Vertical shear instability with dust evolution and consistent cooling times,Astronomy & Astrophysics

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Vertical shear instability with dust evolution and consistent cooling times
Astronomy & Astrophysics ( IF 5.4 ) Pub Date : 2024-06-26 , DOI: 10.1051/0004-6361/202449323
Thomas Pfeil , Til Birnstiel , Hubert Klahr

Context. Gas in protoplanetary disks mostly cools via thermal accommodation with dust particles. Thermal relaxation is thus highly sensitive to the local dust size distributions and the spatial distribution of the grains. So far, the interplay between thermal relaxation and gas turbulence has not been dynamically modeled in hydrodynamic simulations of protoplanetary disks with dust.Aims. We aim to study the effects of the vertical shear instability (VSI) on the thermal relaxation times, and vice versa. We are particularly interested in the influence of the initial dust grain size on the VSI and whether the emerging turbulence is sustained over long timescales.Methods. We ran three axisymmetric hydrodynamic simulations of a protoplanetary disk including four dust fluids that initially resemble MRN size distributions of different initial grain sizes. From the local dust densities, we calculated the thermal accommodation timescale of dust and gas and used the result as the thermal relaxation time of the gas in our simulation. We included the effect of dust growth by applying the monodisperse dust growth rate and the typical growth limits.Results. We find that the emergence of the VSI is strongly dependent on the initial dust grain size. Coagulation also counteracts the emergence of hydrodynamic turbulence in our simulations, as shown by others before. Starting a simulation with larger grains (100 μm) generally leads to a less turbulent outcome. While the inner disk regions (within ∼70 au) develop turbulence in all three simulations, we find that the simulations with larger particles do not develop VSI in the outer disk.Conclusions. Our simulations with dynamically calculated thermal accommodation times based on the drifting and settling dust distribution show that the VSI, once developed in a disk, can be sustained over long timescales, even if grain growth is occurring. The VSI corrugates the dust layer and even diffuses the smaller grains into the upper atmosphere, where they can cool the gas. Whether the instability can emerge for a specific stratification depends on the initial dust grain sizes and the initial dust scale height. If the grains are initially ≳100 μm and if the level of turbulence is initially assumed to be low, we find no VSI turbulence in the outer disk regions.

中文翻译：

垂直剪切不稳定性与粉尘逸出和一致的冷却时间

语境。原行星盘中的气体主要通过与尘埃颗粒的热适应来冷却。因此，热弛豫对局部灰尘尺寸分布和颗粒的空间分布高度敏感。到目前为止，热弛豫和气体湍流之间的相互作用尚未在含尘埃的原行星盘的流体动力学模拟中进行动态建模。我们的目标是研究垂直剪切不稳定性 (VSI) 对热弛豫时间的影响，反之亦然。我们特别感兴趣的是初始尘埃颗粒尺寸对 VSI 的影响以及新出现的湍流是否会长期持续。方法。我们对原行星盘进行了三个轴对称流体动力学模拟，其中包括四种尘埃流体，这些尘埃流体最初类似于不同初始颗粒尺寸的 MRN 尺寸分布。根据当地的灰尘密度，我们计算了灰尘和气体的热适应时间尺度，并将结果用作模拟中气体的热弛豫时间。我们通过应用单分散粉尘生长速率和典型生长极限来考虑粉尘生长的影响。结果。我们发现 VSI 的出现强烈依赖于初始粉尘颗粒尺寸。正如其他人之前所表明的，在我们的模拟中，混凝还抵消了水动力湍流的出现。使用较大颗粒 (100 μm) 开始模拟通常会产生较少的湍流结果。虽然内盘区域（~70 au 内）在所有三个模拟中都会产生湍流，但我们发现较大粒子的模拟不会在外盘中产生 VSI。结论。我们根据漂移和沉降灰尘分布动态计算热适应时间的模拟表明，VSI 一旦在盘中形成，即使发生晶粒生长，也可以在很长的时间内持续。 VSI 使灰尘层起皱，甚至将较小的颗粒扩散到高层大气中，在那里它们可以冷却气体。特定分层是否会出现不稳定取决于初始尘埃颗粒尺寸和初始尘埃尺度高度。如果颗粒最初为 ≳100 μm 并且最初假设湍流水平较低，则我们在外盘区域中发现没有 VSI 湍流。