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

An astrophysical interpretation of the silicon, titanium, and magnesiu m isotope anomalies measured in the mainstream population of single si licon carbide (SiC) grains extracted from carbonaceous meteorites is p resented. 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 ca nnot be explained only by s-processing in the He-burning shell and dil ution with material of solar composition from the envelope. We suggest a multiple, star scenario in which the SiC grains form in stellar env elopes with slightly nonsolar initial Si isotope composition and metal licities from one-half solar to solar. The initial Si abundances are i nferred from considerations of galactic chemical evolution, coupling s pectroscopic observations of abundances in stars of different metal co ntent with current predictions of stellar nucleosynthesis. The isotope s Si-29 and Si-30 are assumed to be entirely produced by short-lived m assive stars exploding as supernovae, which also contribute approximat ely 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 isotope s yields from stars of various mass and initial metallicity are, howev er, required, and a better understanding of how the nucleosynthetic ej ecta by supernovae are well homogenized with the interstellar matter. Even the Ti isotope anomalies in SiC grains cannot be explained as onl y an s-process signature. The linear correlation shown by Ti and Si an omalies 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 non solar isotopic composition initially present in the envelope of carbon stars. The question of the large abundance of extinct Al-26 in many S iC grains is also considered. We find that the production of Al-26 in the H shell of thermally pulsing AGB stars, although followed by subst antial consumption by neutron captures during He thermal pulses, can a ccount 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 en velope during thermal pulses. Finally, the nitrogen isotope anomalies are discussed.