Silicon and Magnesium Isotopes in SiC from AGB Stars

Details Silicon and Magnesium Isotopes in SiC from AGB Stars Silicon and Magnesium Isotopes in SiC from AGB Stars

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.210c&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 210

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Scientific paper

We report the evolution of Si and Mg isotopic abundances in the envelopes of massive (5-8 solar masses) AGB-star models. These have now been demonstrated (Brown and Clayton 1992) to be capable of generating the slope 1.4 correlation between excess ^29Si and excess ^30Si. Figure 1 shows the Si isotopic evolution of a 5.5 solar mass model, which reaches only moderately large delta values in the envelope (paralleling the dashed particle line). The asterisk identifies the transformation to carbon star at low deltas. Figure 1, which in the hard copy appears here, shows AGB 5.5 solar mass Si evolution and mass loss. For the first time we are presenting the mass of envelope lost (in wind) as a function of the ^30Si excess. When this star entered its superwind phase it lost most of its mass, and inefficient dredgeup had resulted in deltas near only 100o/oo, as in the large SiC particles. Because the remaining envelope mass is small, little material is lost subsequently on the road to larger deltas. As a result, values of ^30delta near 100 characterize most of the lost mass, showing that STARDUST SiC need not have huge ^30delta. However, changing the prescription for dredgeup or mass-loss rate can greatly alter this. A 6.5 solar mass AGB star lost its superwind near ^30delta = 1600 even with the same dredgeup prescription, showing that other star parameters can have a large influence on ^30delta through their effect on the dredgeup. This needs much more study before we can conclude the true provenance of the meteoritic SiC particles. Because the Mg isotopes also undergo large isotopic evolution in those models, we ask whether that evolution may somehow obscure the amount of live ^26Al that is inferred to have decayed in situ within these STARDUST particles. To the contrary, we show that this peak in mass loss (Fig. 1) occurs with ^25delta = 2 ^26delta, but finishes with ^26delta = 2 ^25delta, simulating mass fractionation at the end. Most of this Mg isotopic evolution is the result of hot-bottom envelope burning rather than shell dredgeup. But the envelope deltas are very large (20,000), much larger than for Si. This raises a question--did any stellar Mg condense in the SiC particles during the AGB outflow? There is in our models ample ^26Al created during hot-bottom burning in the envelope rather than the shell. The same 5.5 solar mass model has ^26Al/^27Al= 0.36 in the wind, for example, which is ample for the observed ^26Mg enrichments in the SiC particles. Reference: Brown, L.E. and Clayton, D.D. (1992) Astrophys. J.(Lett.) 398, xxx. Figure 1, which appears here in the hard copy, shows AGB 5.5 solar mass Si evolution and mass loss.

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