Interstellar grains in meteorites: III. Graphite and its noble gases

Abstract

Four density fractions of interstellar graphite were analyzed for isotopically anomalous noble gases by stepped heating. As in SiC, the dominant components show the signature of the s-process and apparently come from AGB- (Asymptotic Giant Branch) stars of 1–3 M⊙. But graphite seems to come from a broader range of stars and conditions, comprising at least three and perhaps all four known sources of carbon stardust. Two extreme types of KrS are present, having high or low ( Kr 86 Kr 82 ) s ratio and occurring in high- or low-density graphite, respectively. [ ( Kr 86 Kr 82 ) s = 4.8 in the fraction of 2.15–2.20 g/mL and ≤0.5 in fractions of ≤2.15 g/mL]. These imply two different kinds of AGB-stars, with mean neutron exposures of ≥0.55 mb −1 and ≤0.07 mb −1 . In addition to neon from AGB-star He-shells with its characteristic Ne 20 Ne 22 ∽ 0.09, all graphite fractions also contain monoisotopic Ne 22 from the decay of Na 22 ( t 1 2 = 2.58a ). This suggests the presence of material from novae and supernovae. In contrast to the differences in isotopic ratios, the elemental ratios of the anomalous gases resemble those of SiC, with a generally close match to AGB-star He-shells. But Ne is low by 10–100x, presumably due to diffusion loss.