Does Misalignment between Magnetic Field and Angular Momentum Enhance or Suppress Circumstellar Disk Formation?
Does Misalignment between Magnetic Field and Angular Momentum Enhance or Suppress Circumstellar Disk Formation?
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DOI：
10.3847/1538-4357/aae4dc
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发表时间：
2018-11
期刊：
影响因子：
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通讯作者：
Y. Tsukamoto;S. Okuzumi;K. Iwasaki;M. Machida;S. Inutsuka
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文献类型：
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作者：
Y. Tsukamoto;S. Okuzumi;K. Iwasaki;M. Machida;S. Inutsuka
通过理想和非理想MHD模拟，研究了分子云核角动量与磁场不对准对引力坍缩过程中角动量演化的影响。对于非理想效应，我们考虑欧姆扩散和双极性扩散。先前考虑错位的研究报告了定性矛盾的结果。在不同的研究中，磁制动被报道为因错位而加强或减弱。我们通过改变稳定性参数α（核心的热能与引力能之比），在考虑和不考虑磁扩散的情况下进行了云核坍缩的模拟。非理想MHD模拟表明，当θ = 0°（）时，核心的中心角动量总是大于θ = 90°（）时的中心角动量，与α无关，这意味着在θ = 0°的核心中更容易形成周盘。理想MHD模拟结果表明，当θ = 90°时，小α的中心角动量大于θ = 0°时，大α的中心角动量小于θ = 0°时。对流体元件角动量演化的观察揭示了三种促进角动量演化的机制：(i)等温坍缩阶段的磁制动，（ii）快速（θ = 90°）或缓慢（θ = 0°）旋转的流体元件向中心区域的选择性吸积，以及（iii）第一个核心和磁盘的磁制动。理想和非理想模拟之间的差异源于（iii）的不同效率。
The effect of misalignment between the magnetic field and the angular momentum of molecular cloud cores on the angular momentum evolution during the gravitational collapse is investigated by ideal and non-ideal MHD simulations. For the non-ideal effect, we consider the ohmic and ambipolar diffusion. Previous studies that considered the misalignment reported qualitatively contradicting results. Magnetic braking was reported as being either strengthened or weakened by misalignment in different studies. We conducted simulations of cloud core collapse by varying the stability parameter α (the ratio of the thermal to gravitational energy of the core) with and without including magnetic diffusion. The non-ideal MHD simulations show the central angular momentum of the core, with θ = 0° ( ) being always greater than that with θ = 90° ( ), independently of α, meaning that circumstellar disks form more easily in a core with θ = 0°. The ideal MHD simulations, in contrast, show the central angular momentum of the core with θ = 90° being greater than with θ = 0° for small α and smaller for large α. Inspection of the angular momentum evolution of the fluid elements reveals three mechanisms contributing to the evolution of the angular momentum: (i) magnetic braking in the isothermal collapse phase, (ii) selective accretion of the rapidly (for θ = 90°) or slowly (for θ = 0°) rotating fluid elements to the central region, and (iii) magnetic braking in the first core and the disk. The difference between the ideal and non-ideal simulations arises from the different efficiencies of (iii).