X-wind model

Localized heating nebular shocks and the X-wind model 

Chondrules comprise the major portion of most chondrites, the most abundant type of meteorites. If the achondrites and terrestrial planets formed from chondrite-like precursors, then much, perhaps most of the solid matter in the inner solar system once existed as chondrules. Even if chondrules were restricted to the chondrites, the process that formed them was important in that region. The origin of chondrules is an important unsolved problem in cosmochemistry. Chondrules formed in the Sun s accretion disk through some sort of transient flash-heating event(s). Some CAIs apparently also were melted in the disk. What was the process (or processes) that melted the chondrules and CAIs Whatever it was, it dominated the disk for at least a few million years. 

Sketch of the X-wind model, showing a gap between the protosun and the inner edge of the nebular disk caused by interfering magnetic fields. Materials migrating inward in the disk reach the X-region, where they either accrete onto the star or are ejected outward above the disk (X-wind), only to be accreted back onto the disk farther out. After Shu et al. (1996). 

None of these models has been universally accepted as the mechanism to form chondrules. All of them have their problems. It is possible that we have not yet conceived of the true mechanism that formed chondrules, but whatever it was, it operated pervasively at least in the area where chondrites accreted. 

The shock-wave model may also be applicable to CAIs. Shocks operating under different nebula conditions when the CAIs formed could generate a different temperature profile. Details of this type of model have not been worked out. 

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To summarize, chondrules and CAIs formed by transient heating events that processed a large fraction of the matter in the accretion disk. These heating events appear to overprint the thermal processing that produced the volatile element depletions among chondrites. The exact nature of these events is unknown, although shock waves in the nebula and the X-wind model are currently receiving the most attention. 

Figure 6 The X-wind model for meteoritical processing involves subjecting solids to high temperatures and fluxes of energetic particles from the early Sun during and after being lifted from the midplane of the solar nebula in a bipolar wind, and afterward falling back onto the nebula at greater distances (Shu et al., 2001) (reproduced by permission of University of Chicago Press and American Astronomical Society from Astrophys. J., 2001, 548, 1029-1050).

The X-wind model has also been advanced as a means for thermal processing of chondrules (Shu et al., 1996). While it has not yet been possible to calculate the detailed thermal history of chondrule... 

A second possibility is that CAIs formed elsewhere in the nebula and were subsequently transported to the asteroid belt. In the X-wind model, solids in the disk drifted inwards, emerged from a partially shielded environment and were melted by solar radiation (Shu et al., 1996). Some of these objects were entrained in the wind of material flowing away from the Sun, and millimeter-sized particles would... 

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