10:15 AM - 10:30 AM
[PPS07-10] Requirement of turbulent nebula and dust grains for grain size coarsening in chondrule rims
Keywords:chondrule, protoplanetary disk, fine-grained rims
Chondrules are mm size objects in primitive chondrites. They are coated with fine-grained dust rim (FGRs). The origin of FGRs is still controversial. In a turbulent nebula, free floating chondrules can acquire FGRs through sweeping up dust grains. (nebula origin scenario)
Xiang et al. (2019) examined the internal structures of FGRs formed in such a turbulent nebula. They prepared two different dust populations; monomer grains or dust aggregates. When monomer grains accreted on chondrules, FGRs showed grain size coarsening from the bottom to the top. On the other hand, when aggregates accreted on chondrules, FGRs showed uniform structures. However, Xiang et al. (2019) did not consider dust aggregation and, in what kinds of nebular conditions monomer grains or dust aggregates accrete to form FGRs.
We simulate both aggregation of polydisperse monomer grains and their accretion on chondrules. Aggregate growth can be divided into two stages, monomer-aggregation stage and BCCA-like stage. In the monomer-aggregation stage, smaller monomer grains are incorporated into aggregates faster than larger ones. Most of the monomer grains larger than the average size of aggregates are isolated. In the BCCA-like stage, the evolution of aggregates is similar to the fractal evolution of single size monomer grains. We obtain the requirement for monomer grains to accrete on chondrules. When monomer grains population follows the same power-law size distribution as the Inter Stellar Medium (ISM), the maximum grain size of >1 μm is widely required in a moderately turbulent nebula (α<10-3), while if turbulent intensity is very weak (α=10-5), ~ 10 μm is required. Moreover, size distributions which weigh larger grains more than ISM may be preferable for effective grain size coarsening.
Xiang et al. (2019) examined the internal structures of FGRs formed in such a turbulent nebula. They prepared two different dust populations; monomer grains or dust aggregates. When monomer grains accreted on chondrules, FGRs showed grain size coarsening from the bottom to the top. On the other hand, when aggregates accreted on chondrules, FGRs showed uniform structures. However, Xiang et al. (2019) did not consider dust aggregation and, in what kinds of nebular conditions monomer grains or dust aggregates accrete to form FGRs.
We simulate both aggregation of polydisperse monomer grains and their accretion on chondrules. Aggregate growth can be divided into two stages, monomer-aggregation stage and BCCA-like stage. In the monomer-aggregation stage, smaller monomer grains are incorporated into aggregates faster than larger ones. Most of the monomer grains larger than the average size of aggregates are isolated. In the BCCA-like stage, the evolution of aggregates is similar to the fractal evolution of single size monomer grains. We obtain the requirement for monomer grains to accrete on chondrules. When monomer grains population follows the same power-law size distribution as the Inter Stellar Medium (ISM), the maximum grain size of >1 μm is widely required in a moderately turbulent nebula (α<10-3), while if turbulent intensity is very weak (α=10-5), ~ 10 μm is required. Moreover, size distributions which weigh larger grains more than ISM may be preferable for effective grain size coarsening.