Oxygen Isotopic Compositions of Chondrules as Probes of Solar Protoplanetary Disk Formation

Original Abstract

Chondrules are thought to have formed during transient flash-heating events in dust-enriched regions of the solar protoplanetary disk. Although laboratory studies have characterized the oxygen isotopic compositions of chondritic materials, quantitative interpretations based on simulations of disk formation and evolution remain limited. Here, we perform one-dimensional simulations of disk formation and evolution by solving a diffusion--advection equation with mass infall from the parental cloud core. We compute the temporal evolution of oxygen isotopic compositions using an experimentally derived isotope-exchange model. We examine how the oxygen isotopic signatures of the disk depend on the radial distribution of infalling material and the composition of the parental cloud core. We find that the oxygen isotopic compositions of carbonaceous-chondrite chondrules can be reproduced if either (i) the radial extent of mass infall onto the disk is moderate ($\sim 10~{\rm au}$), or (ii) it is large ($> 10~{\rm au}$) and the parental cloud core was ice-depleted and/or experienced weaker CO self-shielding than is commonly assumed. We further suggest the scenario that the observed bimodal trends in oxygen isotopic composition and redox state reflect the partial escape of H$_{2}$O vapor from chondrule-forming regions during heating. In contrast, if ordinary-chondrite chondrules formed inside the snow line under background temperatures of $\lesssim 500~{\rm K}$, their oxygen isotopic compositions may be difficult to explain within the present disk-evolution model, because oxygen isotopic exchange between silicates and vapor species proceeds efficiently only in the inner disk at $T \gtrsim 500$--$600~{\rm K}$.