Presolar Graphite from the Murchison Meteorite: Imprint of Nucleosynthesis and Grain Formation

Presolar graphite is the carrier of Ne-E(L) and most 22 Ne in Ne-E(L) had long been attributed to radiogenic decay of 22 Na from novae. Of presolar graphite grains with a range of density (1.6-2.2g/cm 3 ), low-density graphite grains extracted from the Murchison meteorite are characterized by 18 O excesses and Si isotopic anomalies and are believed to have formed in supernova ejecta. From noble gas analyses of low-density graphite grains, we conclude that 22 Ne in the grains is from the in situ decay of 22 Na (T1/2=2.6a) produced in the C-burning zone in presupernova stars. The grains also contain Kr that was produced by neutron capture, either in the He-burning zone or the C-burning zone during hydrostatic burning. The 22 Ne of a 22 Na origin indicates that the grains formed shortly after the explosion. The presence of 22 Ne of a 22 Na origin and Kr, and the absence of 22 Ne of a non-radiogenic origin might give us a further clue for graphite formation in supernova ejecta.