Origin of CAIs and of their High 26Al Concentrations

Details Origin of CAIs and of their High 26Al Concentrations Origin of CAIs and of their High 26Al Concentrations

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..499c&link_type=abstract

Meteoritics, vol. 30, no. 5, page 499

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26Al, Calcium-Aluminum-Rich Inclusions, Chondrules, Isotopes, Radionuclides, Extinct, Rays, Cosmic, Solar Nebula

Scientific paper

We propose a new interpretation of the existence of calcium-aluminum-rich inclusions (CAI) and the coincidence that only they appear to have formed from matter bearing a large concentration of 26Al. Our purpose is to unify these coincidences with a new scenario for the formation of CAI. The superrefractory-rich precursor dust is usually assumed to be prepared by thermal evaporation of less refractory elements (esp. Mg, Si and Fe). Instead of seeking an environment at T=1500 C, however, we propose that dust is episodically heated to such temperature by shock-wave heating of the dust in stable surface layers of the solar disk. This occurs in the late passive phase of the disk, when accretion onto the sun has ceased, after the FU Orionis phases, and after the bipolar outflow phases that are fed by that accretion power. At this late passive-disk phase, the sun is in its T Tauri phase and its solar wind sweeps past the surface layers of the disk. This surface wind picks up the gaseous atoms moving upward out of the stable disk surface, leaving the refractory-rich dust behind. Magnetohydrodynamic shocks in the disk surface heat the small dust by its sudden drag (like meteors) through the suddenly fast moving gas. Although hot for only seconds, the Mg, Si, Fe quickly evaporates owing to the small size (submicron) of the dust, which quickly cools by radiating into the vacuum. Because the gas is lost to the stable surface and is therefore not reaccreted, repeated applications of this event leave an Al-rich surface layer of tiny Al-rich dust. The disk surface is a coron-like environment, heated repeatedly by waves that steepen into shocks in the rapidly falling vertical particle density. Gradually over about 1 Myr the same surface shocks collide the Al-rich dust with itself, causing larger aggregates to accumulate. Once mm sized, they sink into the disk and pick up the volatile-element cohort that is also found in the CAI. Some aggregates are later melted (the Type B CAI), probably by the same process within the disk central layers that caused the heating of the chondrules. This finally forms the CAI as we find them in the meteorites. The goemetrical situation is shown in Fig. 1. The surface layers of the disk are continuously bombarded by low-energy cosmic rays which produce 26Al, but only in the surface skin of that disk. Because the disk Is passive (stratified and non-turbulent), that surface skin does not mix with the remainder of the disk. Therefore the 26Al is confined to its skin, which is also the only place where the Al-rich dust is being prepared. The consequence is that only Al-rich solids contain 26Al. Equally primitive (perhaps) chondrules, which form by the heating of normal dust balls in the central plane, do not contain much 26Al because it does not mix from the disk skin downward to the bulk layers of the disk. The cosmic rays causing the 26Al production are accelerated locally, either as solar particles or pickup ions (anomalous cosmic rays) by the solar wind speeding past the neutral disk. If those cosmic rays are mostly tens-of-MeV per nucleon, they penetrate the disk skin to only about 0.2 g cm ^-2. This skin depth is much less than the total surface density near 300 g cm^-2 in the mimimum solar disk. Therefore only about O.l% of the disk (its skin) contains the 26Al produced by those low-energy cosmic rays. This picture distinguishes between the heating events that cause the Al-rich fine dust and the heating events that form the CAIs from aggregates of Al-rich dust and the chondrules from normal dust. It describes the surface of the disk, in effect, as a novel chemical processor of fine dust.

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