Kupier Prize Lecture: Sources of Solar-System Carbon

Abstract

We have tried to deconvolve Solar-System carbon into its sources, on the basis of C 12/C 13 ratios (≡ R). Interstellar SiC in meteorites, representing >4.6-Ga-old stardust from carbon stars, is isotopically heavier ( R̄ = 38 ± 2) than Solar-System carbon (89), implying that the latter contains an additional, light component. A likely source are massive stars, mainly Type II supernovae and Wolf-Rayet stars, which, being O-rich, eject their C largely as CO rather than carbonaceous dust. The fraction of such light C in the Solar System depends on R light in the source. For R light = 180-1025 (as in "Group 4" meteoritic graphite spherules, which apparently came from massive stars >4.6 Ga ago), the fraction of light C is 0.79-0.61. Similar results are obtained for present day data on red giants and interstellar gas. Although both have become enriched in C 13 due to galactic evolution (to R̄ = 20 and 57), the fraction of the light component in interstellar gas again is near 0.7. (Here R̄ represents the mean ratio of a mixture calculated via atom fractions; it is not identical to the arithmetic mean R). Interstellar graphite, unlike SiC, shows a large peak at R ≈ 90, near the solar value. Although some of the grains may be of local origin, others show anomalies in other elements and hence are exotic. Microdiamonds, with R ≈ 93, also are exotic on the basis of their Xe and N. Apparently R ≈ 90 was a fairly common composition 4.6 Ga ago, of stars as well as the ISM.