Molecular cloud formation by compression of magnetized turbulent gas subjected to radiative cooling

Abstract

Complex turbulent motions of magnetized gas are ubiquitous in the interstellar medium (ISM). The source of this turbulence, however, is still poorly understood. Previous work suggests that compression caused by supernova shockwaves, gravity, or cloud collisions, may drive the turbulence to some extent. In this work, we present three-dimensional (3D) magnetohydrodynamic (MHD) simulations of contraction in turbulent, magnetized clouds from the warm neutral medium of the ISM to the formation of cold dense molecular clouds, including radiative heating and cooling. We study different contraction rates and find that observed molecular cloud properties, such as the temperature, density, Mach number, and magnetic field strength, and their respective scaling relations, are best reproduced when the contraction rate equals the turbulent turnover rate. In contrast, if the contraction rate is significantly larger (smaller) than the turnover rate, the compression drives too much (too little) turbulence, producing unrealistic cloud properties. We find that the density probability distribution function evolves from a double lognormal representing the two-phase ISM, to a skewed, single lognormal in the dense, cold phase. For purely hydrodynamical simulations, we find that the effective driving parameter of contracting cloud turbulence is natural to mildly compressive (b ∼ 0.4-0.5), while for MHD turbulence, we find b ∼ 0.3-0.4, i.e. solenoidal to naturally mixed. Overall, the physical properties of the simulated clouds that contract at a rate equal to the turbulent turnover rate, indicate that large-scale contraction may explain the origin and evolution of turbulence in the ISM.