Supernova Reverse Shocks: SiC Growth and Isotopic Composition

We present new mechanisms by which the isotopic compositions of X-type grains of presolar SiC are altered by reverse shocks in Type II supernovae. We address three epochs of reverse shocks: pressure wave from the H envelope near t = 10 6 s, reverse shock from the presupernova wind near 10 8 -10 9 s, and reverse shock from the interstellar medium near 10 10 s. Using one-dimensional hydrodynamics we show that the first creates a dense shell of Si and C atoms near 10 6 s in which the SiC surely condenses. The second reverse shock causes precondensed grains to move rapidly forward through decelerated gas of different isotopic composition, during which implantation, sputtering, and further condensation occur simultaneously. The third reverse shock causes only further ion implantation and sputtering, which may affect trace element isotopic compositions. Using a 25 M ☉ supernova model we propose solutions to the following unsolved questions: Where does SiC condense? Why does SiC condense in preference to graphite? Why is condensed SiC 28 Si-rich? Why is O-richness no obstacle to SiC condensation? How many atoms of each isotope are impacted by a grain that condenses at time t 0 at radial coordinate r 0 ? These many considerations are put forward as a road map for interpreting SiC X grains found in meteorites and their meaning for supernova physics.