Turbulence in virtual: II. Origin of skewness and dual fraction processes

Abstract

Turbulence is a mysterious phenomenon in physical systems and plays a critical role in the interstellar medium (ISM). Previous simulations and observations have shown that the probability density functions (PDFs) of gas densities in supersonic systems tend to be skewed, exhibiting low-density exponential tails on the $s = \ln{\rho}$ scale. In this work, we argue that these exponential tails originate from the convolution of PDF kernels with skewed tails, which can appear at both ends of the PDF. We introduce two density-fraction strategies -- the mass-fraction and volume-fraction approaches -- to explain the physical origins of the low-$s$ and high-$s$ skewed PDF kernels. The PDF kernels constructed in this work satisfy the relation that its structural entropy equals the PDF variance, as we proposed in Paper I. These two types of PDF kernels define two spaces of skewed PDFs, which are dual to each other and possess highly symmetric mathematical structures. We thus speculate that the high-$s$ skewed PDF kernels are physical and may be related to the power-law tails of the column-density PDFs (on the $\rho$ scale) of molecular clouds, which are shaped by both turbulence and gravity. Inspired by this, we further construct a form of an `isothermal' turbulent system that likely favors the volume-fraction strategy.