LARGE AMOUNTS OF EXTINCT AL-26 IN INTERSTELLAR GRAINS FROM THE MURCHISON METEORITE

INTERSTELLAR graphite and silicon carbide grains recovered from the Murchison CM2 chondritic meteorite are known to show large anomalies in the isotopic abundances of neon, xenon, carbon, nitrogen and silicon1-3. These anomalies provide clues to the nucleosynthetic origin of the material from which the grains formed. Here we report that both types of grain also have large abundances of Mg-26 from the decay of extinct Al-26 (half-life 705,000 years). The deduced initial Al-26/Al-27 ratios range up to 0.06 in graphite and 0.2 in SiC-1,200 and 4,000 times the maximum values found in refractory inclusions in primitive meteorites. All proposed stellar sources of carbonaceous dust (red giants, novae, Wolf-Rayet stars and supernovae) also produce Al-26, but the highest Al-26/Al-27 ratios found in these grains seem to rule out Wolf-Rayet stars and supernovae. The aluminium abundance correlates with that of nitrogen, suggesting that the aluminium condensed as aluminium nitride.