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The Hottest Mineral in the Solar System and Irradiation from the Proto-Sun

Wednesday, August 17, 2011
Ming-Chang
Liu (Applicant)

The formation of the Solar System has long fascinated astronomers and planetary scientists alike. Although we cannot travel back 4.5 Gyr to observe how did our own planetary system form, highly sophisticated instrumentation has enabled astronomers to gain a lot of insights into the details of the collapse of molecular cloud cores as the first step of forming new planetary systems. However, direct analysis of samples in terrestrial labs should yield superior data that are impossible to obtain by remote sensing. Therefore, samples of primitive meteorites so far provide the most critical information on the earliest history of Solar System formation. 

Infrared observations have revealed the existence of dust in young disks. Results from Chandra X-ray space telescope also suggest that young stars are associated with strong flare activities, and thus are capable of releasing high flux of energetic particles. To understand the timing of conversion from presolar dust to solar dust in the solar disk, one could use meteorites, as they witnessed the formation of the Solar System. Meteorites consist of less refractory components (T < 1400K), and higher temperature solids (1450K < T < 1750K). Among high temperature phases, hibonite (CaAl12O19) is one of the oldest solids in the Solar System, forming at the temperatures of ~1750K as either direct condensates from the nebula gas, or residues after heating of low temperature material. Two groups of hibonites could be distinguished based on the mineralogy and fossil record of 26Al, a short-lived radionuclide with a half-life of 0.72 Myr: Spinel-HIBonite spherules (SHIBs) contain 26Al/27Al ranging from 1×10-5 to 7×10-5, and PLAty-hibonite Crystals (PLACs) are characterized by very low apparent 26Al/27Al (<<1×10-5). Interestingly, the inferred 26Al abundances are found to be anti-correlated with the magnitude of enrichments or depletions of a neutron-rich isotope, 50Ti. Large d50Ti variations (~30%) are exclusive to PLACs, and SHIBs carry much smaller anomalies (<1%). Such observations suggest that PLACs should have formed in a highly heterogeneous and 26Al-free disk, probably at the very beginning of the Solar System. On the other hand, the relatively homogeneous d50Ti but variable 26Al/27Al in SHIBs indicate that SHIB formation occurred after PLACs, and was contemporaneous with homogenization of 26Al inside the disk, which was delivered from an external source. That PLACs carry fossil records of 10Be suggests that the proto-Sun had become a powerful energetic particle source before the arrival of 26Al. All these data indicate that conversion of presolar dust to solar dust should have started as early as Class I stage.

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