Turbulence in the interstellar medium

Structure formation

Alike large scale structures in the Universe, the self-similar hierarchy of structures observed in interstellar matter may be driven by the gravitational instability but this is not proven yet. We are investigating other processes, possibly responsible for the observed scaling.

Dynamical condensation in a thermally bistable fluid

The interstellar diffuse medium in the pressure conditions prevailing in our Galaxy is thermally bistable and turbulent. The transient overpressures generated locally in convergent flows of trans-sonic turbulence have been shown to be able to nucleate structures of cold gas within the warm neutral medium, and produce long-lived cold condensations at the expense of the warm medium with masses comparable to those observed. The growth of the cold condensations is, in this process, much more efficient than if solely regulated by thermal conduction. (P. Hennebelle, M. Pérault)

Intermittency of turbulence

The spatial and temporal intermittency of viscous dissipation of turbulence is able to deposit large amounts of thermal energy in localized regions of the medium. The subsequent heating of the gas is large enough to trigger a variety of chemical reactions otherwise inactive in the bulk of the cold gas. Once chemically enriched these structures efficiently cool and condense. Ongoing observations with the Plateau de Bure Interferometer (Institut for Millimeter Radioastronomy, IRAM) reveal elongated structures, only a few 100 astronomical units thick, with extremely large internal velocity gradients. These structures might be braided coherent vortices similar to those observed in laboratory turbulence.

Further, statistical studies of the velocity field of large regions mapped in molecular line transitions at high frequency resolution, reveal non-Gaussian statistics which may be interpreted as the signature of intermittemcy of the velocity field, and possibly of viscous dissipation (E. Falgarone & J. Pety in collaboration with Caltech and Paris Observatory).