Gases in presolar silicon carbide and graphite

The G and N components of noble gases are carried by presolar grains of silicon carbide. Diagnostic as far as their origins are concerned, are the isotopic patterns observed in the G component for Ne and Kr/Xe Ne-G [= Ne-E(H)] is dominated by Ne, with only small amounts of while Kr-G and Xe-G show large overabundances of 

For example, at low neutron fluxes Kr (with a half-life of 10.8 years), completely decays before it can capture another neutron and thus Kr is bypassed while at higher neutron densities Kr is produced. Hence the effective neutron density is recorded in the observed Kr/ Kr ratio. Similarly there is a branching at unstable Se, and because the half-life of this nuclide is sensitive to stellar temperature, both neutron density and temperature are recorded in the °Kr/ Kr ratio. 

Silicon carbide. Analyses of grain size separates of SiC have shown that the sensitive Kr isotopic ratios vary as a function of size of the SiC grains (Lewis et al. 

These variations occur coupled together with variations in the elemental composition, where especially the abundance ratio Ne/Xe in the G component (Ne-E(H)/Xe-S) sensitively depends on grain size (Fig. 9 Lewis et al. 1994 cf. also Ott and Merchel 2000). 

Trends observed in noble gas isotopic patterns are, however, contrary to the trends observed in other elements, such as, e.g., Ba (Gallino et al. 1993 Lewis et al. 1994). In addition, the noble gas G component appears little/not elementally fractionated relative to its stellar source, while the N component is [especially within the heavy noble gases and in the ratio of the heavy noble gases relative to He and Ne (cf. Fig. 2 Lewis et al. 1990, 

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