Gas dynamic heating of chondrule precursor grains in the solar nebula

Conditions under which gas dynamic processes operating within the solar nebula could have been responsible for melting of chondrule precursor grains are investigated. Gas-grain energy and momentum transfer are treated in the free molecular flow approximation, including both drag heating due to grain relative motion and heating due to collisions with gas molecules in thermal motion. The effect of thermally emitted radiation on grain heating and cooling is also considered. It is found that melting of chondrulesized grains is most easily achieved in optically thick, dust-rich zones where radiative cooling of a grain is balanced by thermal radiation from surrounding grains. In this circumstance, grain melting can occur for relatively small gas-grain velocities (4 km sec^-1) and gas temperatures (1500 K). The thermal buffering effect of a dust-rich zone would also assist in increasing the cooling time scale, allowing time for outgassing and collapse of vesicles to be more consistent with observed chondrule properties. It may also help to narrow the range of thermal effects on chondrules as compared to those expected for single grains. Nebular shock fronts are suggested as one means of providing the necessary relative velocities and heated gas regions. Shocks of the needed strength and scale size could be generated by several mechanisms including nonaxisymmetric structures in an evolving nebula and interaction of volatile-rich planetesimals in eccentric orbits with the nebula.