Thermal processing of chondrule precursors in planetesimal bow shocks
Thermal processing of chondrule precursors in planetesimal bow shocks
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星子弓激波中球粒前体的热处理
DOI：
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发表时间：
1998
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通讯作者：
L. Hood
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文献类型：
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作者：
L. Hood
摘要：我们检验了这样的假设：球粒（以及 B 型和 C 型富含钙铝的包裹体，CAI）起源于前体通过相对于星云气体进行超音速移动的星子上游的弓形激波。采用二维分段抛物线法 (PPM) 水电编码，并辅以一维绝热激波模型，用于模拟给定星子尺寸、速度以及环境星云密度和温度的激波后气体密度、温度和速度场。通过气体颗粒能量和动量传递的一维方程的积分，在自由分子流近似中计算入射硅酸盐颗粒的热历史。对于气体数密度 >1014 cm−3，在冲击气体厚度为 25-35 km 的过程中，4 至 5 范围内的马赫数足以熔化半径在 0.05 至 0.5 mm 范围内的孤立球形颗粒。最小气体星子相对速度在 5.5-7 km/s 范围内，这意味着轨道偏心率 >0.2 和/或倾角 >15°。厘米级 CAI 前驱体的熔化需要更高的马赫数 (6-7) 或环境气体密度 >1015 cm−3。对于星子轨道偏心率和倾角的恒定径向分布，该模型预测在距太阳的径向距离减小（此时星子速度最大）时，前体粒子的熔化效率更高。为了处理星云中相当大一部分的固体，在球粒形成时期接近~2.5 AU的星子必须具有与目前在残余小行星带中观察到的相当的偏心率和倾角范围。维持必要的气体-星子相对速度的最可能的能源是与形成的外行星（主要是木星）相关的外部引力扰动。
Abstract— We test the hypothesis that chondrules (and Type B and C calcium‐aluminum‐rich inclusions, CAIs) originated during passage of precursors through bow shocks upstream of planetesimals moving supersonically relative to nebula gas. A two‐dimensional piecewise parabolic method (PPM) hydrocode, supplemented by a one‐dimensional adiabatic shock model, is employed to simulate the postshock gas density, temperature, and velocity fields for given planetesimal sizes, velocities, and ambient nebular densities and temperatures. Thermal histories of incident silicate particles are calculated in the free molecular flow approximation by integration of the one‐dimensional equations of gas‐grain energy and momentum transfer. For gas number densities >1014 cm−3, Mach numbers in the range of 4 to 5 are sufficient to melt isolated spherical particles with radii in the range 0.05 to 0.5 mm during passage of shocked gas thicknesses of 25–35 km. Minimum gas‐planetesimal relative velocities are in the range 5.5–7 km/s, implying orbital eccentricities >0.2 and/or inclinations >15°. Melting of centimeter‐sized CAI precursors requires either higher Mach numbers (6–7) or ambient gas densities >1015 cm−3. For a constant radial distribution of planetesimal orbital eccentricities and inclinations, the model predicts more efficient melting of precursor particles at decreasing radial distances from the Sun where planetesimal velocities are largest. In order to process a significant fraction of solids in the nebula, planetesimals near ∼2.5 AU during the chondrule formation epoch must have had a range of eccentricities and inclinations comparable to those presently observed in the residual asteroid belt. The most likely energy source for maintaining the necessary gas‐planetesimal relative velocities is external gravitational perturbations associated with the forming outer planets, primarily Jupiter.