W51North：从热抑制碎裂云中出现的原星系团
W51North: A protocluster emerging out of a thermally inhibited fragmenting cloud
摘要：
大质量恒星形成区的碎裂过程是当前天体物理学研究的热点问题之一，控制碎裂的物理过程包括湍流、磁场、自转、恒星反馈和引力等。然而，在远低于1000 AU的小空间尺度上，对碎片化过程的研究很少。我们的目标是使用ALMA（阿塔卡马大型毫米波和亚毫米波阵列）高角分辨率数据来识别W51 IRS 2中的碎片，并在200 AU的空间尺度上研究碎片的性质。我们使用了来自三个项目的W51 IRS 2的ALMA数据，这些数据在毫米波长下的角分辨率为0.028 $^{\prime\prime}$（144 AU）。我们通过使用{ \it uv}-范围限制的1.3 mm连续数据来识别紧凑的碎片。已进行了伴随物的平均表面密度（MSDC）分析，以研究碎片之间的分离。MSDC分析揭示了两个断裂对应于1845 AU和7346 AU的空间分布，表明存在两级聚类现象，以及1845 AU以下的线性区域，主要与W51 North有关。它的斜率与银河系中其他类星团区域的星团机制的斜率一致。碎片的典型质量和间隔以及密度和碎片数量之间的关系可以通过在200- 400 K的高温下运行的热金斯过程来解释，这与以前对该区域温度的测量一致，并且是由附近的大质量恒星产生的。因此，尽管W51 IRS 2似乎正在经历热抑制碎裂阶段，但这似乎并不能阻止与W51 North相关的原星系团的形成。
英文摘要：
The fragmentation process in massive star-forming regions is one of the contemporary problems in astrophysics, and several physical processes have been proposed to control the fragmentation including turbulence, magnetic field, rotation, stellar feedback, and gravity. However, the fragmentation process has been poorly studied at small spatial scales well below 1000 AU. We aim to use ALMA (Atacama Large Millimeter and Submillimeter Array) high angular resolution data to identify the fragments in W51 IRS2 and to study the fragmentation properties on a spatial scale of 200 AU. We used ALMA data of W51 IRS2 from three projects, which give an angular resolution of 0.028$^{\prime\prime}$ (144 AU) at millimeter wavelengths. We identified compact fragments by using {\it uv}-range constrained 1.3 mm continuum data. A Mean Surface Density of Companions (MSDC) analysis has been performed to study the separations between fragments. A total number of 33 continuum sources are identified and 29 out of them are defined as fragments in the surveyed region.The MSDC analysis reveals two breaks corresponding to spatial sales of 1845 AU and 7346 AU, indicative of a two-level clustering phenomenon, along with a linear regime below 1845 AU, mostly associated with W51 North, whose slope is consistent with the slope for the clustering regime of other cluster-like regions in the Galaxy. The typical masses and separations of the fragments as well as the relation between density and number of fragments can be explained through a thermal Jeans process operating at high temperatures of 200--400 K, consistent with previous measurements of the temperature in the region, and produced by the nearby massive stars. Therefore, although W51 IRS2 seems to be undergoing a thermally inhibited fragmentation phase, this does not seem to prevent the formation of a protocluster associated with W51 North.
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