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

Abstract

Aims. Benefitting from the duality of turbulence (random versus coherent motions), we search for 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-30 m telescope) in a high latitude cloud of the Polaris Flare at unprecedented spatial (11″) and spectral (0.05 km. s-1) resolution for the 12CO(2-1)line. Results. We find that two parsec-scale components of velocities differing by ∼2 kms-1, share a narrow interface (<0.15 pc) that appears to be an elongated, structure of intense velocity-shear, ∼15 to 30 kms-1pc -1. The locus of the extrema of line-centroidvelocity 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 in gas density) occur at the position of the intense velocity shear. Conclusions. These results identify spatial and kinematic coherence on scales of between 0.03 pc and 1 pc. 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 provide support for a link between large-scale turbulence, its intermittent dissipation rate and low-mass dense core formation. © ESO 2009.