Mainstream silicon carbide grains from meteorites

Primitive meteorites contain small quantities (up to several ppm) of presolar silicon carbide (SiC) grains. These grains show highly anomalous isotopic compositions of the major elements C and Si and of the trace elements N, Mg, Ca, Ti, Sr, Zr, Mo, Ba, Nd, Sm, Dy, and the noble gases. The isotopic compositions of most grains (“Mainstream” and the isotopically related minor type A, B, Y, and Z grains; $≈99%$ of total) show imprints of H burning (enhanced $13C$ and $14N;$ production of $26Al$⁠). He burning (enhanced $12C$ and $22Ne$⁠), the s-process (enhancements in the s-process isotopes of the heavy trace elements and in the neutron-rich isotopes of Si, Ca, and Ti), the galactic chemical evolution (variations in Si and Ti), and the galactic cosmic-ray activity (production of $21Ne$⁠). These grains are believed to have formed in the outflows of low-mass $(1–3 M⊙)$ AGB stars. Although many of the isotopic features are satisfactorily explained by current astrophysical models, several serious problems and shortcomings still exist. This includes N-isotopic compositions of some grains, the role of the galactic chemical evolution for Si and Ti, details of the s-process, and the lack of self-consistent stellar models in many cases. From the $21Ne$ abundance presolar cosmic-ray exposure ages of up to 130 (and possibly 2000) million years have been inferred, but uncertainties remain. The isotopic compositions of the mainstream grains from carbonaceous, enstatite, and unequilibrated ordinary chondrites do not indicate significant differences, compatible with the view that the different meteorite parent bodies sampled essentially the same reservoir of presolar SiC grains.