Si and C Isotopes in Presolar Silicon Carbide Grains From AGB Stars

NanoSIMS isotopic analysis of small presolar grains: Search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

“…5 where the predicted 12 C/ 13 C ratios in the envelope of low-metallicity AGB stars reach values in excess of 1000, whereas essentially all Z grains have ratios smaller than 100. As it has been for the low 12 C/ 13 C ratios in RGB stars (Charbonnel, 1995;Abia and Isern, 1997;Boothroyd and Sackmann, 1999), CBP during the AGB phase has been invoked to explain the low 12 C/ 13 C ratios in Z grains Nittler and Alexander, 2003;Nittler et al, 2005b;Zinner et al, 2006b). For oxide grains (Fig.…”

Presolar Grains

“…This process adds 12 C to the envelope, increases the 12 C/ 13 C ratio from the low values resulting from the first dredge-up, and by making C > O, causes the star to become a carbon star. Envelope 12 C/ 13 C ratios predicted by canonical stellar evolution models range from $20 after first dredge-up in the RG phase to $300 in the late TP-AGB phases of solar-metallicity stars (Amari et al, 2001b;El Eid, 1994;Gallino et al, 1994) and to several thousand in low-metallicity stars (Nittler et al, 2005c;Zinner et al, 2006d). Predicted 14 N/ 15 N ratios range from 600 to 1600 (Becker and Iben, 1979;El Eid, 1994), falling short of the range observed in the grains.…”

“…In order to achieve the relatively low 12 C/ 13 C ratios of Z grains, the parent stars must have experienced coolbottom processing Nollett et al, 2003;Wasserburg et al, 1995) during their RG and AGB phase (Nittler et al, 2005c;Zinner et al, 2006d). However, inferred 26 Al/ 27 Al ratios in Z grains Zinner et al, 2007) are not higher than those in mainstream SiC grains ( Figure 5), much lower than those in many presolar oxide grains, for which cool-bottom processing has been invoked (see Section 1.4.9 and Figure 15).…”

“…Accordingly, cool-bottom processing in the parent stars of Z grains must have occurred with a high circulation rate but of low temperature. Nittler et al (2005c) and Zinner et al (2006d) compared the carbon and silicon isotopic compositions of mainstream, Y, and Z grains with new theoretical models of AGB nucleosynthesis. They concluded that cool-bottom burning on the AGB was necessary to explain the carbon ratios and that the recent silicon neutron capture cross sections by Guber et al (2003) yield a better fit to the silicon isotopic ratios.…”

“…They concluded that cool-bottom burning on the AGB was necessary to explain the carbon ratios and that the recent silicon neutron capture cross sections by Guber et al (2003) yield a better fit to the silicon isotopic ratios. From the theoretically inferred metallicities and average silicon isotopic ratios of mainstream, Y, and Z grains, Zinner et al (2001Zinner et al ( , 2006d and Nittler et al (2005c) derived the galactic evolution of the silicon isotopic ratios as function of metallicity. This evolution differs from the results of galactic evolution models based on the yields of supernovae (Timmes and Clayton, 1996) and has important implications concerning the relative contributions from Type II and Type Ia supernovae during the history of our galaxy.…”