Two saturated states of the vertical shear instability in protoplanetary disks with vertically varying cooling times
Two saturated states of the vertical shear instability in protoplanetary disks with vertically varying cooling times
复制标题
冷却时间垂直变化的原行星盘垂直剪切不稳定性的两种饱和状态
DOI：
10.1093/pasj/psac107
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发表时间：
2023
影响因子：
2.3
通讯作者：
Ono Tomohiro
中科院分区：
文献类型：
--
作者：
Fukuhara Yuya;Okuzumi Satoshi;Ono Tomohiro
原行星盘中的湍流在尘埃演化和星子形成中起着重要的作用。垂直剪切不稳定性（VSI）是一种能够在盘外区域产生湍流的流体动力学机制。VSI除了垂直剪切之外还需要快速气体冷却。线性稳定性分析表明，VSI可能不会在气体冷却效率低下的中板周围工作。在这项研究中，我们调查的非线性结果的VSI在磁盘与线性VSI稳定的中平面区域。我们进行二维的全球流体动力学模拟的轴对称磁盘与垂直变化的冷却时间。垂直冷却时间曲线确定线性VSI稳定的中平面层和中平面上方和下方的不稳定层的厚度。我们发现，在非线性饱和状态下，中平面稳定层的厚度决定了VSI驱动湍流的垂直结构。我们确定了两种类型的最终饱和状态：（i）T状态，其特征在于垂直湍流运动渗透到VSI稳定的中面层，和（ii）pT状态，其特征在于湍流运动限制在不稳定层。当中平面VSI稳定层厚于两个气体刻度高度时，实现pT状态。我们还发现，当位于中平面上方和下方的VSI不稳定区域薄于两个气体尺度高度时，在所有高度处，VSI驱动的湍流都在很大程度上被抑制。我们提出的经验公式，预测VSI驱动的湍流的强度作为不稳定和稳定层的厚度的函数。这些公式将有助于研究如何VSI驱动的湍流和尘埃颗粒控制磁盘冷却效率同时演变。
Turbulence in protoplanetary disks plays an important role in dust evolution and planetesimal formation. The vertical shear instability (VSI) is one of the candidate hydrodynamic mechanisms that can generate turbulence in the outer disk regions. The VSI requires rapid gas cooling in addition to vertical shear. A linear stability analysis suggests that the VSI may not operate around the midplane where gas cooling is inefficient. In this study, we investigate the nonlinear outcome of the VSI in disks with a linearly VSI-stable midplane region. We perform two-dimensional global hydrodynamical simulations of an axisymmetric disk with vertically varying cooling times. The vertical cooling time profile determines the thicknesses of the linearly VSI-stable midplane layer and unstable layers above and below the midplane. We find that the thickness of the midplane stable layer determines the vertical structure of VSI-driven turbulence in the nonlinear saturated state. We identify two types of final saturated state: (i) T states, characterized by vertical turbulent motion penetrating into the VSI-stable midplane layer, and (ii) pT states, characterized by turbulent motion confined in the unstable layers. The pT states are realized when the midplane VSI-stable layer is thicker than two gas scale heights. We also find that the VSI-driven turbulence is largely suppressed at all heights when the VSI-unstable region lying above and below the midplane is thinner than two gas scale heights. We present empirical formulas that predict the strength of VSI-driven turbulence as a function of the thicknesses of the unstable and stable layers. These formulas will be useful for studying how VSI-driven turbulence and dust grains controlling the disk cooling efficiency evolve simultaneously.