Large amounts of extinct26AI in interstellar grains from the Murchison meteorite

Abstract

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 26Mg from the decay of extinct 26A1 (half-life 705,000 years). The deduced initial 26Al/27Al ratios range up to 0.06 in graphite and 0.2 in Si C—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 26Al, but the highest 26Al/27A1 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.