Sodium-22 from Supernovae: a Meteorite Connection

There are several meteoritic noble gas components whose isotopic abundances are distinctly different from the solar abundance. One of these components, Ne-E(L), is a 22Ne-rich component carried by presolar graphite. Neon-22 is dominantly from the radiogenic decay of 22Na (T1/2 = 2.6 a), but it also contains 22Ne produced by 14N(α, γ )18F(e+ν)18O(α, γ )22Ne. The proportions of 22Ne from the two different origins vary with graphite density. Many low-density graphite grains (1.65–1.72 g cm−3) exhibit 18O and 28Si excesses, signatures of supernova origin. We revisited noble gas data on bulk (= aggregates) and single-grain analyses of presolar graphite to better understand the origin of Ne-E(L). We conclude that essentially all 22Ne in low-density graphite grains originated from 22Na produced in the O/Ne zone in supernovae. The initial presence of 22Na in these grains indicates that grains formed within a few years after the supernova explosion. The absence of implanted 22Ne can be explained if the grains and the gas in the ejecta became decoupled when they encountered the reverse shock and sputtering eroded the grains in the outer H-rich zone and in the interstellar medium.