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Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

机译：星体塌陷的3D辐射化学流体力学模拟中的磁扩散率

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Context. Both theory and observations of star-forming clouds require simulations that combine the co-evolving chemistry, magneto-hydrodynamics, and radiative transfer in protostellar collapse simulation. A detailed knowledge of self-consistent chemical evolution for the main charge carriers (both gas species and dust grains) allows us to correctly estimate the rate and nature of magnetic dissipation in the collapsing core. This knowledge is critical to answer one of the most significant issues of star and planet formation: what is the magnitude and spatial distribution of magnetic flux as the initial condition to protoplanetary disk evolution? Aims. We use a chemo-dynamical version of RAMSES , which is described in a companion publication, to follow the chemo-dynamical evolution of collapsing dense cores with various dust properties and interpret differences that occur in magnetic diffusivity terms. These differences are crucial to circumstellar disk formation. Methods. We performed 3D chemo-dynamical simulations of 1 M _(⊙) isolated dense core collapse for a range in dust size assumptions. The number density of dust and its mean size affect the efficiency of charge capturing and the formation of ices. The radiative hydrodynamics and dynamical evolution of chemical abundances were used to reconstruct the magnetic diffusivity terms for clouds with various magnetisation. Results. The simulations are performed for a mean dust size ranging from 0.017 μ m to 1 μ m, and we adopt both a fixed dust size and a dust size distribution. The chemical abundances for this range of dust sizes are produced by RAMSES and serve as inputs to calculations of Ohmic, ambipolar, and Hall diffusivity terms. Ohmic resistivity only plays a role at the late stage of the collapse in the innermost region of the cloud where gas density is in excess of a few times 10~(13) cm~(-3) . Ambipolar diffusion is a dominant magnetic diffusivity term in cases where mean dust size is a typical ISM value or larger. We demonstrate that the assumption of a fixed dominant ion mass can lead to a one order of magnitude mismatch in the ambipolar diffusion magnitude. The negative Hall effect is dominant during the collapse in case of small dust, i.e. for the mean dust size of 0.02 μ m and smaller; we connect this effect to the dominance of negatively charged grains. We find that the Hall effect reverses its sign for mean dust size of 0.1 μ m and smaller. The phenomenon of the sign reversal strongly depends on the number of negatively charged dust relative to the ions and the quality of coupling of the charged dust to the magnetic fields. We have adopted different strengths of magnetic fields, β = P _(gas)/ P _(mag) = 2,5,25 . We observe that the variation on the field strength only shifts the Hall effect reversal along the radius of the collapsing cloud, but does not prevent it. Conclusions. The dust grain mean size appears to be the parameter with the strongest impact on the magnitude of the magnetic diffusivity, dividing the collapsing clouds in Hall-dominated and ambipolar-dominated clouds and affecting the size of the resulting disks. We propose to link the dust properties and occurrence and size of disk structures in Class 0 young stellar objects. The proper accounting for dust grain growth in the radiative magneto-hydrodynamical collapse models are as important as coupling the dynamics of the collapse with the chemistry.
机译：上下文。对恒星形成云的理论和观测都需要进行模拟，这些模拟将共同演化的化学，磁流体动力学和辐射转移结合在一起，进行了原恒星坍缩模拟。对主要电荷载流子（气体种类和尘埃粒子）的自洽化学演化的详细了解，使我们能够正确估计塌陷核中的磁耗散率和性质。这一知识对于回答恒星和行星形成这一最重要的问题至关重要：作为原行星盘演化的初始条件，磁通量的大小和空间分布是什么？目的我们使用RAMSES的化学动力学版本（在同伴出版物中进行了描述）来跟踪具有各种粉尘特性的致密核的化学动力学演变，并解释在磁扩散率方面出现的差异。这些差异对于星际盘的形成至关重要。方法。在一定的粉尘尺寸假设范围内，我们对1 M _（⊙）隔离的致密芯塌陷进行了3D化学动力学模拟。灰尘的数量密度及其平均大小会影响电荷捕获和结冰的效率。辐射流体动力学和化学丰度的动力学演化被用来重建具有各种磁化强度的云的磁扩散率项。结果。针对平均灰尘尺寸范围为0.017μm至1μm的情况执行模拟，我们同时采用了固定的灰尘尺寸和灰尘尺寸分布。 RAMSES会产生此尘埃尺寸范围的化学丰度，并用作欧姆，双极性和霍尔扩散系数计算的输入。欧姆电阻率仅在气体密度超过10〜（13）cm〜（-3）几倍的云的最内层区域崩溃的后期才起作用。在平均粉尘尺寸为典型ISM值或更大的情况下，双极扩散是主要的磁扩散率术语。我们证明了固定主导离子质量的假设可能导致双极性扩散幅度中的一个数量级失配。在粉尘较小的情况下（即平均粉尘尺寸为0.02μm及以下），在倒塌过程中负霍尔效应最为明显;我们将此效应与带负电的晶粒的优势联系起来。我们发现霍尔效应将其符号反转，平均灰尘尺寸为0.1μm或更小。符号反转的现象在很大程度上取决于带负电的灰尘相对于离子的数量以及带电灰尘与磁场的耦合质量。我们采用了不同的磁场强度，β= P _（gas）/ P _（mag）= 2,5,25。我们观察到，场强的变化只会使霍尔效应的反转沿塌陷云的半径移动，而不能阻止这种变化。结论。尘埃颗粒的平均大小似乎是对磁扩散率大小影响最大的参数，它将散乱的云划分为以霍尔为主和双极为主的云，并影响了所得磁盘的大小。我们建议将0级年轻恒星物体的尘埃特性与磁盘结构的出现和大小联系起来。在辐射磁流体动力学崩溃模型中正确考虑粉尘颗粒的生长与将崩溃动力学与化学过程耦合在一起一样重要。

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