Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains from Meteorites

Abstract

We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are 29 Si and 30 Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of di erent mass move the silicon isotopic composition? (4) Why do present calculations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley & Weaver (1995) supernova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains.

Isotopic compositions of different presolar silicon carbide size fractions from the Murchison meteorite

“…The X grains have large deficiencies in 29Si and JOSi (or 28Si excesses) and do not plot in the region shown in the figure. The spread in the Si-isotopic ratios of individual Sic grains have been interpreted to reflect both differences in the initial compositions of the parent stars of the grains (Clayton, 1988;Alexander, 1993;Gallino et al, 1994;Timmes and Clayton, 1996;Clayton and Timmes, 1997;Alexander and Nittler, 1999;Lugaro ef al., 1999) and (to a lesser extent) neutron-capture reactions in the He shell of AGB stars during their thermal pulse stage (Gallino et al, 1990;1994;Brown and Clayton, 1992;Lugaro et al, 1999).…”

AGB Stars Migration fom the Central of the Milky Way Galaxy to Our Sun’s Birthplace and Its Relation with Silicon Carbide Grains

“…1, where galactic time in a chemical evolution model is shown on the right margin (Timmes and Clayton, 1996). The obvious correlation trend seen there is called the mainstream.…”

“…The corresponding three-dimensional density distribution ρ (R,z) can be derived from the Poisson's equation (Timmes and Clayton, 1996) as…”

Reservoir for Comet Material: Circumstellar Grains

“…5). The preferred interpretation of this line is that it reflects primarily the GCE, both in time and space, of the Si isotopes and represents a range of Si starting compositions of a large number of stars and not the effects of the stellar nucleosynthesis in the grain's parent stars (Alexander 1993;Gallino et al 1994;Timmes and Clayton 1996;Lugaro et al 1999). …”