a white dwarf, in the case of CNe
and SNIa; a neutron star, for XRBs), and a large Main
Sequence (or a more evolved) star. The companion overfills
its Roche lobe and matter flows through the inner
Lagrangian point, leading to the formation of an accretion
disk around the compact star. A fraction of this H- (He-)
rich matter ultimately ends up on top of the compact star,
where it is gradually compressed up to the point when
ignition conditions to drive a thermonuclear runaway
(hereafter, TNR) are reached.
Classical Novae
Nuclear physics plays a crucial role in the course of
nova explosions. As material from the accretion disk
piles up on top of the (CO or ONe) white dwarf, the first
nuclear reactions take place. This is followed by a rise
in temperature since degenerate conditions prevent the
star from readjusting the hydrostatic equilibrium by an
envelope expansion and, as a result, a TNR ensues.
At very early stages of the explosion, the main nuclear
activity is driven by 12 C(p,γ) 13 N(β + ) 13 C(p,γ) 14 N. But as
the temperature rises, the characteristic time for proton
capture reactions on 13 N becomes shorter than its
β + -decay time, initiating the hot CNO cycle. Very little
CNO breakout is expected. Instead, the nuclear activity
in the Ne-Ca region, characteristic of ONe novae, is
probably driven by mixing at the core-envelope interface
during the TNR. The likely nucleosynthetic endpoint is
located around Ca.
134 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010