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 μm. 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 ¹²C/¹³C ratios between 18 and 6800, ¹⁴N/¹⁵N ratios from 13 to 200, δ²⁹Si/²⁸Si between -750 and +60‰, δ³⁰Si/²⁸Si from -770 to -10‰, and ⁵⁴Fe/⁵⁶Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic ²⁶Mg (from the radioactive decay of ²⁶Al, half-life ≈ 7 × 10⁵ years) and radiogenic ⁴⁴Ca (from the radioactive decay of ⁴⁴Ti, half-life = 60 years). While all X grains but one have radiogenic ²⁶Mg, only ∼20% of them have detectable amounts of radiogenic ⁴⁴Ca. Initial ²⁶Al/²⁷Al ratios of up to 0.36 and initial ⁴⁴Ti/⁴⁸Ti 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 ²⁶Al/²⁷Al ratios, except for the highest ²⁶Al/²⁷Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed ²⁹Si/³⁰Si 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 ¹⁴N/¹⁵N ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 M_⊙ type II SN if rotational mixing, preferential trapping of N, or both from ¹⁵N-rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial ²⁶Al/²⁷Al ratios in small X grains are remarkably similar to those of large X grains (2-10 μm). 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.