投影对导出的质量-尺寸和线宽-尺寸关系的影响
The Effect of Projection on Derived Mass-Size and Linewidth-Size Relationships
摘要：
幂律质量尺寸和线宽尺寸相关性，两个“拉尔森定律”，经常被研究，以评估分子云内团块的动力学状态。利用分子云的流体动力学模拟结果，我们研究几何投影如何影响推导的Larson关系。我们发现柱密度图中的大尺度结构与3D模拟（PPP）中的结构具有相似的质量和尺寸。在柱状密度图中，较小尺度的团块被测量为比PPP团块更大，这是由于所有排放气体沿着视线的投影。此外，由于投影效应，合成光谱观测（PPV）中的结构可能不一定与模拟中的物理结构相关。仅考虑湍流速度时，PPV立方体中的线宽-尺寸关系与模拟测量的线宽-尺寸关系明显不同。在模拟线宽中包括热压力施加了最小线宽，这导致线宽-尺寸关系的斜率更好地一致，尽管偏移量仍然存在差异，以及相当大的分散。采用维里分析中常用的假设，我们发现在PPV和PPP立方体的结构计算维里参数的相似之处。然而，由于PPP和PPV立方体中线宽和质量大小关系的差异，我们警告说，由于几何投影效应，将维里分析应用于观察到的云可能会产生误导。我们推测，要准确评估复杂的云，如那些具有高度非线性结构的云，是否被束缚，需要考虑动力学和重力压力之外的物理过程。
英文摘要：
Power law mass-size and linewidth-size correlations, two of "Larson's laws," are often studied to assess the dynamical state of clumps within molecular clouds. Using the result of a hydrodynamic simulation of a molecular cloud, we investigate how geometric projection may affect the derived Larson relationships. We find that large scale structures in the column density map have similar masses and sizes to those in the 3D simulation (PPP). Smaller scale clumps in the column density map are measured to be more massive than the PPP clumps, due to the projection of all emitting gas along lines of sight. Further, due to projection effects, structures in a synthetic spectral observation (PPV) may not necessarily correlate with physical structures in the simulation. In considering the turbulent velocities only, the linewidth-size relationship in the PPV cube is appreciably different from that measured from the simulation. Including thermal pressure in the simulated linewidths imposes a minimum linewidth, which results in a better agreement in the slopes of the linewidth-size relationships, though there are still discrepancies in the offsets, as well as considerable scatter. Employing commonly used assumptions in a virial analysis, we find similarities in the computed virial parameters of the structures in the PPV and PPP cubes. However, due to the discrepancies in the linewidth- and mass- size relationships in the PPP and PPV cubes, we caution that applying a virial analysis to observed clouds may be misleading due to geometric projection effects. We speculate that consideration of physical processes beyond kinetic and gravitational pressure would be required for accurately assessing whether complex clouds, such as those with highly filamentary structure, are bound.
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