PNNL

Abstract

We show that the growth rate of dust grains in the cold interstellar medium is enhanced by the high degree of compressibility of a turbulent, dilute gas. By means of high resolution (1024^3) numerical simulations, we confirm the theory that the spatial mean growth rate is proportional to the gas-density variance. This also results in broadening of the grain-size distribution (GSD) due to turbulence-induced variation of the grain-growth rate. We show, for the first time in a detailed numerical simulation of hydrodynamic turbulence, that the GSD evolves towards a shape which is a reflection of the gas-density distribution, regardless of the initial distribution. That is, in case of isothermal, rotationally forced turbulence, the GSD tends to be a lognormal distribution. This is clearly different from the widely used power-law distribution with a power index of -3.5. We also show that in hypersonic turbulence, decoupling of gas and dust becomes important and that this leads to an even furtheraccelerated grain growth.