Extreme hydrodynamic losses of Earth-like atmospheres in the habitable zones of very active stars
Extreme hydrodynamic losses of Earth-like atmospheres in the habitable zones of very active stars
复制标题
非常活跃恒星的宜居带中类地大气的极端流体动力损失
DOI：
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发表时间：
2019
影响因子：
6.5
通讯作者：
M. Güdel
中科院分区：
文献类型：
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作者：
C. Johnstone;M. Khodachenko;T. Lüftinger;K. Kislyakova;H. Lammer;M. Güdel
目标。在这封信中，我们第一次计算了类地大气的跨音速流体动力逃逸。我们考虑的情况下，地球质量的行星与大气成分相同，目前的地球轨道在1 Au周围的年轻和非常活跃的太阳质量的星星。
方法.为了模拟高层大气，我们使用了Kompot代码，这是一个计算行星高层大气物理结构的第一原理模型，考虑到流体动力学以及在行星高层大气中发生的主要化学和热过程。这个模型使我们能够计算一维垂直结构的大气作为输入的高能量光谱的年轻和活跃的太阳。
结果大气具有跨音速流体动力学帕克风的形式，其在我们的计算域的上边界处具有超过逃逸速度的流出速度。流出的气体主要由原子氮和氧以及它们的离子当量组成，并且具有20%的最大电离分数。质量流出速率为1.8 × 109 g s−1，这将在不到100万年的时间内侵蚀现代地球的大气层。
结论.这种极端的质量损失率表明，当行星在一颗非常活跃的星星的宜居带内运行时，不可能形成类似地球的大气层。相反，这样的大气层只能在星星的活动降低到低得多的水平后才能形成。这发生在地球早期的大气层中，可能是由其他气体如二氧化碳主导的。由于一颗星星的活动衰减所需的时间与其质量高度相关，这对于理解围绕低质量恒星运行的行星可能形成的时间尺度非常重要。
Aims. In this Letter, we calculate for the first time the full transonic hydrodynamic escape of an Earth-like atmosphere. We consider the case of an Earth-mass planet with an atmospheric composition identical to that of the current Earth orbiting at 1 AU around a young and very active solar mass star.
Methods. To model the upper atmosphere, we used the Kompot Code, which is a first-principles model that calculates the physical structures of the upper atmospheres of planets, taking into account hydrodynamics and the main chemical and thermal processes taking place in the upper atmosphere of a planet. This model enabled us to calculate the 1D vertical structure of the atmosphere using as input the high-energy spectrum of a young and active Sun.
Results. The atmosphere has the form of a transonic hydrodynamic Parker wind, which has an outflow velocity at the upper boundary of our computational domain that exceeds the escape velocity. The outflowing gas is dominated by atomic nitrogen and oxygen and their ion equivalents and has a maximum ionization fraction of 20%. The mass outflow rate is found to be 1.8 × 109 g s−1, which would erode the modern Earth’s atmosphere in less than 0.1 Myr.
Conclusions. This extreme mass loss rate suggests that an Earth-like atmosphere cannot form when the planet is orbiting within the habitable zone of a very active star. Instead, such an atmosphere can only form after the activity of the star has decreased to a much lower level. This happened in the early atmosphere of the Earth, which was likely dominated by other gases such as CO2. Since the time it takes for the activity of a star to decay is highly dependent on its mass, this is important for understanding possible formation timescales for planets orbiting low-mass stars.