Nitrogen Isotopes in Silicon Carbide: Stellar Nucleosynthesis?

Nitrogen in presolar SiC varies over a wide range of concentrations and is typically ^(14)N-rich relative to solar N, consistent with ^(15)N being consumed during CNO processing in stellar envelopes [e.g., 1]. Although C is also heavily processed in the envelopes [1], no clear isotopic correlation exists between C and N [e.g., 2], making N compositions difficult to interpret. Although the same general N features are seen in SiC from many meteorites, clear differences between meteorites have also been observed. In particular, Murchison SiC appears to have systematically higher ^(15)N/^(14)N ratios than Orgueil SiC [2,3]. Among ISN-poor SiC grains for both meteorites, ^(15)N/^(14)N and ^(28)Si/^(l4)N exhibit a positive correlation (Fig. 1).