Chondrule formation and evolution of the early solar system

摘要

Chondrules are millimeter to sub-millimeter size silicate spherules that formed during localized and transient high-temperature events in the early solar system. Although it is not yet understood how chondrules formed, recent studies have provided important clues in understanding the physical conditions of chondrule formation. In this paper I review recent developments in studies of chondrules to provide constraints on high-temperature processes during the evolution of protoplanetary disks, including: (1) the timing and duration of the chondrule-forming high-temperature period, based on long-lived and short-lived chronometers; (2) the thermal history of chondrule melts during each chondrule-forming event, as constrained from laboratory experiments and petro-logical and isotopic studies of chondrules, including shock-wave heating as a plausible heat source for chondrule formation; and (3) sources for shock waves and chondrule-forming regions in the protoplanetary disk. Given our current knowledge, I provide the following possible answers to the above four points: (1) the period of chondrule formation began shortly after the formation of calcium-aluminum-rich inclusions and lasted for a few million years; (2) chondrule precursors were heated at a rate of > 10~4 K/h in the temperature range of ～ 1400-1600 K, and melt droplets were cooled from a peak temperature of ～ 1800-2200 K at a rate of ～ 5-1000 K/h; (3) high-velocity shock waves ( > 20 km/s) in a low-density gas region ( < 10~(19) particles/m~3) may be appropriate for localized transient heating events associated with chondrule formation; and (4) X-ray flares from the young Sun in its T-Tauri stage might be the source of the shock waves.