THE PUZZLE OF SILICON, TITANIUM, AND MAGNESIUM ANOMALIES IN METEORITIC SILICON-CARBIDE GRAINS

An astrophysical interpretation of the silicon, titanium, and magnesium isotope anomalies measured in the mainstream population of single silicon carbide (SiC) grains extracted from carbonaceous meteorites is presented. The condensation site of the grains is envisaged in the cool atmospheres of carbon stars. The Si isotope anomalies show a general linear correlation between Si-29/Si-28 and Si-30/Si-28, whose slope cannot be explained only by s-processing in the He-burning shell and dilution with material of solar composition from the envelope. We suggest a multiple, star scenario in which the SiC grains form in stellar envelopes with slightly nonsolar initial Si isotope composition and metallicities from one-half solar to solar. The initial Si abundances are inferred from considerations of galactic chemical evolution, coupling spectroscopic observations of abundances in stars of different metal content with current predictions of stellar nucleosynthesis. The isotopes Si-29 and Si-30 are assumed to be entirely produced by short-lived massive stars exploding as supernovae, which also contribute approximately 70% of the solar Si-28 abundance, the remaining approximately 30% coming from long-lived stars, evolving in binary systems, and leading to supernovae of Type la. More detailed calculations of the Si isotopes yields from stars of various mass and initial metallicity are, however, required, and a better understanding of how the nucleosynthetic ejecta by supernovae are well homogenized with the interstellar matter. Even the Ti isotope anomalies in SiC grains cannot be explained as only an s-process signature. The linear correlation shown by Ti and Si anomalies indicates that a similar approach can be used to interpret the Ti anomalies as a mixture of a pure s-component and of a variable nonsolar isotopic composition initially present in the envelope of carbon stars. The question of the large abundance of extinct Al-26 in many SiC grains is also considered. We find that the production of Al-26 in the H shell of thermally pulsing AGB stars, although followed by substantial consumption by neutron captures during He thermal pulses, can account for the high Al-26/Al-27 ratios. The spread of carbon anomalies is interpreted as a consequence of an initial spread of C-12/C-13 as observed in M stars and of the subsequent enrichment in C-12 of the envelope during thermal pulses. Finally, the nitrogen isotope anomalies are discussed.