The presence of relatively short lived r-process chronometers in primitive solar system matter (e.g. the radioactive isotopes 107Pd, 129I, 182Hf, and 244Pu, appended to our Table 1) constrains the history of r-process synthesis to include significant recent production (a single early r-process event is excluded). The history of star formation and nucleosynthesis activity thus becomes a significant consideration. Meyer & Schramm (1986) concluded that "the effective nucleosynthesis rate was relatively constant over most of the duration of nucleosynthesis...," based upon considerations of 244Pu. Wasserburg, Busso, & Gallino (1996) have recently argued for such uniform production of the heavy r-process nuclei (A > 140). They find consistency for 182Hf, 244Pu, 235U, 238U, and 232Th, with a uniform nucleosynthesis history. The consistency with 182Hf is, however, achieved at an expense: the lighter r-process chronometers 107Pd and 129I cannot be correlated with this source "at the usual yields," as they would be greatly over-produced. Wasserburg et al. suggest a further complication: a second r-process environment for the mass range A < 140. This possibility has yet to be investigated in any great detail.
Given the fact that a rather uniform rate of nucleosynthesis seems most consistent with observations of short lived r-process chronometers, we can utilize the 232Th / 238U ratio to arrive at an (admittedly) model dependent age estimate. With the assumption of a constant nucleosynthesis rate over galactic history, the appropriate equation for the case of the 232Th-238U pair is
|
where, again, the primordial solar system ratio is (232Th /
238U)SS = 2.32 and the
r-process production ratio is (232Th /
238U)r-process = 1.65 ± 0.20. This yields a
timescale
for the epoch of nucleosynthesis of T = 8.2 + 3 Gyr, and an age
for the Galaxy (T +
SS) of
|
This estimate is intriguingly consistent with galactic age determinations from globular clusters. It may also be noted that an approximately uniform star formation and nucleosynthesis history is consistent with such other observed galactic abundance trends as the relatively 'flat' age-metallicity relation for disk stars (Edvardsson et al. 1993).