行星形成的确定性模型V.冰线附近的聚集
Toward a Deterministic Model of Planetary Formation V. Accumulation Near the Ice Line
摘要：
我们解决两个悬而未决的问题，在连续吸积的情况下，气体巨行星形成，保留尘埃颗粒存在的气体阻力和核心，尽管I型迁移。这些过程的效率由磁盘结构决定。理论模型表明，行星形成于原恒星盘区域，在中平面附近有一个不活跃的中性“死区”，被部分电离的表面层夹在一起，磁旋转不稳定性活跃。由于尘埃颗粒丰度的转变，活动层的厚度在冰线附近突然减小。在一个范围内的适度的吸积率（$\sim 10^{-9}-10^{-8} M_\odot$年$^{-1}$），角动量转移率的变化导致当地的表面密度和压力分布最大值附近的冰线。方位角速度变为超开普勒，并且颗粒在该过渡区中积聚。这个屏障局部保留原行星核心，并将重元素表面密度提高到启动有效气体吸积所需的临界值。它导致了一个气体巨星形成的首选地点和时代。我们模拟和再现了观测到的频率和质量周期分布的气体巨星周围的太阳型恒星，而不必大大降低I型迁移强度。短周期行星的质量函数可以用来校准I型迁移的效率，并外推具有可居住类地行星的恒星比例。
英文摘要：
We address two outstanding issues in the sequential accretion scenario for gas giant planet formation, the retention of dust grains in the presence of gas drag and that of cores despite type I migration. The efficiency of these processes is determined by the disk structure. Theoretical models suggest that planets form in protostellar disk regions with an inactive neutral ``dead zone'' near the mid plane, sandwiched together by partially ionized surface layers where magnetorotational instability is active. Due to a transition in the abundance of dust grains, the active layer's thickness decreases abruptly near the ice line. Over a range of modest accretion rates ($\sim 10^{-9}-10^{-8} M_\odot$ yr$^{-1}$), the change in the angular momentum transfer rate leads to local surface density and pressure distribution maxima near the ice line. The azimuthal velocity becomes super-Keplerian and the grains accumulate in this transition zone. This barrier locally retains protoplanetary cores and enhances the heavy element surface density to the critical value needed to initiate efficient gas accretion. It leads to a preferred location and epoch of gas giant formation. We simulate and reproduce the observed frequency and mass-period distribution of gas giants around solar type stars without having to greatly reduce the type I migration strength. The mass function of the short-period planets can be utilized to calibrate the efficiency of type I migration and to extrapolate the fraction of stars with habitable terrestrial planets.
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