The sources of shortlived radionuclides

Along with the record of the evolution of stable and long-lived isotopes, matter from the interstellar medium and circumstellar disks has preserved vestiges of short-lived nuclides (Section 3.4). Recalling the 26Al-26Mg isotopic systematics, it has been shown that 26Mg excesses correlate perfectly with the Al/Mg ratios in relevant meteoritic minerals, and therefore 26Al was present in the early solar system (see Fig. 10.7).

Even though the history of short-lived isotopes is by definition short, it is in fact rather complicated because it includes the ejection of freshly synthesized nuclides from the stellar source(s), propagation of these nuclides in the interstellar medium, their mixing with relevant stable isotopes in the interstellar medium, their condensation or implantation in solid grains and mixing within the circumstellar disk. It is also possible that some of these nuclides may be (partially) generated directly in the interstellar medium (e.g. a presolar cloud) and/or in the planetary nebula itself. Identification of the process responsible for generating a given isotope is especially important in cases where the data could yield a chronological result.

Generation of short-lived radionuclides in the circumstellar disk

The interaction of energetic particles emitted by a young star with matter in the circumstellar disk could yield short-lived nuclides. In this regard the observation of decay products of the fragile 10Be (Table 3.3) and especially of the very short-lived 7Be (t7 = 53 days) is of extreme importance. The latter isotope could only be produced in situ in existing solid grains in the nebula. Chaussidon et al. (2006) investigated the systematics of 7Be ^ e-capture ^ 7Li (using 6Li as a stable reference isotope) in high-temperature inclusions separated from the Allende chon-drite and observed a weak 7Li/6Li versus 9Be/6Li correlation, which these authors interpreted as an indication of the in situ decay of 7Be.

Gounelle et al. (2001, 2006) modelled the irradiation of early solar system rocks by intense solar radiation including impulsive and gradual components, and demonstrated that several short-lived nuclides could have been produced by in situ spallation reactions. The 7Be/10Be ratio observed in the inclusions is close to the production ratio modelled for spallation reactions between accelerated protons and O-atoms. Within the framework of such a model the canonical SOS ratios of 10Be/9Be, 26Al/27Al, 41Ca/40Ca and 53Mn/55Mn can be reproduced (Table 3.3). The compositions and energies of the irradiating fluxes used in the modelling are supported by observations of nascent stars. Modifications of these fluxes (mainly relating to gradual or impulsive events) might explain the observed decoupling between different short-lived families, such as the decay products of 10Be and 26Al (e.g. Marhas et al., 2002).

The irradiation scenario was also developed by Leya et al. (2003). In order to explain the SOS "canonical" ratios, e.g. 26Al/27Al ~ 5 x 10-5, found in most samples (Fig. 10.7) it was suggested that the targets were gaseous (and therefore homogeneous) and were irradiated by similar fluxes. Young etal. (2005) presented new supporting evidence for this scenario: the earliest SOS rocks stay in a high-temperature environment for a relatively long time interval, comparable with the irradiation time scale, during which time the shortest-lived nuclides such as 41Ca mainly decayed before the host object was "closed" (able to preserve both parent and daughter nuclides) while those having a somewhat longer life, e.g. 26Al, survived until the closure time (Section 10.6). This removes an important argument against the irradiation scenario, namely that if 26Al had been produced in the early solar nebula by the reaction 24Mg(3He, p)26Al, the reaction 40Ca(3He, 2p)41Ca would have vastly overproduced 41Ca compared with observations (Sahijpal et al., 1998; Srinivasan and Bischoff, 2001). The heavy O-isotope compositions in presolar grains (Fig. 3.2), which are not readily reproduced in stellar models, are also best explained by irradiation of the early solar nebular gas (O, Ne) by an impulsive solar flare (p, 3He, 4He) (Aleon et al., 2005).

Even though there is thus strong evidence favouring the irradiation hypothesis, several radionuclides (e.g. 244Pu, 182Hf and even rather short-lived 60Fe) can only be yielded by stellar nucleosynthesis, the contribution of which thus appears unavoidable.

