A PRESOLAR GALACTIC MERGER SPAWNED THE SiC-GRAIN MAINSTREAM

摘要

The merger of a metal-poor satellite galaxy with the Milky Way about 5-6 Gyr ago is postulated to resolve three great unexplained conflicts presented by mainstream presolar Stardust SiC grains. The model allows all of the asymptotic giant branch (AGB) carbon stars that donated these grains to have been formed nearly simultaneously in a starburst generated by gaseous mixing, despite their great apparent age differences when evaluated in terms of Galactic chemical evolution (GCE). The model explains why a precisely measured linear correlation exists between the ratios ~(29)Si/~(28)Si and ~(30)Si/~(28)Si in the initial compositions of those AGB stars. It suggests why the slope of that normalized correlation line is m = 4/3 rather than unity, as predicted by GCE. It also suggests why the solar silicon isotopes lie near the bottom of that mainstream correlation line rather than near its top, as expected by current astrophysical ideas. By addressing many isotopic puzzles found within the solar composition, the model also yields a fresh view of the origin of the Sun and of its relationship to the Galaxy. The model is remarkable in reading dynamic events of the presolar history of the Milky Way from precise isotopic ratios measured in terrestrial laboratories within individual micron-sized presolar grains that have been extracted from meteorites that formed 4.56 Gyr ago but that fell only recently to Earth.