太阳星云中的混合：同位素异质性和难熔颗粒大尺度传输的意义
Mixing in the Solar Nebula: Implications for Isotopic Heterogeneity and Large-Scale Transport of Refractory Grains
摘要：
在星尘号航天器从81 P/Wild 2彗星收集的颗粒中发现了难熔颗粒（Brownlee et al. 2006），这为原太阳附近高温区域形成的物质向外大规模迁移到太阳星云的彗星形成区域提供了基本事实。虽然一般湍流粘性驱动的吸积盘模型被用作解释这种大规模传输的一种手段，但这种“α”粘性背后的详细物理学仍然不清楚。在这里，我们提出了一个替代的物理机制，在太阳星云大规模运输：引力扭矩与瞬态螺旋臂在一个轻微的引力不稳定盘，类型似乎是必要的形成气体巨行星。三维模型的时间演化的自引力盘，包括辐射转移和详细的状态方程，表明小尘埃颗粒将被输送上游和下游（相对于平均向内流动的气体和尘埃被吸积的中央原恒星）的磁盘内的时间尺度小于1000年内10 AU。这些模型还表明，存在的任何初始空间异质性（例如，在短寿命的同位素，如26铝）将通过磁盘混合均匀化到约10%的水平，保留使用短寿命的同位素作为准确的星云计时器，同时允许稳定的氧同位素比的传播。这种有限的星云空间异质性似乎与螺旋臂实现的粗混合有关，其径向宽度为1 AU，时间尺度为~1000年。
英文摘要：
The discovery of refractory grains amongst the particles collected from Comet 81P/Wild 2 by the Stardust spacecraft (Brownlee et al. 2006) provides the ground truth for large-scale transport of materials formed in high temperature regions close to the protosun outward to the comet-forming regions of the solar nebula. While accretion disk models driven by a generic turbulent viscosity have been invoked as a means to explain such large-scale transport, the detailed physics behind such an ``alpha'' viscosity remains unclear. We present here an alternative physical mechanism for large-scale transport in the solar nebula: gravitational torques associated with the transient spiral arms in a marginally gravitationally unstable disk, of the type that appears to be necessary to form gas giant planets. Three dimensional models are presented of the time evolution of self-gravitating disks, including radiative transfer and detailed equations of state, showing that small dust grains will be transported upstream and downstream (with respect to the mean inward flow of gas and dust being accreted by the central protostar) inside the disk on time scales of less than 1000 yr inside 10 AU. These models furthermore show that any initial spatial heterogeneities present (e.g., in short-lived isotopes such as 26Al) will be homogenized by disk mixing down to a level of ~10%, preserving the use of short-lived isotopes as accurate nebular chronometers, while simultaneously allowing for the spread of stable oxygen isotope ratios. This finite level of nebular spatial heterogeneity appears to be related to the coarse mixing achieved by spiral arms, with radial widths of order 1 AU, over time scales of ~1000 yrs.
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