Condensation and mixing in supernova ejecta

Low-density graphite spherules from the Murchison carbonaceous chondrite contain TIC grains and possess excess Si-28 and Ca-44 (from decay of short-lived Ti-44). These and other isotopic anomalies indicate that such grains formed by condensation from mixtures of ejecta from the interior of a core-collapse supernova with those from the exterior. Using homogenized chemical and isotopic model compositions of the eight main burning zones as end-members, Travaglio et al. (1999) attempted to find mixtures whose isotopic compositions match those observed in the graphite spherules, subject to the condition that the atomic C/O ratio = 1. They were partially successful, but this chemical condition does not guarantee condensation of TiC at a higher temperature than graphite, which is indicated by the spherule textures. In the present work, model compositions of relatively thin layers of ejecta within the main burning zones computed by Rauscher et al. (2002) for Type II supernovae of 15, 21 and 25 M-circle dot are used to construct mixtures whose chemical compositions cause equilibrium condensation of TiC at a higher temperature than graphite in an attempt to match the textures and isotopic compositions of the spherules simultaneously. The variation of pressure with temperature and the change in elemental abundances with time due to radioactive decay were taken into account in the condensation calculations. Layers were found within the main Ni, O/Ne, He/C and He/N zones that, when mixed together, simultaneously match the carbon, nitrogen and oxygen isotopic compositions, Ti-44/Ti-48 ratios and inferred initial Al-26/Al-27 ratios of the low-density graphite spherules, even at subsolar C-12/C-13 ratios. Due to the relatively large proportion of material from the Ni zone and the relative amounts of the two layers of the Ni zone required to meet these conditions, predicted Si-28 excesses are larger than observed in the low-density graphite spherules, and large negative delta Ti-46/Ti-48, delta Ti-47/Ti-48, delta Ti-49/Ti-48 and delta Ti-50/Ti-48 are produced, in contrast to the observed normal delta Ti-46/Ti-48 and delta Ti-47/Ti-48, large positive delta Ti-49/Ti-48 and smaller positive delta Ti-50/Ti-48. Although better matches to the observed delta Ti-46/Ti-48, delta Ti-47/Ti-48 and Si-28 excesses can be found using much smaller amounts of Ni zone material and some Si/S zone material, it is very difficult to match simultaneously the Ti and Si isotopic compositions in any mixtures of material from these deep layers with He/C and He/N zone material, regardless of the condensation sequence. The occurrence of Fe-rich, Si-poor metal grains inside the graphite spherules does not have a satisfactory explanation. (C) 2010 Elsevier Ltd. All rights reserved.