Presolar Grains from Novae

We report the discovery of five SiC grains and one graphite grain isolated from the Murchison carbonaceous meteorite whose major-element isotopic compositions indicate an origin in nova explosions. The grains are characterized by low C/C (4-9) and N/N (5-20) ratios, large excesses in Si (Si/Si ratios range to 2.1 times solar) and high Al/Al ratios. These isotopic signatures are theoretically predicted for the ejecta from ONe novae and cannot be matched by any other stellar sources. Previous studies of presolar grains from primitive meteorites have shown that the vast majority formed in red giant outflows and supernova ejecta. Although a classical nova origin was suggested for a few presolar graphite grains on the basis of Ne enrichments, this identification is somewhat ambiguous since it is based only on one trace element. Our present study presents the first evidence for nova grains on the basis of major element isotopic compositions of single grains. We also present the results 1Present address: The Scripps Research Institute, La Jolla, CA 92037, USA; [email protected] 2Present address: Laboratory for Extraterrestrial Physics, Code 691, NASA Goddard Spaceflight Center, Greenbelt MD 20771, USA; [email protected] 3Departament de Fı́sica i Enginyeria Nuclear, Universitat Politècnica de Catalunya, E-08800 Barcelona, Spain; [email protected]