Speaker
Ernst Zinner
(Washington University)
Description
Primitive meteorites contain graphite spherules whose anomalous isotopic compositions indicate a stellar
origin [1]. Because the isolation of presolar graphite grains is difficult, they have been less well studied than
presolar SiC and presolar oxide grains. It has been known that the isotopic compositions of presolar graphite
grains depends on their density, but detailed isotopic measurements have been made only on low-density (<2
g.cm-3) individual grains [2]. The NanoSIMS ion microprobe has enabled us to measure O and Si isotopic
ratios in graphite grains with a range of densities from the carbonaceous chondrites Murchison and Orgueil
[3-5]. These measurements confirm that low-density grains originated from supernovae and indicate that
most high-density (>2 g.cm-3) grains come from C-rich AGB stars of low metallicity. Low-density grains and
a few grains of higher density are characterized by large 18O and 28Si excesses that are signatures of Type II
supernovae. Many high-density grains have high 12C/13C ratios (>300) and large excesses in 30Si and
smaller ones in 29Si. These are best explained by low-metallicity AGB stars. In these stars the enrichments of
the envelope in the heavy Si isotopes and in 12C, products of nucleosynthesis in the He shell, are much larger
than those expected for solar-metallicity parent stars. The high 12C/13C ratios imply also high C/O ratios,
which cause the preferential condensation of graphite grains over SiC grains. The Ne-E(L) component, almost
pure 22Ne, which is characteristic of presolar graphite and led to its discovery [6], apparently has two sources
[7]. In SN grains it is due to the decay of short-lived (T1/2 = 2.6 yr) 22Na, which condenses into the grains,
whereas in AGB grains it is due to the abundant 22Ne in the He shell, produced by α-captures on 14N, the
result of previous H burning in the CNO cycle.
[1] Hoppe P. et al. (1995) Geochim. Cosmochim. Acta 59, 4029-4056.
[2] Travaglio C. et al. (1999) Astrophys. J. 510, 325-354.
[3] Amari S. et al. (2004) Meteorit. Planet. Sci. 39, A13.
[4] Amari S. et al. (2005) Meteorit. Planet. Sci. 40, A15.
[5] Jadhav M. et al. (2006) New Astron. Rev. submitted.
[6] Amari S. et al. (1990) Nature 345, 238-240.
[7] Amari S. (2006) New Astron. Rev. submitted.
Author
Ernst Zinner
(Washington University)
Co-authors
Ms
Manavi Jadhav
(Washington University)
Dr
Sachiko Amari
(Washington University)
