A TRANSMISSION ELECTRON MICROSCOPY STUDY OF PRESOLAR HIBONITE

作者：Zega Thomas J^*; Alexander Conel M O'D; Nittler Larry R; Stroud Rhonda M

来源：Astrophysical Journal, 2011, 730(2): 83.

DOI：10.1088/0004-637X/730/2/83

• circumstellar matter
• stars: AGB and post-AGB
• stars: evolution
• stars: formation
• stars: fundamental parameters
• supernovae: general

We report isotopic and microstructural data on five presolar hibonite grains (KH1, KH2, KH6, KH15, and KH21) identified in an acid residue of the Krymka LL3.1 ordinary chondrite. Isotopic measurements by secondary ion mass spectrometry (SIMS) verified a presolar circumstellar origin for the grains. Transmission electron microscopy (TEM) examination of the crystal structure and chemistry of the grains was enabled by in situ sectioning and lift-out with a focused-ion-beam scanning-electron microscope (FIB-SEM). Comparisons of isotopic compositions with models indicate that four of the five grains formed in low-mass stars that evolved through the red giant/asymptotic giant branches (RGBs/AGBs), whereas one grain formed in the ejecta of a Type II supernova. Selected-area electron-diffraction patterns show that all grains are single crystals of hibonite. Some grains contain minor structural perturbations (stacking faults) and small spreads in orientation that can be attributed to a combination of growth defects and mechanical processing by grain-grain collisions. The similar structure of the supernova grain to those from RGB/AGB stars indicates a similarity in the formation conditions. Radiation damage (e.g., point defects), if present, occurs below our detection limit. Of the five grains we studied, only one has the pure hibonite composition of CaAl12O19. All others contain minor amounts of Mg, Si, Ti, and Fe. The microstructural data are generally consistent with theoretical predictions, which constrain the circumstellar condensation temperature to a range of 1480-1743 K, assuming a corresponding total gas pressure between 1 x 10(-6) and 1 x 10(-3) atm. The TEM data were also used to develop a calibration for SIMS determination of Ti contents in oxide grains. Grains with extreme O-18 depletions, indicating deep mixing has occurred in their parent AGB stars, are slightly Ti enriched compared with grains from stars without deep mixing, most likely reflecting differences in grain condensation conditions.

• 出版日期2011-4-1