Stars form inside molecular cloud filaments from the competition of gravitational forces with turbulence and magnetic fields. The exact orientation of these filaments with the magnetic fields depends on the strength of these fields, the gravitational potential, and the line of sight (LOS) relative to the mean field. To disentangle these effects we employ three-dimensional magnetohydrodynamical numerical simulations that explore a wide range of initial turbulent and magnetic states, i.e. sub-Alfvénic to super-Alfvénic turbulence, with and without gravity. We use histogram of relative orientation (HRO) and the associated projected Rayleigh statistics (PRS) to study the orientation of density and, in order to compare with observations, the integrated density relative to the magnetic field. We find that in sub-Alfvénic systems the initial coherence of the magnetic is maintained inside the cloud and filaments form perpendicular to the field. This trend is not observed in super-Alfvénic models, where the lines are dragged by gravity and turbulence and filaments are mainly aligned to the field. The PRS analysis of integrated maps shows that LOS effects are important only for sub-Alfvénic clouds. When the LOS is perpendicular to the initial field orientation most of the filaments are perpendicular to the projected magnetic field. The inclusion of gravity increases the number of dense structures perpendicular to the magnetic field, reflected as lower values of the PRS for denser regions, regardless of whether the model is sub- or super-Alfvénic. The comparison of our results with observed molecular clouds reveals that most are compatible with sub-Alfvénic models.

中文翻译：

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自引力湍流分子云中的磁场取向

由于重力与湍流和磁场的竞争，恒星在分子云细丝内形成。这些细丝与磁场的确切方向取决于这些场的强度、引力势和相对于平均场的视线 (LOS)。为了解开这些影响，我们采用三维磁流体动力学数值模拟来探索广泛的初始湍流和磁状态，即亚阿尔芬到超阿尔芬湍流，有和没有重力。我们使用相对方向的直方图 (HRO) 和相关的投影瑞利统计 (PRS) 来研究密度的方向，并且为了与观察结果比较，相对于磁场的积分密度。我们发现在亚阿尔芬系统中，磁的初始相干性保持在云内部，并且细丝形成垂直于场。在超阿尔芬模型中没有观察到这种趋势，其中线被重力和湍流拖曳，细丝主要与场对齐。综合地图的 PRS 分析表明，LOS 效应仅对亚阿尔芬云很重要。当 LOS 垂直于初始磁场方向时，大多数灯丝垂直于投射磁场。包含重力会增加垂直于磁场的致密结构的数量，这反映为更密集区域的 PRS 值较低，无论模型是亚阿尔芬尼克还是超阿尔芬尼克。