Condensation from supernova gas made of free atoms

Silicon carbide grains and graphite spherules containing TiC inclusions are found in the Murchison carbonaceous chondrite. The high Ca-44 (from Ti-44) and Si-28 contents of these grains are strong evidence that ail three mine rals originated in the deep zones of Type II supernova ejecta. We present e quilibrium calculations for SN shells, and show that TiC, but not graphite or SIG, is a stable condensate in the innermost shells where [Ti + Si] >> [ C + O], even though C/O < 1 in these shells. Because of the great stability of gaseous CO, however, neither carbides nor graphite can survive at chemi cal equilibrium in the massive O-rich shells which separate the heavy eleme nt-rich inner sources of Ti-44 and Si-28 from outer C-rich zones where thes e minerals would be stable. Clayton et al. (1999) found that, under circums tances where all gaseous molecules and particularly CO are completely disso ciated by Compton electrons, specific choices of kinetic parameters enable the prediction of graphite formation, even in an O-rich supernova shell. Fo llowing up on this hypothesis, we calculated high-temperature equilibrium c ondensation sequences in the absence of polyatomic molecules for gases havi ng canonical solar, and supernova shell compositions. Graphite is indeed pr edicted to be stable in O-dominated supernova zones in the absence of gaseo us molecules. But the complementary phases found in meteorites, TIC and SiC , are not stable under these conditions, while SiO2, which is not found, is produced in abundance. Without resolving these discrepancies between theor y and observation, the problem of reconciling the zone where the mineralogi cal identities of supernova grains were established with the zone implied b y their isotopic compositions remains unsolved. Copyright O 2001 Elsevier S cience Ltd.