标题： Clumpfind的危险：分子云中亚结构的质量谱 显示英文标题

标题： The Perils of Clumpfind: The Mass Spectrum of Sub-structures in Molecular Clouds

摘要： 我们研究了由 13CO (1-0) 跟踪的英仙座分子云复合体中的亚结构质量谱，发现对于标准的 Clumpfind 参数，得到的结果为 $dN/dM\propto M^{-2.4}$。这一结果与经典的 $dN/dM\propto M^{-1.6}$不一致。为了理解这种差异，我们研究了使用 Clumpfind 算法得出的质量谱的稳健性。分别使用 850 $\mu$米尘埃辐射和英仙座的 13CO 光谱线观测数据测试了二维和三维版本的 Clumpfind。阈值的变化影响不大，但步长的变化对二维和三维情况产生了不同的影响。在二维情况下，发射相对孤立（仅与云中最密集的峰相关联），质量谱的变化相对于质量函数拟合不确定性可以忽略不计。然而，在三维情况下，由于 13CO 发射追踪了分子云的主要部分，聚类数量和导出的质量谱高度依赖于使用的步长。这里“二维”和“三维”的区别也反映了“稀疏”和“拥挤”发射之间的差异。在任何“拥挤”的情况下，不应盲目使用 Clumpfind 来推导质量函数。在“拥挤”的情况下，Clumpfind 的输出仍然可以提供有用的发射统计描述，用于相互比较，但聚类列表不应被视为适合生成具有物理意义的质量函数的稳健区域分解。我们得出结论，由于 MC 的分层结构，13CO 的质量谱取决于观测分辨率。 显示英文摘要

摘要： We study the mass spectrum of sub-structures in the Perseus Molecular Cloud Complex traced by 13CO (1-0), finding that $dN/dM\propto M^{-2.4}$ for the standard Clumpfind parameters. This result does not agree with the classical $dN/dM\propto M^{-1.6}$. To understand this discrepancy we study the robustness of the mass spectrum derived using the Clumpfind algorithm. Both 2D and 3D Clumpfind versions are tested, using 850 $\mu$m dust emission and 13CO spectral-line observations of Perseus, respectively. The effect of varying threshold is not important, but varying stepsize produces a different effect for 2D and 3D cases. In the 2D case, where emission is relatively isolated (associated with only the densest peaks in the cloud), the mass spectrum variability is negligible compared to the mass function fit uncertainties. In the 3D case, however, where the 13CO emission traces the bulk of the molecular cloud, the number of clumps and the derived mass spectrum are highly correlated with the stepsize used. The distinction between "2D" and "3D" here is more importantly also a distinction between "sparse" and "crowded" emission. In any "crowded" case, Clumpfind should not be used blindly to derive mass functions. Clumpfind's output in the "crowded" case can still offer a statistical description of emission useful in inter-comparisons, but the clump-list should not be treated as a robust region decomposition suitable to generate a physically-meaningful mass function. We conclude that the 13CO mass spectrum depends on the observations resolution, due to the hierarchical structure of MC.