Meteoritic Silicon Carbide Grains with Unusual Si-Isotopic Compositions: Evidence for an Origin in Low-Mass, Low-Metallicity Asymptotic Giant Branch Stars

摘要

Nine silicon carbide grains of the rare type Z separated from the Murchison CM2 meteorite have been analyzed for the isotopic compositions of C, Si, N (seven grains), and Mg-Al (two grains) by ion microprobe mass spectrometry. These grains have ~12C/~13C ratios from 11 to 120, ~14N/~15N ratios between 1100 and 19000, initial ~26Al/~27Al ratios of less than 0.003, and, relative to solar system Si, deficits in ~29Si of up to 150 /1000 and enrichments in ~30Si of up to 510/1000. These isotopic signatures rule out the previously postulated nova or Type II supernova origin of the Z grains. Based on the predictions from a new asymptotic giant branch (AGB) star model it appears likely that the Z grains formed in the outflows of low-mass (<2.3 M), low-metallicity AGB stars that experienced strong cool bottom processing during the red giant phase.