Correlated Nanoscale Characterization of a Unique Complex Oxygen-rich Stardust Grain: Implications for Circumstellar Dust Formation
J. Leitner, P. Hoppe, C. Floss, F. Hillion, T. Henkel
Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 11 May 2017
“We report the light to intermediate-mass element abundances as well as the oxygen, magnesium, silicon, and titanium isotope compositions of a unique and unusually large (0.8 µm × 3.75 µm) presolar O-rich grain from the Krymka LL3.2 chondrite. The O-, Al-, and Ti-isotopic compositions are largely compatible with an origin from an asymptotic giant branch (AGB) star of 1.5 solar masses with a metallicity that is 15% higher than the solar metallicity. The grain has an elevated 17O/16O ratio (8.40 ± 0.16 × 10–4) compared to solar, and slightly sub-solar 18O/16O ratio (1.83 ± 0.03 × 10–3). It shows evidence for the presence of initial 26Al, suggesting formation after the first dredge-up, during one of the early third dredge-up (TDU) episodes. Titanium isotopic data indicate condensation of the grain before significant amounts of material from the He-burning shell were admixed to the stellar surface with progressive TDUs. We observed a small excess in 30Si (δ30Si = 41 ± 5 ‰), which most likely is inherited from the parent star’s initial Si-isotopic composition. For such stars stellar models predict a C/O-ratio <1 even after the onset of TDU, thus allowing the condensation of O-rich dust. The grain is an unusual complex presolar grain, consisting of an Al-Ca-Ti-oxide core, surrounded by an Mg-Ca-silicate mantle, and resembles the condensation sequence for a cooling gas of solar composition at pressures and dust/gas ratios typically observed for circumstellar envelopes around evolved stars. We also report the first observation of phosphorus in a presolar grain, although the origin of the P-bearing phase remains ambiguous."