Disk Winds Driven by Magnetorotational Instability and Dispersal of Proto-planetary Disks
Disk Winds Driven by Magnetorotational Instability and Dispersal of Proto-planetary Disks
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T. Suzuki
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作者：
T. Suzuki
通过对分层吸积盘进行局部三维MHD模拟，我们研究了由MHD湍流驱动的盘风。在初始给定的弱垂直磁场中，磁致旋转不稳定性和差动旋转引起的绕组有效地放大了垂直磁场。在磁压与气体压力相当但略小于气压的1.5-2倍尺度高度，大尺度槽流的发展最为有效。这些通道流的破裂通过将坡印亭通量输送到气体中来驱动结构圆盘风。这些特征在各种初始场的模拟中被普遍观察到。这种盘风过程应该在原行星盘的动力蒸发中起到至关重要的作用。沟道流的破裂还激发了与AlfvéNic波和(磁)声波有关的动量通量向中面移动，这可能有助于原行星盘中小尘埃颗粒的沉积。1.磁旋转不稳定性(MRI；Balbus&Hawley 1991)被认为是为吸积盘中角动量的有效向外传输提供湍流的一种强有力的机制。已经在局部剪切箱中进行了MHD模拟(例如，Hawley，Gammie和Balbus 1995；Brandenburg等人)。1995年；Sano et al.2004)，以研究核磁共振驱动的湍流特性。Miller&Stone(2000)研究了具有自由边界的垂直分层局部磁盘，以允许质量和磁场泄漏。虽然他们的主要目的是研究层状圆盘的一般性质，如圆盘日冕而不是圆盘风，但他们得出的结论是，在初始环向和零净垂直磁通磁场的情况下，流出的质量通量很小。另一方面，年轻恒星周围的原行星盘应该具有与其母分子云相连的净垂直磁场。在这种情况下，圆盘表面的物理条件类似于太阳的开放冕洞，在那里太阳风是由磁力线的湍流足点运动驱动的(Sakao等人。2007年；Tsuneta等人。2008年)。显然，盘中由磁共振激发的MHD湍流也有望驱动吸积盘表面的风。尽管这样的盘风机制可能在吸积盘的演化中起到重要作用(Ferreira，Dougados&Cabrit2006)，但由于数值处理的困难，迄今还没有进行定量研究：风过程的长期计算需要准确的…
By performing local three-dimensional MHD simulations of stratified accretion disks, we investigate disk winds driven by MHD turbulence. Initially given weak vertical magnetic fields are effectively amplified by magnetorotational instability and winding due to differential rotation. Large scale channel flows develop most effectively at 1.5-2 times the scale heights where the magnetic pressure is comparable to but slightly smaller than the gas pressure. The breakup of these channel flows drives structured disk winds by transporting the Poynting flux to the gas. These features are universally observed in the simulations of various initial fields. This disk wind process should play an essential role in the dynamical evaporation of proto-planetary disks. The breakup of channel flows also excites the momentum fluxes associated with Alfvénic and (magneto-)sonic waves toward the mid-plane, which possibly contribute to the sedimentation of small dust grains in protoplanetary disks. 1. INTRODUCTION Magnetorotational instability (MRI; Balbus & Hawley 1991) is regarded as a robust mechanism to provide turbulence for an efficient outward transport of angular momentum in accretion disks. MHD simulations in a local shearing box have been carried out (e.g., Hawley, Gammie, & Balbus 1995; Brandenburg et al. 1995; Sano et al. 2004) to study the properties of MRI-driven turbulence. Miller & Stone (2000) studied vertically stratified local disks with free boundaries to allow leaks of mass and magnetic field. While their main purpose is to study general properties of stratified disks such as disk coronae rather than disk winds, they concluded that the mass flux of the outflows is small in the cases of initially toroidal and zero-net vertical flux magnetic fields. On the other hand, protoplanetary disks around young stars should have net vertical magnetic fields that are connected to their parental molecular clouds. In this case, physical conditions of the surface of the disk is analogous to the open coronal holes of the sun where the solar wind is driven by turbulent footpoint motions of the magnetic field lines (Sakao et al. 2007; Tsuneta et al. 2008). Obviously, MHD turbulence excited by MRI in the disk is also expected to drive winds from the surfaces of the accretion disk. Although such a disk wind mechanism may play a significant role in the evolution of accretion disks (Ferreira, Dougados & Cabrit 2006), quantitative studies have not been carried out so far because of difficulties of numerical treatment: a long-term calculation of the wind process requires accurate …