Carbon, nitrogen, and oxygen are among the most abundant nuclei in the universe. The origins of their main isotopes, 12C, 14N, and 16O, in stars are reasonably well-understood. The minor isotopes, 13C, 15N, 17O, and 18O, are much less abundant, and the stories of their origins are murkier. Supernovae, low-mass stars, and novae probably all participate. The abundances of all of these nuclides are well-measured in presolar grains embedded in meteorites, and most of those discovered so far have clear origins in low-mass stars. The grains must have solidified in the outflows from these stars, survived for some time in the interstellar medium, and been incorporated into solid bodies when the solar system formed. The simplest models of low-mass stars end with the ejection of material enriched in 13C and 17O, heavily depleted in 15N, and relatively unchanged in 18O. The grain data present a number of deviations from this picture, pointing toward physical processes absent in stellar evolution models, as I will discuss. I will concentrate on the puzzling case of nitrogen isotopic ratios in the grains, and connections to more general issues in the nuclear evolution of the Galaxy.
ANL Physics Division Colloquium Schedule