标题： 棕矮星和系外行星大气中的电离 第二部分 灰尘引起的碰撞电离 显示英文标题

标题： Ionisation in atmospheres of Brown Dwarfs and extrasolar planets II Dust-induced collisional ionization

摘要： 观测表明，在光谱类型M之后的单一、低质量天体中，连续射电辐射以及间歇性的H-α和X射线辐射是显著的。这些活动特征被解释为电离大气与恒星磁场耦合的结果。然而，仍然令人困惑的是，如此低温的大气如何能产生所需的电离水平。在这些低温下，热气体过程不足以产生足够的电离，但云的形成开始发生。云粒子可以作为流注中电子雪崩的种子，从而电离周围气体，并可能导致闪电和间接地导致磁场耦合，这种过程的组合也预期出现在原行星盘中。然而，前提是云粒子带电。我们利用Drift-Phoenix模型大气的结果来研究碰撞过程。我们表明，由湍流引起的尘埃-尘埃碰撞产生的电离是最有效的动能过程。仅尘埃-尘埃碰撞不足以改善云层内部大气的磁场耦合，但通过颗粒或气体相中的分离电子提供的电荷可以触发二次非线性过程。宇宙射线可能增加整体的电离水平，但如果存在强而大尺度的磁场，如棕矮星上的磁场，它们的影响会减弱。我们认为，尽管在跨越完全对流边界的所有天体中热气体电离下降，但通过碰撞过程的尘埃充电可能在最低质量的天体中起重要作用。因此，大气尘埃的出现可能与那些温度较低但比纯热电离所预期的更带电的大气中的异常X射线和射电辐射相关。 显示英文摘要

摘要： Observations have shown that continuous radio emission and also sporadic H-alpha and X-ray emission are prominent in singular, low-mass objects later than spectral class M. These activity signatures are interpreted as being caused by coupling of an ionised atmosphere to the stellar magnetic field. What remains a puzzle, however, is the mechanism by which such a cool atmosphere can produce the necessary level of ionisation. At these low temperatures, thermal gas processes are insufficient, but the formation of clouds sets in. Cloud particles can act as seeds for electron avalanches in streamers that ionise the ambient gas, and can lead to lightning and indirectly to magnetic field coupling, a combination of processes also expected for protoplanetary disks. However, the precondition is that the cloud particles are charged. We use results from Drift-Phoenix model atmospheres to investigate collisional processes. We show that ionisation by turbulence-induced dust-dust collisions is the most efficient kinetic process. Dust-dust collisions alone are not sufficient to improve the magnetic coupling of the atmosphere inside the cloud layers, but the charges supplied either on grains or within the gas phase as separated electrons can trigger secondary non-linear processes. Cosmic rays are likely to increase the global level of ionisation, but their influence decreases if a strong, large scale magnetic field is present as on Brown Dwarfs. We suggest that although thermal gas ionisation declines in objects across the fully-convective boundary, dust charging by collisional processes can play an important role in the lowest mass objects. The onset of atmospheric dust may therefore correlate with the anomalous X-ray and radio emission in atmospheres that are cool, but charged more than expected by pure thermal ionisation.