History of individual presolar SiC grains revealed by stellar winds.

Asymptotic giant branch (AGB) stars, which are evolved low- to intermediate-mass stars, rank among the most prolific producers of interstellar dust in the Galaxy. Presolar silicon carbide (SiC) grains in meteorites are mainly produced by AGB stars. These presolar grains contain abundant noble gases, believed to have been implanted by stellar winds from the central stars of planetary nebulae (CSPNe). Here we report 3-D maps of the helium distribution in individual presolar SiC grains. The maps reveal that the implantation energy of 4He for individual grains varies from about 2 to about 4 keV/nucleon, which falls within the range of the CSPN winds. The variation of 4He fluence (1×1012-2×1015 atom cm-2) shows that the SiC grains form during the last about 1 My period of the AGB star phase. Then, they migrate outwards and become exposed to CSPN winds at distances of 0.3-32 light-years from the CSPNe.