Dependence of the initial internal structure of chondrule rim on dust size distribution

TL;DR

This study models dust aggregation processes in protoplanetary nebulae to understand how initial chondrule rim structures depend on dust size distribution, revealing conditions for grain size coarsening consistent with meteoritic observations.

Contribution

It introduces a detailed dust aggregation model considering polydisperse grains and identifies conditions for monomer accretion and grain size coarsening on chondrules.

Findings

Maximum grain size > 1 μm needed for monomer accretion at typical turbulence levels.

Extremely weak turbulence (α < 10^{-5}) allows larger grains (~10 μm) to facilitate coarsening.

Larger mass fractions of big grains in size distribution promote effective grain size coarsening.

Abstract

Coarse objects in chondrites such as chondrules and CAIs are mostly coated with fine-grained rims (FGRs). FGRs can be formed on the surface of free floating chondrules in a turbulent nebula, where dust aggregation also occurs. A former study has reported that the morphology of the dust populations accreting onto chondrules affects the initial structures of FGRs. It was revealed that, if monomer grains accrete onto chondrules, the smaller grains tend to accumulate near the surface of chondrules, and FGRs exhibit grain size coarsening from the bottom to the top. However, the study did not consider the effect of temporal growth of dust aggregates on FGRs formation. In this study, we calculate the aggregation of polydisperse monomer grains and their accretion onto chondrules. The following two different stages of dust aggregation can be identified: the monomer-aggregation stage and the BCCA-like stage. In the monomer-aggregation stage, monomer grains are incorporated into aggregates when the average aggregate size reaches the size of the monomer. In the BCCA-like stage, aggregates evolve fractally in a fashion similar to that of single size monomer grains. Based on the results of the previous study, we obtain the requisite conditions for chondrules to acquire monomer-accreting FGRs with grain size coarsening observed in some chondrites. In the case of similar size distribution as that of Inter Stellar Medium (ISM), the maximum grain size of $>$ $1$ $\mu$m is widely ($\alpha$ $<$ $10^{-3}$) required for monomer accretion, while if turbulent intensity in a nebula is extremely weak ($\alpha$ $<$ $10^{-5}$), a maximum grain size $\sim$ $10$ $\mu$m is required. The monomer size distributions having larger mass fraction in the large grains compared to ISM might be necessary for the effective grain size coarsening.