Iron and Nickel Isotopic Ratios in Presolar SiC Grains

• Published in: The Astrophysical journal Vol. 689; no. 1; pp. 622 - 645
• Main Authors: Marhas, Kuljeet K, Amari, Sachiko, Gyngard, Frank, Zinner, Ernst, Gallino, Roberto
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10-12-2008; IOP
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; dust, extinction; nuclear reactions, nucleosynthesis, abundances; Supernovae; Nucleosynthesis; stars: AGB and post-AGB; Abundance; Asymptotic giant branch; supernovae: general; Isotope ratio
• DOI: 10.1086/592599
• EISSN: 1538-4357
• ISSN: 0004-637X

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae and 53 SiC grains of other types. The Ni/Fe and Co/Fe ratios in grains of all types are much higher than in the gas from which the grains are believed to have condensed. A majority of the X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in super(57)Fe, super(61)Ni, and super(62)Ni. super(60)Ni excesses are small and the super(54)Fe/ super(56)Fe ratios of almost all X grains are normal. These isotopic compositions are best explained by mixing of material from the He/N zone of Type II supernovae with material from the He/C zone. The lack of any super(54)Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the super(28)Si excesses in X grains, is very rich in super(54)Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The super(57)Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed super(62)Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding super(61)Ni excesses. A Y grain and a Z grain have excesses in super(54)Fe and super(62)Ni, but close to normal super(57)Fe/ super(56)Fe and super(60,61)Ni/ super(58)Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars.