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 (Kr8682)s ratio and occurring in high- or low-density graphite, respectively. [(Kr8682)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 Ne2022 # 0.09, all graphite fractions also contain monoisotopic Ne22 from the decay of Na22 (t1 = 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.