Placing the Sun and Mainstream SiC Particles in Galactic Chemodynamic Evolution

This work continues to seek a possible paradigm for the existence of the mainstream of presolar SiC particles having 29Si/28Si and 30Si/28Si isotopic ratios larger than those found in solar material. The isotopic trend of the mainstream SiC presolar particles extracted from the meteorites and the isotopic composition of the Sun are both interpreted within a framework of Galactic evolution in which the stars diffuse substantial distances but the interstellar gas remains homogeneous at each value of Galactocentric radius. Viewed from the radial position of solar birth, which lies outside the bulk of the molecular-cloud scattering centers, the presolar cloud (having solar composition) picks up asymptotic giant branch (AGB) star dust from AGB carbon stars that were primarily more metal-rich when they were born but have overwhelmingly diffused outward to reach the solar birthplace instead of diffusing inward. This would solve the fundamental problem facing any satisfactory paradigm, namely, the metal-richness of the presolar carbon stars in the solar-birth neighborhood. Other aspects of the mainstream characteristics are also commented on in this model.

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