Intermittency of interstellar turbulence: Parsec-scale coherent structure of intense velocity-shear

Aims. Guided by the duality of turbulence (random versus coherent motions), we seek coherent structures in the turbulent velocity field of molecular clouds, anticipating their importance in cloud evolution. Methods. 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 (0.05km s−1) resolutions in the 12CO(2 − 1) line. Results. We find that two parsec-scale components of velocities differing by ∼ 2 km s−1, share a narrow interface (< 0.15 pc) that appears as an elongated structure of intense velocity-shear, ∼ 15 to 30 km s−1 pc−1. 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. Conclusions. 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.