Mass spectrometry of stardust : experimental tests of nucleosynthesis

Primitive meteorites contain grains of diamond, graphite and silicon carbide which condensed from the outflows of red-giant stars, which existed before the formation of our Sun and Solar System. Nuclear processes within the parent star have modified the isotopic compositions of all elements in the grains. By means of a very sensitive and selective microanalytical technique, Resonant Ionization Mass Spectrometry, we can measure isotopic abundances of trace metals in individual 3-micrometer-sized crystals. The technique uses multi-photon photoionization with tuned lasers to ionize a single element selectively and efficiently. This allows isotopic measurements of trace constituents in complex systems. Results from zirconium and molybdenum in presolar silicon carbide grains show large enhancements in the s-process isotopes, and depletion by factors >2 in the r-process and p-process isotopes. The observations are in good agreement with recent calculations of s-process nucleosynthesis in Asymptotic Giant Branch stars. These results confirm the classical subdivision of nucleo-synthetic processes into p-, r-, and s-processes and show that the principal site of the s-process is in carbon-rich red giant stars. (author)