Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale

TL;DR

This study observes small-scale, high-velocity shear structures in interstellar turbulence, revealing sharp CO edges and suggesting turbulence intermittency as a key process in molecular cloud formation.

Contribution

It provides the first direct detection of intermittency in interstellar turbulence through high-resolution CO observations of tiny, high-shear structures.

Findings

Discovery of 3 mpc thick CO structures with sharp edges.

Measurement of the highest velocity-shears far from star-forming regions.

Identification of thin CO layers as turbulence dissipation sites.

Abstract

The condensation of diffuse gas into molecular clouds occurs at a rate driven largely by turbulent dissipation. This process still has to be caught in action and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with the IRAM-PdB interferometer in the translucent environment of two low-mass dense cores. The large size of the mosaic compared to the resolution (4 arcsec) is unprecedented in the study of the small-scale structure of diffuse molecular gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600 AU) and lengths up to 70mpc are found. These are not filaments because once merged with short-spacing data, they appear as the sharp edges 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 ever measured far from star forming regions (780 km/s/pc). Because the large scale structures have sharp edges, with little or no overlap, they have to be thin CO-layers. Their edges mark a sharp transition between a CO-rich component and a gas undetected in the CO line because of its low CO abundance, presumably the cold neutral medium. We propose that these sharp edges 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.