Chondrule fine-grained mantle formation by hypervelocity impact of chondrules with a dusty gas

At some stage during the chondrule (and refractory inclusion) formation process, many of these objects accreted fine-grained mantles of dust. Observations suggest that the mantle thickness is directly proportional to the chondrule-core radius. Following the proposal of H. C. Connolly, Jc, and S. G. Love (1998, Science 280, 62-67), we demonstrate numerically how this effect can be produced by the hypersonic interaction between a chondrule and a mixture of gas and dust. This result is of relevance to the shock and jet models of chondrule formation, and places limits on both models. In particular, we use this result to constrain our version of the jet theory of chondrule formation, where chondrules are formed at the base of a bipolar solar jet and then ejected to the outer regions of a magnetically confined solar nebula, where they impact at hypersonic speeds into the nebula. We find that the observed linear correlation between mantle thickness and chondrule-core radius requires a dust-to-gas mass ratio of approximately 0.5-1.0, provided that the dust-chondrule sticking coefficient, Q, was in the range 0.5-1.0. We suggest that the settling of dust ejected from the jet could produce such high ratios in the inner regions of the nebula. Another constraint on the chondrule formation process is the observed structure of fine-grained rims around igneous rims, but not the other way around. We argue that this observation can be readily explained by the jet model, but poses a challenge for the shock model. As a consequence of this study, we show that the standard drag coefficient for a sphere moving through rarefied gas is approximately 70% of the physically correct value. We also derive a simple form for the drag coefficient which describes the interaction between dust grains and a macroscopic sphere. (C) 2000 Academic Press.