星际turbulence的阵发性：强速度切变的秒差距尺度相干结构
Intermittency of interstellar turbulence: Parsec-scale coherent structure of intense velocity-shear
摘要：
在湍流的二元性（随机与相干）的指导下，我们在分子云的湍流速度场中寻找相干结构，预测它们在云演化中的重要性。我们分析了在12 CO（2-1）线上用HERA多波束接收器（IRAM-30米望远镜）以前所未有的空间（11”）和光谱（0.05 km/s）分辨率在北极星耀斑的高纬度云中获得的大地图（40'乘20'）。我们发现两个秒差距尺度的速度分量相差约2 km/s，共享一个狭窄的界面（$<0.15$ pc），该界面表现为一个细长的强烈速度剪切结构，约15至30 km/s/pc。该场中的线质心速度增量（E-CVI）极值的轨迹遵循这种强剪切结构以及12 CO（2-1）高速线翼的轨迹。两个秒差距尺度分量的投影中的微小空间重叠意味着它们是CO排放片，并且在强烈速度剪切的位置处发生气体性质（CO富集和/或气体密度增加）的不连续性。这些结果揭示了小至0.03秒差距尺度和秒差距尺度之间的空间和运动学相干性。他们证实，偏离高斯的概率密度函数的E-CVIs是一个强大的统计示踪剂的湍流的不稳定性。他们揭示了大尺度湍流、其间歇性耗散率和低质量致密核心形成之间的联系。
英文摘要：
Guided by the duality of turbulence (random versus coherent we seek coherent structures in the turbulent velocity field of molecular clouds, anticipating their importance in cloud evolution. We analyse a large map (40' by 20') obtained with the HERA multibeam receiver (IRAM-30m telescope) in a high latitude cloud of the Polaris Flare at an unprecedented spatial (11") and spectral (0.05 km/s) resolutions in the 12CO(2-1) line. We find that two parsec-scale components of velocities differing by ~2 km/s, share a narrow interface ($<0.15$ pc) that appears as an elongated structure of intense velocity-shear, ~15 to 30 km/s/pc. The locus of the extrema of line--centroid-velocity increments (E-CVI) in that field follows this intense-shear structure as well as that of the 12CO(2-1) high-velocity line wings. The tiny spatial overlap in projection of the two parsec-scale components implies that they are sheets of CO emission and that discontinuities in the gas properties (CO enrichment and/or increase of gas density) occur at the position of the intense velocity shear. These results disclose spatial and kinematic coherence between scales as small as 0.03 pc and parsec scales. They confirm that the departure from Gaussianity of the probability density functions of E-CVIs is a powerful statistical tracer of the intermittency of turbulence. They disclose a link between large scale turbulence, its intermittent dissipation rate and low-mass dense core formation.
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