扩散分子气体III的耗散结构--强速度剪切的小尺度不稳定性
Dissipative structures of diffuse molecular gas III -- Small-scale intermittency of intense velocity-shears
摘要：
我们进一步表征的结构，初步确定的热和化学的理由，作为网站的耗散湍流分子云（论文I和II）。我们的研究是基于两点统计的线质心速度（CV），计算从三个大的12 CO地图的两个字段。计算了CO线质心速度增量（CVI）随滞后变化一个数量级的概率密度函数（PDF）和线CV的结构函数，直到第6阶。我们证明了CV线在湍流速度场中具有三个不连续性特征：（1）随着滞后的减小，CVIPDF中的非高斯尾增加;（2）高阶结构函数偏离Kolmogorov标度的情况在湍流度更大的场中更为明显;（3）CVIPDF尾的位置描绘出狭窄的边界结构（厚度约0.02 pc），与致密气体结构无关，与较大尺度上观察到的较厚气体结构（约0.18 pc）在空间上一致。与理论预测的对抗导致我们确定这些小规模的对流结构与极端的速度剪切与气体温暖比大部分。最后，它们的平均方向平行（或接近）于局部磁场投影的方向。在这些半透明领域的湍流表现出的统计和结构的小尺度和惯性范围的不稳定性的签名。在30 pc尺度上更湍流的场在小尺度上也更间歇。小规模的间歇性结构与以前确定的增强耗散的网站相吻合。它们被组织成秒差距尺度的相干结构，耦合了广泛的尺度。
英文摘要：
We further characterize the structures tentatively identified on thermal and chemical grounds as the sites of dissipation of turbulence in molecular clouds (Papers I and II). Our study is based on two-point statistics of line centroid velocities (CV), computed from three large 12CO maps of two fields. Probability density functions (PDF) of the CO line centroid velocity increments (CVI) over lags varying by an order of magnitude and structure functions of the line CV, up to the 6th order, are computed. We show that the line CV bear the three signatures of intermittency in a turbulent velocity field: (1) the non-Gaussian tails in the CVI PDF grow as the lag decreases, (2) the departure from Kolmogorov scaling of the high-order structure functions is more pronounced in the more turbulent field, (3) the positions contributing to the CVI PDF tails delineate narrow filamentary structures (thickness ~ 0.02 pc), uncorrelated to dense gas structures and spatially coherent with thicker ones (~0.18 pc) observed on larger scales. The confrontation with theoretical predictions leads us to identify these small-scale filamentary structures with extrema of velocity-shears associated with gas warmer than the bulk. Last, their average direction is parallel (or close) to that of the local magnetic field projection. Turbulence in these translucent fields exhibits the statistical and structural signatures of small-scale and inertial-range intermittency. The more turbulent field on the 30 pc-scale is also the more intermittent on small scales. The small-scale intermittent structures coincide with those formerly identified as sites of enhanced dissipation. They are organized into parsec-scale coherent structures, coupling a broad range of scales.
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