Hiroaki Kaneko

Among the several candidate models for chondrule formation, the lighting model has been recognized to be less likely than the other two major models, shock-wave heating and planetesimal collision. It might be because we have believed that the lightning model predicts cooling rates of chondrules that are too fast to reproduce their textures with the assumption that the discharge channels must be optically thin. However, the previous works revealed that the buildup of a strong electric field to generate the lightning in protoplanetary disks requires the enhancement of the solid density. Moreover, some properties of chondrules indicate their formation in environments with such a high solid density. Therefore, the discharge channels may be optically thick, and the lightning model can potentially predict the proper cooling rates of chondrules. In this study, we reinvestigate the cooling rates of chondrules produced by the lightning in the solid-rich environments considering the radiative transfer and the expansion of the hot channel. Chondrules must interact dynamically with the surrounding gas and dust via the drag force. We consider two limiting cases for the dynamics of chondrules: the drag force is ignored in the first case, and chondrules are completely coupled with their surroundings in the second case. In both cases, the lightning model predicts the proper cooling rates of chondrules under the optically thick conditions with high solid enhancement. Therefore, the lightning model is worth further investigation to judge its reliability as the source of chondrule formation.

Fine-grained rims (FGRs) are frequently found around chondrules in primitive chondrites. The remarkable feature of FGRs is their submicron-sized and nonporous nature. The typical thickness of FGRs around chondrules is 10–100 μm. Recently, a novel idea was proposed for the origin of FGRs: high-speed collisions between chondrules and fine dust grains called the kinetic dust aggregation process. Experimental studies revealed that (sub)micron-sized ceramic particles can stick to a ceramic substrate in a vacuum when the impact velocity is approximately in the range of 0.1–1 km s⁻¹. In this study, we examine the possibility of FGR formation via kinetic dust aggregation in chondrule-forming shock waves. When shock waves are created by undifferentiated icy planetesimals, fine dust grains would be released from the planetary surface due to the evaporation of icy planetesimals. We consider the dynamics of chondrules behind the shock front and calculate the growth of FGRs via kinetic dust aggregation based on simple one-dimensional calculations. We found that nonporous FGRs with a thickness of 10–100 μm would be formed in shock waves around evaporating icy planetesimals.