Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale
Résumé
Aims: The condensation of diffuse gas into molecular clouds and dense cores occurs at a rate driven largely by turbulent dissipation. This process still has to be caught in action and characterized. Methods: We observed a mosaic of 13 fields with the IRAM-PdB interferometer (PdBI) to search for small-scale structure in the 12CO(1-0) line emission of the turbulent and translucent environment of a low-mass dense core in the Polaris Flare. The large size of the mosaic (1' × 2') compared to the resolution (4'') is unprecedented in the study of the small-scale structure of diffuse molecular gas. Results: The interferometer data uncover eight weak and elongated structures with thicknesses as small as ≈3 mpc (600 AU) and lengths up to 70 mpc, close to the size of the mosaic. These are not filaments because once merged with short-spacings data, the PdBI-structures appear to be the sharp edges, in space and velocity-space, of larger-scale structures. Six out of eight form quasi-parallel pairs at different velocities and different position angles. This cannot be the result of chance alignment. The velocity-shears estimated for the three pairs include the highest values ever measured in regions that do not form stars (up to 780 km s-1 pc-1). The CO column density of the PdBI-structures is in the range N(CO) = 1014 to 1015 cm-2 and their H2 density, estimated in several ways, does not exceed a few 103 cm-3. Because the larger scale structures have sharp edges (with little or no overlap for those that are pairs), they have to be thin layers of CO emission. We call them SEE(D)S for sharp-edged extended (double) structures. These edges mark a transition, on the milliparsec scale, between a CO-rich component and a gas undetected in the 12CO(1-0) line because of its low CO abundance, presumably the cold neutral medium. Conclusions: We propose that these SEE(D)S are the first directly-detected manifestations of the intermittency of interstellar turbulence. The large velocity-shears reveal an intense straining field, responsible for a local dissipation rate several orders of magnitude above average, possibly at the origin of the thin CO layers. Based on observations obtained with the IRAM Plateau de Bure interferometer and 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).