Chondrule Formation in Radiative Shock
Chondrule Formation in Radiative Shock
复制标题
辐射冲击中的球粒形成
DOI：
10.1006/icar.1994.1196
复制
发表时间：
1994
期刊：
影响因子：
3.2
通讯作者：
W. Ip
中科院分区：
文献类型：
--
作者：
T. Ruzmaikina;W. Ip
太阳星云激波中固体颗粒熔化的先前模型忽略了激波前沿后面的气体冷却，即，他们考虑了绝热冲击。本文研究了激波后区的气体冷却效应。结果表明，偶极分子和小尘埃粒子有效地冷却了激波后的气体，并导致激波后区域气体密度的急剧增加。亚毫米和更大的颗粒在减速之前穿过冷却区，并且被阻力（通过冷却和压缩的激波后气体）加热，比绝热激波更强烈。这种效应打开了毫米级尘埃聚集体（球粒前体）在较冷区域熔化的可能性，即使当物质对聚集体的热辐射是透明的。推测了在坍缩的日前云中形成球粒前体和在太阳星云形成过程中的辐射吸积激波中熔化球粒前体的可能性。在这样的激波中，如果下落气体的密度仅比对应于10 - 5 M·year-1吸积率的平均下落气体密度大几倍，那么颗粒可以在日心半径处熔化，直到小行星带的内部;熔化的极限半径随密度而增加。落尘气体的离心半径附近的星团吸积和密度增加提供了必要的落尘气体密度。由于激波后气体的快速冷却而导致的拖曳加热的增强对于太阳星云中其他来源的激波也是重要的。在最小质量太阳星云的中心平面，球粒前体可以在速度≥10 km sec-1的冲击中熔化。熔融状态的持续时间和晶粒的冷却速率由晶粒的动能和衰变速率决定（而不是晶粒辐射冷却的时间尺度！）。毫米大小的致密颗粒的温度≥1600 K 3秒，≥700 K 1010 4秒，时间适合球粒。当颗粒动能较大时，这些时间尺度在较快的冲击中可能较大，在日前云坍缩之前和坍缩过程中，球粒前体可能以蓬松的聚集体形式形成。后来他们必须增加他们的密度。讨论了通过相互碰撞或由于表面张力当它们开始熔化时增加聚集体密度的可能性;讨论了通过相互碰撞或由于表面张力当它们开始熔化时增加聚集体密度的可能性。
Abstract Previous models of solid grain melting in solar nebula shocks have neglected gas cooling behind the shock front; i.e., they considered adiabatic shocks. The effect of gas cooling in the postshock region is studied in this article. It was found that shocked gas is cooled efficiently by dipole molecules and small dust particles, and this results in a sharp increase in gas density in the postshock region. Submillimeter and larger grains cross the region of cooling before being decelerated, and are heated by the drag (through the cooled and compressed postshock gas) more strongly than in an adiabatic shock. This effect opens the possibility of melting of millimeter-size dust aggregates (chondrule precursors) in that cooler region, even when the matter is transparent for the thermal radiation of the aggregates. The possibility of formation of chondrule precursors in the collapsing presolar cloud and of melting of the precursors in the radiative accretional shock during formation of the solar nebula is speculated. In such a shock, grains could be melted at heliocentric radii up to the inner part of asteroidal belt if the density of infalling gas were only few times larger than the average density of infalling gas corresponding to an accretion rate of 10 -5 M ⊙ year -1 ; the limiting radius for melting increases with density. Clumpy accretion and enhancement of density in the vicinity of the centrifugal radius of infalling gas provide the necessary densities of infalling gas. Enhancement of drag heating due to fast cooling of postshock gas is also significant for shocks of other origin in the solar nebula. At the central plane of a minimum-mass solar nebula, chondrule precursors could be melted in shocks with velocity ≥10 km sec -1 . The duration of the molten state and the cooling rate of a grain are determined by the grain's kinetic energy and the rate of its decay (not the time scale of radiational cooling for the grain!). Millimeter-size compact grams have temperatures ≥1600 K for 3 sec, and ≥700 K for ∼10 4 sec, times that are appropriate for chondrules. These time scales could be larger in faster shocks when grains have larger kinetic energy, Chondrule precursors could be formed as fluffy aggregates before and during the collapse of presolar cloud. Later they must increase their density. The possibility of increasing the density of the aggregates by means of mutual collisions or as a result of surface tension when they began to melt is discussed possibility of increasing the density of the aggregates by means of mutual collisions or as a result of surface tension when they began to melt is discussed.