Laboratory Analysis of Presolar Silicate Stardust: Constraints on Nucleosynthesis and Dust Formation and Processing

Details Laboratory Analysis of Presolar Silicate Stardust: Constraints on Nucleosynthesis and Dust Formation and Processing Laboratory Analysis of Presolar Silicate Stardust: Constraints on Nucleosynthesis and Dust Formation and Processing

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0605n&link_type=abstract

American Astronomical Society, AAS Meeting #212, #6.05; Bulletin of the American Astronomical Society, Vol. 40, p.196

Astronomy and Astrophysics

Astrophysics

1

Scientific paper

Silicates are the most abundant dust species observed around the atmospheres of oxygen-rich red giant stars. These condensates have recently been identified in interplanetary dust particles (IDPs), primitive meteorites, and Antarctic micrometeorites. Their discovery allows for laboratory analysis, on the nanometer scale, of a representative component of stellar outflows.
We have identified >100; presolar silicate grains in meteorites and IDPs. The oxygen and silicon isotopic compositions of these silicates were measured with the Carnegie NanoSIMS 50L ion microprobe. These compositions constrain the grains’ parent stellar sources, and provide new constraints on astrophysical models. The majority of presolar silicates originate around RG and AGB stars. The parent stars of some silicates have undergone a deep mixing process involving H-burning regions. This process explains several astronomical observations. Theoretical modeling of this deep mixing predicts O isotopic ratios in agreement with the measured compositions of presolar silicates and oxides.
The chemical compositions and mineralogies of the grains reflect the physical conditions and compositions of stellar atmospheres during grain condensation. We have applied two complementary techniques. In focused ion beam transmission electron microscopy (FIB-TEM), electron transparent sections of circumstellar grains are extracted in situ to obtain structural and chemical information. A crystalline forsterite has been identified, as well as amorphous silicates. Auger microscopy is the newest technique used to obtain quantitative chemical compositions. Most of the 76 silicate grains analyzed have nonstoichiometric compositions. About 60% of the grains have Mg/Fe 1, 20% are Mg-rich, and 20% are Fe-rich. Surprisingly, astronomical spectra indicate the vast majority of circumstellar condensates are Mg-rich silicates. The high Fe content of presolar silicates could indicate condensation under non-equilibrium conditions, or secondary processing of the grains. The Auger microprobe can also produce elemental maps. Clearly, complementary laboratory analyses of circumstellar silicates provide a novel method to study astrophysics.

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