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The Astrophysical Journal


The American Astronomical Society


We suggest that the large 26Al/27Al ϭ 5 ϫ 10Ϫ5 abundance ratio found in calcium-aluminum–rich inclusions (CAIs) in meteorites is produced by energetic particle irradiation in the early solar system but only in a thin (0.2 g cmϪ2) skin of the solar preplanetary disk that stops the energetic particles. Buildup of that 26Al concentration happens only during the quiescent, or passive, phase of the solar disk, after accretion and associated turbulence has ceased. We propose that CAIs also originate in the form of fine Al-rich dust within a coronal-type environment atop the disk. In this model only the CAIs among planetary materials would contain this high 26Al/27Al ratio. Chronological ordering based on 26Al content within planetary materials would be invalid in this model, which would allow chondrules of ordinary chondrites to be as old as CAIs despite their lack of 26Al. This temporal reordering could resolve a growing crisis in planetary disk physics. We also outline other problems in planetary history that will be alleviated or reinterpreted if our model is correct. We describe four sources for the energetic particles, noting that for each the power requirements are reduced to credible values by the smallness of the irradiated mass, about 10Ϫ5 MJ of disk matter.