The sonic scale of interstellar turbulence

Understanding the physics of turbulence is crucial for many applications, including weather, industry and astrophysics. In the interstellar medium(1,2), supersonic turbulence plays a crucial role in controlling the gas density and velocity structure, and ultimately the birth of stars(3-8). Here we present a simulation of interstellar turbulence with a grid resolution of 10,0483 cells that allows us to determine the position and width of the sonic scale (l(s))-the transition from supersonic to subsonic turbulence. The simulation simultaneously resolves the supersonic and subsonic cascade, with the velocity as a function of scale, v(l) proportional to l(p), where we measure p(sup) = 0.49 +/- 0.01 and p(sub) = 0.39 +/- 0.02, respectively. We find that l(s) agrees with the relation l(s) = phi sLM-1/psup, where M is the three-dimensional Mach number, L is either the driving scale of the turbulence or the diameter of a molecular cloud, and phi(s) is a dimensionless factor of order unity. If L is the driving scale, we measure phi(s) = 0.42(-0.09)(+0.12), primarily because of the separation between the driving scale and the start of the supersonic cascade. For a supersonic cascade extending beyond the cloud scale, we get phi(s) = 0.91(-0.20)(+0.25). In both cases, phi(s) less than or similar to 1, because we find that the supersonic cascade transitions smoothly to the subsonic cascade over a factor of 3 in scale, instead of a sharp transition. Our measurements provide quantitative input for turbulence-regulated models of filament structure and star formation in molecular clouds.

Automated summary

Understanding the physics of turbulence is crucial for various applications, including weather, industry, and astrophysics. By presenting a simulation of interstellar turbulence, researchers were able to determine the sonic scale transition and velocity structure, providing quantitative input for models of filament structure and star formation in molecular clouds. Key findings included the measurements of the supersonic and subsonic cascade with velocity as a function of scale, where the transition from supersonic to subsonic turbulence was shown to occur smoothly over a factor of 3 in scale.