Accretion-Driven Turbulence as Universal Process: Galaxies, Molecular Clouds, and Protostellar Disks

Résumé : When cosmic structures form, they grow in mass via accretion from their surrounding environment. The energy associated with this transport of material provides a ubiquitous source of internal turbulence. We propose that accretion will drive turbulent motions in a wide range of astrophysical objects and study this process in the case of galaxies, molecular clouds and protoplanetary disks. We use a combination of numerical simulations and analytical arguments to predict the level of turbulence as a function of the accretion rate, the dissipation scale, and the density contrast, and compare with observational data. We find that in Milky Way type galaxies the turbulence in the non star-forming outer parts of the disk can be explained by accretion, provided that the galaxies accrete at a rate comparable to the rate at which they form stars. We note that the extended outer disk carries the bulk of the turbulent energy in the galaxy. Our approach fails for dwarf galaxies and we expect other sources to dominate. We calculate the rate at which molecular clouds grow in mass when they build up from the atomic component of the galactic gas. The very process of cloud formation can drive turbulent motions on small scales via establishing the turbulent cascade. In the case of T-Tauri disks, we show that accretion can drive subsonic turbulence at the observed level if the rate at which gas falls onto the disk is comparable to the rate at which disk material accretes onto the central star. This also explains the observed relation of accretion rate and stellar mass, dM/dt ~ M^1.8. The efficiency required to convert infall motion into turbulence is of order of a few percent in all three cases. We conclude that accretion-driven turbulence is a universal concept with far-reaching implications for a wide range of astrophysical objects.