Formation Scenario for Wide and Close Binary Systems
Formation Scenario for Wide and Close Binary Systems
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DOI：
10.1086/529133
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发表时间：
2007-09
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影响因子：
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通讯作者：
M. Machida;K. Tomisaka;Tomoaki Matsumoto;S. Inutsuka
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作者：
M. Machida;K. Tomisaka;Tomoaki Matsumoto;S. Inutsuka
用三维电阻MHD嵌套网格模拟研究了碎裂和二元形成过程。从均匀磁场中旋转的Bonnor-Ebert等温云开始，我们计算了从分子云核心（n = 104 cm −3）到恒星核心（n <$1022 cm −3）的云演化，其中n表示中心密度。我们计算了147个模型，不同的初始磁场，旋转和热能和振幅的非轴对称扰动。在坍缩云中，碎裂主要由转动能与磁能的初始比值控制，而与初始热能和非轴对称扰动的振幅无关。云的旋转促进了碎片化，而磁场在云演化的所有阶段延迟或在某些情况下抑制了碎片化。结果分为三种类型。当云的旋转能相对于磁能较大时，碎裂发生在低密度相（1012 cm −3 Au），间距为3-300 AU。在这一阶段出现的碎片有望演化成宽双星系统。另一方面，当初始云具有相对于旋转能量更大的磁能时，碎裂仅发生在高密度阶段（n <$1017 cm −3），在云由于欧姆耗散而经历磁场的显著减小之后。出现在这个阶段的碎片有10.3 Au的相互分离，预计将演变成密切的二元系统。在磁场足够强的情况下，不会发生碎裂，而在这种情况下，可能会诞生单颗恒星。两种类型的破碎时代反映了广泛和密切的分离。我们也许能够观察到一个双峰分布的径向分离的原恒星在非常年轻的恒星群。
Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n = 104 cm −3) to the stellar core (n≃ 1022 cm −3), where n denotes the central density. We calculated 147 models with different initial magnetic, rotational, and thermal energies and the amplitudes of the nonaxisymmetric perturbation. In a collapsing cloud, fragmentation is mainly controlled by the initial ratio of the rotational to the magnetic energy, regardless of the initial thermal energy and amplitude of the nonaxisymmetric perturbation. The cloud rotation promotes fragmentation, while the magnetic field delays or in some cases suppresses fragmentation through all phases of cloud evolution. The results are categorized into three types. When the clouds have larger rotational energies in relation to magnetic energies, fragmentation occurs in the low-density phase (1012 cm −3≲ n≲ 1015 cm −3) with separations of 3-300 AU. Fragments that appeared in this phase are expected to evolve into wide binary systems. On the other hand, when initial clouds have larger magnetic energies in relation to the rotational energies, fragmentation occurs only in the high-density phase (n≳ 1017 cm −3) after the clouds experience a significant reduction of the magnetic field owing to the ohmic dissipation. Fragments appearing in this phase have mutual separations of ≲0.3 AU and are expected to evolve into close binary systems. No fragmentation occurs in the case of sufficiently strong magnetic field, in which single stars are expected to be born. Two types of fragmentation epoch reflect wide and close separations. We might be able to observe a bimodal distribution for the radial separation of the protostar in extremely young stellar groups.