The Kelvin-Helmholtz instability in Orion: a source of turbulence and chemical mixing

Olivier Berne, Yosuke Matsumoto

Published 2012-10-20, updated 2012-10-24Version 2

Hydrodynamical instabilities are believed to power some of the small scale (0.1-10 pc) turbulence and chemical mixing in the interstellar medium. Identifying such instabilities has always been difficult but recent observations of a wavelike structure (the Ripples) in the Orion nebula have been interpreted as a signature of the Kelvin-Helmholtz instability (KHI), occurring at the interface between the HII region and the molecular cloud. However, this has not been verified theoretically. In this letter, we investigate theoretically the stability of this interface using observational constraints for the local physical conditions. A linear analysis shows that the HII/molecular cloud interface is indeed KH unstable for a certain range of magnetic field orientation. We find that the maximal growth-rates correspond to typical timescales of a few 1e4 years and instability wavelengths of 0.06 to 0.6 pc. We predict that after 2e5 years the KHI saturates and forms a turbulent layer of thickness ~0.5 pc. The KHI can remain in linear phase over a maximum distance of 0.75 pc. These spatial and time scales are compatible with the Ripples representing the linear phase of the KHI. These results suggest that the KHI may be crucial to generate turbulence and to bring heavy elements injected by the winds of massive stars in HII regions to colder regions where planetary systems around low mass stars are being formed. This could apply to the transport of 26Al injected by a massive star in an HII region to the nascent solar-system.