CIRCUMSTELLAR Fe OXIDE FROM THE ACFER 094 CARBONACEOUS CHONDRITE

摘要

ceous chondrite. The grain has a close-to-solar ~(17)O/~(16)O ratio and an ~(18)O/~(16)O ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100%o-200%o in both ~(54)Fe and ~(57)Fe, relative to ~(56)Fe. No evidence of excess ~(60)Ni from the decay of extinct ~(60)Fe was observed. Auger elemental spectra show that the grain is compositionally similar to wuestite (FeO), but may contain a small amount of Mg in addition to Fe and O. The solid solution series magnesiowuestite, (Mg,Fe)O, is predicted to form under nonequilibrium conditions in oxygen-rich asymptotic giant branch (AGB) stars with low mass-loss rates, and Fe-rich magnesiowustite has been proposed as the carrier of the 19.5 μm feature observed in the spectra of certain low-mass-loss AGB stars that show little silicate emission. Although the isotopic data are ambiguous, these observations argue in favor of an AGB source for this Fe oxide grain.