2009ApJ...690.1424A

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 (T_1/2= 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/cm3) 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.