CLUMPY DISK ACCRETION AND CHONDRULE FORMATION

Chondrules are the major constituent of most of the primitive meteorites, yet the mechanism of their formation is largely unknown. Chondrule textures and compositions place tight constraints on the thermal processing necessary to turn precursor dust aggregates into chondrules-temperatures between about 1500 and 2000 K and cooling times of hours are indicated. As suggested by Hood and Horanyi (1991, Icarus 93, 259-269), shock heating within the nebula may be capable of matching the thermal constraints, provided that a suitable source of nebular shocks can be found. We suggest here that the source of the shocks possibly responsible for chondrule formation was episodic accretion onto the solar nebula of low-mass clumps (∼1022 g) of interstellar gas. Three types of astronomical observations are best explained by the existence of opaque clumps that block stellar radiation while moving at high velocities within a few astronomical units of young stellar objects: (a) rapid variations in stellar luminosity, independent of wavelength; (b) rapid changes in surface brightness of nearby reflection nebulae; and (c) rapid variations in spectral lines excited in circumstellar regions. While the origin of these clumps is presently unknown, they might arise from the inhomogeneous infall of the residual molecular cloud core, from the return of matter ejected by the stellar wind, or from a combination of these two effects. Infalling clumps will impact the protoplanetary disk at high velocity, producing a localized, intense source of heat suitable for chondrule formation. As the resulting shock wave propagates into the nebula, preexisting aggregates of dust grains formed in the nebula will be melted by the shock front, producing chondrules. For reasonable estimates of the clump impact rate onto a minimum mass nebula, we find that a nonnegligible fraction of the nebula mass undergoes the required thermal processing. Clumpy disk accretion is a mechanism that is capable of providing a long-lived, late-phase, episodic, and variable intensity source of cyclical thermal processing for chondrule precursor grains. © 1993 by Academic Press, Inc.