来自垂死世界的混沌风：演化大气的一维映射
Chaotic winds from a dying world: a one-dimensional map for evolving atmospheres
AI总结：
针对小体积高温行星蒸发风的无规律变化，构建了基于蒸汽压、大气滞后性及温室与核冬天效应交替的一维时间映射模型，揭示了其质量损失率呈现混沌或周期性爆发的物理机制。
摘要：
体积小于水星且被加热至约2000 K升华温度的行星，会以充满尘埃的蒸发风形式灾难性地损失质量。观测表明，这些风在很大程度上无规律地阵发与消退；Kepler任务的凌星深度在不同轨道间随机变化，变化幅度高达10倍或更多。我们通过构建一个风质量损失率随时间变化的映射，解释了混沌外流是如何产生的。该映射基于三个陈述构建：（1）风质量损失率与表面平衡蒸汽压成正比，随地面温度呈指数上升。（2）由于风逃离行星引力势阱需要有限的时间，任意时刻的表面质量损失率决定了其后某一时刻的风光学深度——即大气具有滞后性。（3）当大气为光学薄时，地面温度随光学深度增加而上升（温室效应）；当大气为光学厚时，地面温度随光学深度增加而下降（核冬天效应）。陈述（3）源于尘埃在强烈恒星辐射下凝结的方式。如近期所讨论，在恒星面前初始裸露的凝结物必定富含硅酸盐而贫铁，它们通过在可见光波段弱吸收和在红外波段强发射来保持足够低的温度以维持凝结。随后，当尘埃颗粒数量多到足以自屏蔽星光时，它们可能吸积更多铁并逆转其可见光至红外的不透明度比率。根据参数的不同，风的映射可以规律地在温室效应与核冬天之间繁荣与萧条交替，或者爆发成混沌状态。Lyapunov时间以轨道周期为单位衡量，这是风在Coriolis力作用下从行星昼侧偏离并离开Hill球所需的时间。
英文摘要：
Planets which are smaller than Mercury and heated to sublimation temperatures of $\sim$2000 K lose mass catastrophically in dusty evaporative winds. The winds are observed to gust and recede largely without pattern; transit depths from the Kepler mission vary randomly from orbit to orbit by up to a factor of 10 or more. We explain how chaotic outflows may arise by constructing a map for the wind mass-loss rate as a function of time. The map is built on three statements: (1) The wind mass-loss rate scales in proportion to the surface equilibrium vapor pressure, rising exponentially with ground temperature. (2) Because the wind takes a finite time to escape the planet's gravity well, the surface mass-loss rate at any time determines the wind optical depth at a later time -- the atmosphere has hysteresis. (3) The ground temperature increases with optical depth (greenhouse effect) when the atmosphere is optically thin, and decreases with optical depth when the atmosphere is optically thick (nuclear winter). Statement (3) follows from how dust condenses in the face of intense stellar irradiation. As discussed recently, condensates initially naked before the star must be silicate-rich and iron-poor, staying cool enough for condensation by absorbing weakly in the visible and emitting strongly in the infrared. Later, when grains are numerous enough to self-shield from starlight, they may accrete more iron and reverse their visible-to-infrared opacity ratio. Depending on parameters, the map for the wind can regularly boom and bust between a greenhouse and a nuclear winter, or erupt into chaos. Lyapunov times are measured in orbital periods, the time for the wind to turn by Coriolis forces away from the planet's dayside, out of the Hill sphere.
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