Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5-1.5 μm

We report isotopic abundances for C, N, Mg-Al, Si, Ca-Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 mum. Carbon was measured in 41 X grains, N in 37 grains, Mg-Al in 18 grains, Si in 87 grains, Ca-Ti in 25 grains, and Fe in 8 grains. These X grains have C-12/C-13 ratios between 18 and 6800, N-14/N-15 ratios from 13 to 200, delta Si-29/Si-28 between -750 and +60 parts per thousand, delta Si-30/Si-28 from -770 to -10 parts per thousand, and F-54/Fe-56 ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic Mg-26 (from the radioactive decay of Al-26, half-life approximate to 7 x 10(5) years) and radiogenic Ca-44 (from the radioactive decay of Ti-44, half-life = 60 years). While all X grains but one have radiogenic Mg-26, only similar to 20% of them have detectable amounts of radiogenic Ca-44. Initial Al-26/Al-27 ratios of up to 0.36 and initial Ti-44/Ti-48 ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf-Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for Al-26/Al-27 ratios, except for the highest Al-26/Al-27 ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed Si-29/Si-30 ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured N-14/N-15 ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Mo type II SN if rotational mixing, preferential trapping of N, or both from N-15-rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial Al-26/Al-27 ratios in small X grains are remarkably similar to those of large X grains (2-10 mum). Titanium-44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti-bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain-size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.