![]() | Annu. Rev. Astron. Astrophys. 1997. 35:
503-556 Copyright © 1997 by Annual Reviews. All rights reserved |
In this review I refer to heavy elements as those elements beyond the
iron peak, with nuclear charge Z
31. The reader is directed
to
Meyer (1994),
Busso et al (1995),
Lambert et al
(1995b),
Käppeler et al (1989)
for more detailed discussions of heavy element synthesis. Elements beyond
the iron peak cannot be efficiently produced by charged-particle
interactions owing to the large Coulomb repulsion between nuclei;
temperatures high enough to overcome the Coulomb barrier tend to
photodisintegrate even the iron-peak nuclei (e.g.
Woosley & Weaver
1995).
Burbidge et al
(1957) showed that heavy elements can be synthesized by successive
neutron captures onto iron-peak nuclei, followed by
decays.
The neutron captures can occur on a time scale long enough for all
decays to
occur, which is called the s-process (for slow neutron capture), or on a
time scale that is short compared to
decay,
called the r-process (for rapid neutron capture); these two processes
lead to two characteristic abundance patterns.
In a steady flow of neutrons the abundance of each isotope is inversely proportional to its neutron capture cross section. The closed neutron shells with 50, 82, and 126 neutrons have small neutron capture cross sections, leading to abundance peaks for these nuclei. Similarly, even-numbered nuclei have smaller neutron capture cross sections than odd-numbered nuclei, resulting in higher abundances for the even nuclei; this is called the odd-even effect. The s-process abundance pattern is characterized by abundance peaks near mass numbers 87, 138, and 208 neutrons and a strong odd-even effect. The r-process abundance pattern is characterized by the abundance peaks shifted to mass numbers near 80, 130, and 195 with no odd-even effect.
Seeger et al (1965)
showed that observed abundances of s-process-only isotopes can be
represented
by an exponential distribution of neutron exposures. For the Solar System
material, the heavy element abundance pattern is best fit by a combination
of two s-process exponentials (e.g.
Käppeler et al
1989): (a) the weak component, which corresponds to the light
elements A
85 (thought to occur in the
cores of massive stars, M
10
M
; see
Raiteri et al 1991,
1992,
1993), and
(b) the main component, which fits the region approximately
between Rb and Pb.
The s-process main component is thought to occur during the thermal
pulse stage of low-mass (1-3
M) AGB stars at
neutron densities of 107-109
cm-3. Quantitative calculations of AGB (asymptotic giant
branch) nucleosynthesis were first performed by
Iben (1975),
Truran & Iben
(1977).
Iben & Truran
(1978)
estimated that AGB nucleosynthesis in intermediate mass AGB stars could
account for a significant fraction of the Galactic abundances of carbon
and s-process elements. Since that time, much observational and
theoretical work has converged
on the idea that the s-process occurs during the AGB phase of low-mass stars
(e.g. see
Busso et al 1995,
Lambert et al 1995b),
between the H and He burning shells with neutrons liberated by the
13C(
,
n)16O reaction.
Smith & Lambert
(1990)
showed that the observed s-process abundances in M, MS, and S stars
indicate a mean neutron exposure of
0 ~
0.3 at 30 keV. Because this is equal to the Solar System neutron exposure,
it is consistent with AGB s-process nucleosynthesis as a major supply of
the main component of the Solar System s-process elements. Recent
observational
information on the conditions of AGB s-process nucleosynthesis has come from
neutron densities inferred from measurements of Rb and Zr isotopic
abundances
(Lambert et al 1995b).
Another exciting area of AGB nucleosynthesis research involves the study of presolar grains, embedded in meteorites (see Zinner 1996). Anomalous isotopic abundances of carbon and s-process elements in SiC grains indicate a carbon star origin (e.g. Anders & Zinner 1993). Boothroyd et al (1994), Wasserburg et al (1995) infer the presence of deep circulation currents in AGB stars from the 12C / 13C and 18O / 16O ratios in these grains.
The site of the r-process is still in debate, although SN have been suspected from the beginning (Burbidge et al 1957). The most popular model is due to Meyer et al (1992), who suggested that the r-process occurs in the hot high-entropy bubble surrounding the nascent neutron star during the SN explosion. In this region, the high photon-to-baryon ratio favors photodissociation, thus keeping the number of free neutrons high and the number of nuclei low. Once the material cools, the nuclei are exposed to a sea of neutrons, at neutron densities of ~ 1020 cm-3, which drives the r-process.