An interstellar dust component rich in 12C

Chondritic meteorites contain grains which condensed in circumstellar environments and carry unusual isotope signatures1providing clues to the location in which grain formation took place. Isotope measurements on grains that are composed entirely of carbon or carbon compounds have, up to now, given compositions either close to typical Solar System values or enriched in 13C (refs 2–5). Despite the existence of astrophysical environments where 12C production predominates, highly 12C-enriched carbon com-ponents in meteorites are uncommon. Here we report the results of an analysis of an acid-resistant residue from the Allende (CV3) meteorite1,6, extensively combusted in oxygen at low temperatures (<500 °C) to remove isotopically normal carbon, which revealed the presence of a small amount (<0.1 p.p.m.) of carbon with a 12C/13C ratio close to 120, well above the value of 89 ± 3 found in terrestrial samples or bulk meteorites. This component is denoted Cλ. Stellar evolutionary processes favour lowering of the 12C/13C ratio; hence Cλ, which has a combustion temperature consistent with graphite, might be a primitive type of interstellar material.