棕矮星和太阳系外行星II大气中的电离
Ionisation in atmospheres of Brown Dwarfs and extrasolar planets II Dust-induced collisional ionization
摘要：
观测结果表明，在光谱类别M之后的单一低质量物体中，连续的无线电发射以及零星的H-α和X射线发射都很突出。这些活动的签名被解释为是由电离的大气耦合到恒星磁场。然而，仍然令人困惑的是，这样一个凉爽的大气层能够产生必要水平的电离的机制。在如此低的温度下，热气体过程不足，但云的形成开始。云粒子可以作为电子雪崩的种子在流光电离周围的气体，并可能导致闪电和间接的磁场耦合，一个组合的过程也预计原行星盘。然而，前提是云粒子是带电的。我们使用的结果从漂移凤凰模型大气研究碰撞过程。我们表明，电离的诱导尘埃尘埃碰撞是最有效的动力学过程。尘埃与尘埃的碰撞本身并不足以改善云层内部大气的磁耦合，但是颗粒上或气相中作为分离电子提供的电荷可以触发二次非线性过程。宇宙射线可能会增加全球的电离水平，但如果像棕矮星那样存在强大的大规模磁场，它们的影响就会减弱。我们认为，虽然热气体电离下降的对象在整个完全对流边界，碰撞过程中的尘埃充电可以发挥重要作用，在最低质量的对象。因此，大气尘埃的出现可能与大气中的异常X射线和无线电发射有关，这些大气是冷的，但由于纯热电离而比预期的要多。
英文摘要：
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.
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