PRODUCTION OF CARBON-RICH PRESOLAR GRAINS FROM MASSIVE STARS

About a year after core-collapse supernova, dust starts to condense in the ejecta. In meteorites, a fraction of C-rich presolar grains (e.g., silicon carbide (SiC) grains of Type-X and low density graphites) are identified as relics of these events, according to the anomalous isotopic abundances. Several features of these abundances remain unexplained and challenge the understanding of core-collapse supernovae explosions and nucleosynthesis. We show, for the first time, that most of the measured C-rich grain abundances can be accounted for in the C-rich material from explosive He burning in core-collapse supernovae with high shock velocities and consequent high temperatures. The inefficiency of the super(12)C( alpha , gamma ) super(16)O reaction relative to the rest of the alpha -capture chain at T > 3.5 x 10 super(8) K causes the deepest He-shell material to be carbon-rich and silicon-rich, and depleted in oxygen. The isotopic ratio predictions in part of this material, defined here as the C/Si zone, are in agreement with the grain data. The high-temperature explosive conditions that our models reach at the bottom of the He shell can also be representative of the nucleosynthesis in hypernovae or in the high-temperature tail of a distribution of conditions in asymmetric supernovae. Finally, our predictions are consistent with the observation of large super(44)Ca/ super(40)Ca observed in the grains. This is due to the production of super(44)Ti together with super(40)Ca in the C/Si zone and/or to the strong depletion of super(40)Ca by neutron captures.