Letter to the Editor

Intermittency of interstellar turbulence: parsec-scale coherent structure of intense, velocity shear^*

¹ LAOG, CNRS & Université Joseph Fourier, UMR 5571, 414 rue de la Piscine, BP 53, 38041 Grenoble Cedex 09, France e-mail: pierre.hilyblant@obs.ujf-grenoble.fr
² LRA/LERMA, CNRS & École normale supérieure & Observatoire de Paris, UMR 8112, 24 rue Lhomond, 75231 Paris Cedex 05, France

Received: 7 April 2009
Accepted: 12 May 2009

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 ¹²(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 to be 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 ¹²(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.