Age constraints from nucleocosmochronology

4. ABUNDANCE CLUES TO R-PROCESS HISTORY

Significant constraints on the site of r-process nucleosynthesis are provided by observations of the heavy element patterns in halo stars. Early abundance studies of metal poor stars (see, e.g., the reviews by Pagel et al. 1968; Spite & Spite 1978, 1985; Wheeler, Sneden, & Truran 1989) revealed that the abundances of nuclei normally attributable to the s-process were systematically depleted relative to r-process nuclei. The recognition that the heavy element abundance patterns in extremely metal deficient stars ([Fe/H] ~ -3) involve exclusively r-process products (Truran 1981) is now strongly supported by spectroscopic studies (Sneden & Pilachowski 1985; Gilroy et al. 1988; Cowan et al. 1996; Sneden et al. 1996) of an increasing number of such stars. Analysis of the data concerning the Ba/Eu ratio (equivalently, the s-process/r-process ratio) in halo stars (Mathews, Bazan, & Cowan 1992) indicates that significant s-process production from AGB stars first occurred in the metallicity range -2.5 ltapprox [Fe/H] -2.0. The abundance pattern determined for the metal-poor halo star CS 22892-052 ([Fe/H] = -3.2) by Sneden et al. (1996) is compared to the solar system r-process abundance pattern in Figure 1. Note particularly the excellent agreement obtained over the range of elements from Ba to Os.

Figure 1. The heavy element abundance pattern obtained for the metal poor halo star CS 22892-052 is compared to the r-process abundance pattern. Note particularly the excellent agreement obtained over the range of elements from Ba to Os. (This figure was kindly provided by J. J. Cowan.)

The straightforward conclusion to be drawn from these observational behaviors is that r-process nucleosynthesis, and the associated production of the critical actinide nuclear chronometers we have identified, first occurs during the very earliest stages of galactic evolution. r-process nucleosynthesis, therefore, is most likely associated with the environments provided by the evolution of massive stars (M 10 M) and Type II supernovae. This supports the viewpoint that the nucleosynthesis history we are probing with the actinide radioactive isotopes is indeed the entire history of the Galaxy. The production history of the ²³²Th / ²³⁸U and ²³⁵U / ²³⁸U chronometers produced by the r-process should trace the rate of star formation activity in the Galaxy. This implies that ²³²Th / ²³⁸U and ²³⁵U / ²³⁸U chronometer dating should therefore provide an excellent measure of the age of the Galaxy.

Several features of the abundance data regarding r-process abundances in metal deficient stars warrant special mention:

Ground based observations have revealed that the heavy element abundance patterns characteristic of the most metal poor halo stars are generally consistent with the r-process abundances in solar system matter. This includes the recent study of the star CS 22892-052 ([Fe/H] = -3.2) by Sneden et al. (1996), in which thorium has also been detected (Figure 1).
HST observations (Cowan et al. 1996) have further substantiated this behavior as, with the first detection of platinum, osmium, and lead in a metal-poor halo star, they have shown that nuclei in the r-process peak at mass A ~ 195 are also formed in solar r-process proportions.
The level of abundance of thorium in the star CS 22892-052 again confirms that the r-process occurring in the earliest stages of evolution of our Galaxy was generally consistent with that which formed the bulk of the r-process heavy elements in solar system matter: the relative abundance levels in the barium region and immediately beyond, in the mass region A ~ 195, and in the actinide region are compatible with the corresponding solar ratios. We note that this is true in spite of the fact that the r-process/Fe ratio in the star CS 22892-052 is ~ 10-50 times the solar system value. This does not, however, guarantee that the ²³²Th abundance in this star can provide a good age estimate. We will return to this issue in Section 7.
No significant data yet exists regarding the r-process pattern below barium. This includes the important r-process abundance peak at mass number A ~ 130.