Presolar SiC grains of type A and B: Their isotopic compositions and stellar origins

A total of 124 presolar SiC grains of type A and B (defined as having C-12/C-13 < 10) were identified by ion imaging in an acid-resistant residue of the Murchison carbonaceous meteorite. Their isotopic ratios, together with those of 28 previously analyzed A+B grains, are reported here. The N-14/N-15 ratios range from 39 to 10(4), with one-third of the grains having N-14/N-15 ratios lower than the solar value of 272. Inferred Al-26/Al-27 ratios of A+B grains range up to 10(-2). These isotopic compositions clearly distinguish A+B grains from other presolar SiC populations. Among the A+B grains, grains with lower C-12/C-13 ratios tend to have lower N-14/N-15 and higher Al-26/Al-27 ratios. Silicon of A+B grains is enriched in the neutron-rich isotopes (Si-29/Si-28 up to 1.20 times solar, Si-30/Si-28 Up to 1.13 times solar), and in an Si three-isotope plot the distribution of the isotopic ratios is very similar to that of mainstream grains, indicating that the parent stars of the A+B grains had close-to-solar metallicity. Titanium isotopic ratios of 13 grains, out of 30 analyzed, deviate from solar by more than 2 in at least one isotopic ratio and span the same range as those of mainstream grains. Trace element abundance patterns of 20 previously measured A+B grains indicate that seven condensed from an atmosphere without s-process enrichments, while 13 did so from an atmosphere enriched in s-process elements by 3-5 times solar. Observationally, the most likely sources of A+B grains with solar s-process abundances are J-type carbon stars, but the origin of these stars is unclear. Born-again asymptotic giant branch (AGB) stars, typified by Sakurai's object (V4334 Sgr), are possible sources of A+B grains with enhanced s-process elemental abundances. Other C-rich stars with low C-12/C-13 ratios including R stars and CH stars are less likely stellar sources. Whatever the stellar sources, both H and He burning as well as mixing must have occurred in the proper combination to produce both low C-12/C-13 ratios and C>O in the envelope. A special nucleosynthetic problem is posed by the N-14/N-15 ratios of the grains. High ratios can be explained by hot bottom burning and by cool bottom processing in thermally pulsing AGB stars. Another proposed scenario that possibly yields this signature is extensive mixing of He-burning material into the H-rich envelope during the core He flash. However, the spread of the N-14/N-15 ratios and lower-than-solar N-14/N-15 ratios remains unexplained. The isotopic and elemental compositions of A+B grains can provide new information about nucleosynthesis in their possible parent stars that cannot be obtained in any other way.