Generation of short-lived radionuclides in interstellar clouds (cloud core)

Another scenario was suggested by Clayton and Jin (1995): the occurrence of 26Al and several other short-lived radionuclides could be accounted for by bombardment of the proto-solar cloud with cosmic rays. Proposed mechanisms include: (1) the trapping of freshly synthesized particles (with 26Al/27Al ~ 0.01) accelerated from local SNe ejecta or by stellar wind; (2) the trapping of low-energy galactic cosmic rays with very high 26Al/27Al ~ 0.1; (3) the production of radionu-clides within the cloud (cloud core) by the interaction of low-energy cosmic ray nuclei (O, Na, Mg, Si) with cloud constituents, e.g. 12C(16O, pn)26Al. The discovery of the interaction of heavy cosmic ions with interstellar hydrogen supports the latter suggestion (Bloemen et al., 1994). Clayton (1994) showed that the reactions 26Mg(p, n)26Al and 28Si(p, ppn)26Al could yield a ratio 26Al/27Al « 2 x 10-5, which is quite similar to the observed initial ratio in the solar nebula. Further, the reaction 38Ar(a, n)41Ca could yield 41Ca. In such a scenario, the initially heterogeneous distribution of the short-lived species within the cloud or, later on, in the circumstellar disk could have resulted from the propagation of heavy ions through variable magnetic fields (Podosek and Cassen, 1994). However, very specific heavy-ion abundances and narrow limits for their energy are required to reproduce the observed abundances of several short-lived radionuclides.

Stellar sources for short-lived radionuclides

The third proposal envisages stellar production of the nuclides followed by ejection of stellar debris into the interstellar medium. Sources would be chiefly supernovae (SNe II) or low-mass AGB stars (Cameron et al., 1995; Srinivasan et al., 1996; Wasserburg et al., 1994, 1995b; Woosley and Weaver, 1995). In this scenario, relatively long-lived species would have different histories from short-lived ones. The former (146Sm, 244Pu, 129I and even 53Mn) could have been produced by several generations of massive stars well before the formation of the solar system and stored in an interstellar cloud. The very short-lived nuclides (26Al, 36Cl, 41Ca, 60Fe, 182Hf) would have to be synthesized and ejected into the interstellar medium not earlier than ~ 1 Myr prior to the formation of solid objects in the planetary nebula (Meyer and Clayton, 2000).

Models of low- to middle-mass stars allow quantitative estimates of the yield of 26Al. Dredge-up is expected to bring 26Al from the H-burning shell just outside the helium-exhausted core, where 26Al is produced via 25Mg(p, y )26Al, to the surface envelope of AGB stars. The 26Al/27Al ratios observed in presolar oxide grains are ~ 10-2 to 10-4, in agreement with those derived from AGB stellar models. The oxygen-isotope composition of these grains also points to this source. The amount of 26Al in the grains indicates the time interval between mass loss in AGB stars and grain formation (Nittler, 1997; Busso et al., 1999).

An AGB source is, however, inconsistent with the observed initial solar system abundance of 182Hf: the yield predicted by AGB models, 182Hf/180Hf - 10—6, is a factor — 100 below the observed ratio listed in Table 3.3 (Wasserburg et al., 1994; Arnould et al., 1997). Also it is highly unlikely that AGB star wind and an SNe II would occur nearly simultaneously in the vicinity of a pre-collapse cloud.

Cameron (2001a, c) suggested that all extinct radionuclides except 7Be could have been produced in SNe II explosive burning and the r-process (Section 6.3). Cameron's unifying approach envisages dominant production of the lighter species (26Al to 60Fe) in explosive nucleosynthesis, the medium species (107Pd and 129I) via the weak (low-A) r-process, and the heavy radionuclides (146Sm, 182Hf, 244Pu) in the main (high-A) r-process. Cameron argued that even light fragile elements, e.g. 10Be, could be synthesized in the expanding SNe envelope.

Summarizing, recent progress, particularly the identification of a 7Li excess related to 7Be decay and the development of adequate models, tilts the balance in favour of nebula irradiation, although this cannot be the sole producer of short-lived nuclides. Regardless of the precise model, however, it appears that in all cases these nuclides were produced early enough to allow a chronological interpretation of the observations (Sections 10.4, 11.6, 12.4).

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