Astrophysical and Nuclear Physics Aspects of the r-Process
F.-K. Thielemann, C. Freiburghaus, T. Rauscher, K.-L. Kratz, B. Pfeiffer,
in "Fission and Neutron-Rich Nuclei", eds. J. Hamilton, A.V. Ramayya
(World Scientific, Singapore 1998), p. 47.
The formation of elements as heavy as Th and U and beyond
is due to the rapid neutron-capture process (r-process), which causes the
production of highly unstable nuclei near the neutron drip-line and functions
via neutron captures, $(\gamma,n)$-photodisintegrations, $\beta^-$-decays
and beta-delayed processes. Neutrino-induced reactions may also play a possible
role. Observations of abundances in old (low metallicity) stars indicate
that it operates very early in galactic evolution, and is thus related to
massive star progenitors forming supernovae and/or neutron stars.
The abundances, being in all individual observations identical to solar
r-abundances (at least for $A$>130), indicate a unique process and witness
the interplay between nuclear structure far from beta-stability and the
appropriate astrophysical environment. In previous studies we pursued a
model-independent approach for the r-process as a function of neutron number
densities $n_n$, temperatures $T$, and durations times $\tau$.
The present study follows the expansion of matter with an initial entropy $S$
and an expansion timescales $\tau$ (more suited for astrophysical environments),
which can also follow the freeze-out of reactions with declining temperatures
and densities explicitely.
We compare the similarities and differences between the two approaches.
Special emphasis is given to constraints on nuclear properties far from
stability, resulting from a comparison with solar r-process abundances in
either approach. In particular the behavior of shell closures is examined.
In addition, investigations are presented to test whether some features can
also provide clear constraints on the permitted astrophysical conditions.
We discuss supernova and neutron star merger scenarios with respect to these