15.6 Absent species

15.6.1 Cm–U

²⁴⁷Cm (curium) decays to ²³⁵U with a half-life of 16 Myr. The significance of this species is that, like ²⁴⁴Pu, it is formed by the r-process only, but unlike ²⁴⁴Pu it has a comparatively short half-life. Hence the abundance of ²⁴⁷Cm can indicate conclusively whether late additions of material to the nebula (called for above) were r-process products or not. Since the strongest evidence for late addition is represented by ²⁶Al and ⁴¹Ca anomalies in CAI, these same samples have the best chance of being enriched in the daughter product of curium, which is ²³⁵U. The search for ²⁴⁷Cm therefore consists of looking for very small variations in uranium isotope composition. Unfortunately, ²³⁵U is normally used as an enriched isotope in U abundance determination by isotope dilution, and hence most laboratories are susceptible to artificial perturbations in this ratio. Perhaps for this reason, numerous claims have been made for excess ²³⁵U, but all are suspect.

In an attempt to conclusively resolve this problem, Chen and Wasserburg (1981) made a very careful investigation of U isotope compositions in samples for which large ²⁶Al signatures had been demonstrated, using a double 233/236 uranium spike to correct for mass fractionation during analysis (section 2.4.2). This study placed a maximum limit of " 0.4 % on uranium isotope variation in these very favourable samples, from which Chen and Wasserburg calculated a maximum initial ²⁴⁷Cm/²³⁸U ratio of 1.5 H 10^!3.

This value can be compared with the analogous ²⁴⁴Pu/²³⁸U ratio, which sets an upper limit on the dilution factor for late r-process material. Since inferred initial ²⁴⁷Cm abundances are substantially lower than ²⁴⁴Pu, it is deduced that the late ²⁶Al addition to the solar nebula was not r-process material. Wasserburg (1985) argued that the low ²⁴⁷Cm abundance places a strong lower limit on ) (100 Myr, Wasserburg and Papanastassiou, 1982) for the last r-process addition to the solar nebula. However, it should be pointed out that this argument only excludes late heterogenous additions of ²⁴⁷Cm to the solar nebula (accompanying ²⁶Al). This is because it rests on the observed homogeneity of ²³⁵U/²³⁸U isotope ratios rather than their absolute value.

15.7 Conclusions

In order to achieve a realistic model for the formation and early evolution of the solar nebula it is necessary to use the results of several extinct nuclide tracers. In an early review of the subject, Wasserburg and Papanastassiou (1982) drew attention to the relative similarity between the ‘hot/cold’ isotope ratios of ca. 1 H 10^!4, 0.5 H 10^!4 and 0.2 H 10^!4 for iodine, aluminium and palladium in chondrites. They pointed out that if the additions of hot material occurred comparatively early (e.g. ) = ca. 200 Myr), as suggested by Schramm and Wasserburg (1970), then their different half-lives would have attenuated the short-lived nuclides to very different degrees by the time of solar-system condensation. In contrast, the comparatively similar abundance ratios actually observed may imply a late addition with similar degrees of dilution by cold material.

More recent evidence should have narrowed the options for the origin of the solar-system, but a unique model is still not available. The presence of very short-lived species can be explained by the ‘trigger hypothesis’ (Cameron and Truran, 1977; Cameron et al., 1995), by which a single event caused late nuclide injection into a molecular cloud, and also triggered the collapse of the cloud. However, it is not clear whether this event was a Red Giant or supernova. On the other hand, ¹²⁹I and ¹⁸²Hf are best explained by a supernova, while actinides must be produced in supernovae. However, more than one type of supernova may be required to fit all of the production ratios.

Wasserburg et al. (1996) argued that the effective frequency by which supernovae could contribute to galactic nuclide production could be much higher than the one per 100 Myr commonly assumed. In addition, they suggested that different types of supernovae could have contributed ‘spikes’ of different nuclides to the pre-solar nebula. In the first place, the low level of ¹²⁹I implies an early supernova source () around 100 Myr). Wasserburg et al. then grouped ¹⁸²Hf with a late actinide addition from a second supernova () around 10 Myr, but this can increase in the light of new Hf/W data). Finally, they attributed ⁴¹Ca, ²⁶Al and ⁶⁰Fe, to very late addition, either from a third supernova source or from a Red Giant. A model of multiple supernova sources was also proposed by Harper (1996), who envisaged star birth in a large molecular cloud with a complex series of injection and mixing events (Fig. 15.30).

According to the model of Shu et al. (1997) and Lee et al. (1998), some of the very short-lived species (⁴¹Ca, ²⁶Al, and also ⁵³Mn) could also have been formed by spallation reactions when the sun was an embedded protostar. However, this mechanism cannot explain the presence of ⁶⁰Fe in the early Solar System, so it seems likely that at least some of the ⁴¹Ca, ²⁶Al, and ⁵³Mn were introduced, along with ⁶⁰Fe, by a late supernova.

Fig. 15.30. Schematic illustration of a possible scenario for the formation of new star systems (including the solar-system) in a giant molecular cloud containing supernova remnants Stippled). After Harper (1996).

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