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*Astron. Astrophys. 346, 798-804 (1999)*
## 2. Th cosmochronometry and the r-process
Assuming that the whole r-abundance distribution observed in CS
22892-052 and HD115444 is essentially solar, it is straighforward to
relate the star age to the Th
abundances,
where is the characteristic
-decay timescale of Th and the
subscripts *obs* and *r* refer to the observed and universal
r-process abundance ratios, respectively. As classically done, the Th
abundance is here expressed relative to the spectroscopically relevant
Eu r-dominant element. The recent accurate observation of Th at the
surface of CS 22892-052 amounts to
(Sneden et al. 1996; Cowan et al. 1997). Th has also been observed at
the surface of HD 115444, but its precise abundance remains to be
determined (Pfeiffer et al. 1998). Assuming that a solar-like mix of
the r-elements ingested in these halo stars originates from a small
number of nucleosynthetic events that took place just before the
formation of the stars, the age of the star can be estimated from Eq.
(1) without calling for a complex model of the chemical evolution of
the Galaxy. The only difficulty of the methodology is therefore
related to the theoretical estimate of the r-production ratio
.
Unfortunately, the r-process remains the most complicated
nucleosynthetic process to model from the astrophysics as well as
nuclear physics point of view (for a review see Arnould &
Takahashi 1999). On the nuclear physics side, the nuclear structure
properties (such as the nuclear masses, deformation, ...) of thousands
of nuclei located between the valley of
-stability and the neutron drip line
have to be known, as well as their interaction properties, i.e the
() and
() rates,
- and
-decay half-lives and the fission
probabilities. Despite much recent experimental effort, those
quantities for most of the nuclei involved in the r-process remain
unknown, so that they have to be extracted on theoretical grounds and
are subject to the associated uncertainties. On top of these nuclear
difficulties, the question of the astrophysical conditions under which
the r-process can develop is far from being settled. The site(s) of
the r-process is (are) not identified yet, all the proposed scenarios
facing serious problems. For this reason, only parametric approaches,
such as the so-called canonical model (Seeger et al. 1965) can be used
to estimate the Th production. We use in the present study the
multi-event
model ^{1}
(Bouquelle et al. 1996; Goriely & Arnould 1996) in which the best
fit to the solar abundances is derived from a superposition of
canonical events with the aid of an iterative inversion procedure.
Compared with other treatments of the canonical model (e.g Pfeiffer et
al. 1998), a major advantage of the multi-event approach is to provide
an efficient tool for a systematic study of the various uncertainties
affecting the model (Goriely 1999). The iterative inversion method
works in such a way that the modification of a given (nuclear or
astrophysics) input in the r-process model leads to an automatic
renormalization of the thermodynamic conditions necessary to optimize
the fit to the solar r-abundance distribution. Therefore, the
uncertainties affecting the input data of the parametric model, as
well as their impact on the Th production can be studied
systematically within the multi-event approach, as shown in the next
section.
Our standard calculation is performed under the following
thermodynamic conditions: ,
and
(where *T* is the temperature,
the neutron density and
the number of neutrons captured per
seed nucleus). Note that the r-process calculations are performed
making use of the waiting point approximation, since under the
thermodynamic conditions considered here, an almost complete
equilibrium is established (Goriely
& Arnould 1996). When not available experimentally, the nuclear
data are taken from the ETFSI nuclear masses of Aboussir et al. (1995)
and from the gross theory (GT2) of
decay (and -delayed neutron emission)
of Tachibana et al. (1990). In addition,
-decay and fission processes are also
considered (before and after the neutron irradiation freeze-out). The
fission processes include spontaneous,
-delayed and neutron-induced fission,
the probabilities of which are calculated according to the
prescriptions of Kodoma & Takahashi (1975) with the ETFSI fission
barriers (Mamdouh et al. 1998). The procedure used to fit the solar
r-abundance distribution is similar to the one described in Bouquelle
et al. (1996) though each isotope is now given a weight inversely
proportional to the error affecting its solar r-abundance (Goriely,
1999). Since we are mainly concerned with Th cosmochronometry, no
details are given for the representative thermodynamic conditions
required to fit the solar system r-abundance distribution (Such
details can be found in Goriely & Arnould, 1996).
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999
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