双温吸积盘冕的局域模型。II. 离子热传导与盘蒸发缺失
Local models of two-temperature accretion disc coronae. II. Ion thermal conduction and the absence of disc evaporation
AI总结：
本文通过局部分层剪切盒模拟研究双温吸积盘冕耦合，发现离子热传导不会导致盘蒸发，净垂直磁通量的不确定性才是决定盘是否蒸发为辐射低效吸积流的关键因素。
摘要：
我们使用带有沿磁场方向热传导的局部分层剪切盒模拟，研究一个理想化模型，该模型描述冷的、辐射高效的吸积盘与其上方热的两温度冕之间的耦合。热冕通过热传导导致冷盘蒸发，已被提出作为调节X射线双星系统中观测到的软态到硬态跃迁的一种机制。我们将局域盘块中的冕等离子体建模为MHD流体，该流体同时受到自由流离子传导和一个参数化冷却函数的作用，该冷却函数刻画了从热离子到较冷、快速冷却的轻子的碰撞性能量转移。在我们所有的模型中，无论穿过盘的初始净垂直磁通量（NF）如何，我们都没有发现盘蒸发的证据。进入盘的离子热流在传导能够加热盘表面层之前就被辐射掉了。当存在初始NF时，我们模型冕中的稳态温度、密度和外流速度不受传导影响。热传导非但没有促进盘蒸发，反而更可能为盘提供从冕中凝结出来的等离子体，尤其是在没有NF的流动中。我们的工作表明，与热传导相比，穿过盘的NF量的不确定性对盘是否会蒸发成辐射低效吸积流具有更大的影响。我们推测，净磁通量的变化介导了盘的截断/蒸发。
英文摘要：
We use local stratified shearing-box simulations with magnetic field-aligned thermal conduction to study an idealized model of the coupling between a cold, radiatively efficient accretion disc, and an overlying, hot, two-temperature corona. Evaporation of a cold disc by conduction from the hot corona has been proposed as a means of mediating the soft-to-hard state transitions observed in X-ray binary systems. We model the coronal plasma in our local disc patch as an MHD fluid subject to both free-streaming ion conduction and a parameterized cooling function that captures the collisional transfer of energy from hot ions to colder, rapidly cooling leptons. In all of our models, independent of the initial net vertical magnetic flux (NF) threading the disc, we find no evidence of disc evaporation. The ion heat flux into the disc is radiated away before conduction can heat the disc's surface layers. When an initial NF is present, steady-state temperature, density, and outflow velocities in our model coronae are unaffected by conduction. Instead of facilitating disc evaporation, thermal conduction is more likely to feed the disc with plasma condensing out of the corona, particularly in flows without NF. Our work indicates that uncertainties in the amount of NF threading the disc hold far greater influence over whether or not the disc will evaporate into a radiatively inefficient accretion flow compared to thermal conduction. We speculate that a change in net flux mediates disc truncation/evaporation.
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