A Presolar Galactic Merger Spawned the SiC‐Grain Mainstream

The mainstream SiC grains have presented unsolved puzzles. Whereas their origin in AGB carbon stars seems correct, based on C isotopes and s-process trace elements, the Si isotopes present two dominating puzzles that are quite extreme in the picture [1,2] that chemical evolution of the Galaxy has generated the SiC mainstream correlation line. The first is that most of the donor AGB stars appear to have evolved in higher-metallicity regions of the Galaxy than has the sun [1,3]; and the second is that the correlation line δ(29) vs. δ(30) has slope m= 4/3 rather than unity, the slope expected if the solar Si composition reflects that of the local interstellar medium when the sun was born [1,3]. In this work I present a new dynamics-based interpretation of these puzzles and discuss associated features that are thrown into a new light by it. Mergers: It is widely believed now that galactic mergers played a large role in the growth of the total mass of the Galaxy. I propose that the Si-isotope correlation line is but a two-component mixing line between galactic-disk gas and a satellite galaxy that was cannibalized by the Galaxy about 6.5 Gyr ago. I stress that this is not improbable, but is rather the current view of how much of the Galaxy grew. This mixing occurred along a hydrodynamic stream generated by the gaseous collision of these two earlier galaxies. Fig. 1 illustrates the idea succinctly. I take the upper end of the mixing line in the Si three-isotope