Koskinen Tommi T, Aylward Alan D, Miller Steve
Atmospheric Physics Laboratory, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
Nature. 2007 Dec 6;450(7171):845-8. doi: 10.1038/nature06378.
Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically. Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere. Jupiter's atmosphere is stable, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere-ionosphere model and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.
对行星HD209458b的最新观测表明，它被一层不断膨胀的氢原子大气所包围，且该大气正在以流体动力学方式逸出。从理论上来说，已经证明这种逸出至少在0.1天文单位的轨道范围内是可能的（参考文献4和5），并且H₃⁺离子在冷却高层大气中起着关键作用。木星的大气是稳定的，所以在5到0.1天文单位之间，必定存在一个从稳定到不稳定的转变点。在此我们表明，对于围绕太阳型恒星运行的类木行星，在0.14到0.16天文单位之间，大气稳定性会出现急剧的破坏。这些结果与早期的模型不同，早期模型认为热层温度要高得多，且离恒星更远的地方蒸发更为显著。（我们使用了一个三维、时间相关的耦合热层 - 电离层模型，并恰当地考虑了H₃⁺离子的冷却作用，这使我们能够对全球的热量重新分布和分子组成变化进行建模。）在0.2到0.16天文单位之间，H₃⁺离子的冷却作用与恒星的加热作用相平衡，但在0.16天文单位以内，分子氢会发生热解离，抑制了H₃⁺的形成，并有效地关闭了这种冷却模式。
