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Complex turbulent motions of magnetized gas are ubiquitous in theinterstellar medium. The source of this turbulence, however, is still poorlyunderstood. Previous work suggests that compression caused by supernovashockwaves, gravity, or cloud collisions, may drive the turbulence to someextent. In this work, we present three-dimensional (3D) magnetohydrodynamic(MHD) simulations of contraction in turbulent, magnetized clouds from the warmneutral medium (WNM) of the ISM to the formation of cold dense molecularclouds, including radiative heating and cooling. We study different contractionrates and find that observed molecular cloud properties, such as thetemperature, density, Mach number, and magnetic field strength, and theirrespective scaling relations, are best reproduced when the contraction rateequals the turbulent turnover rate. In contrast, if the contraction rate issignificantly larger (smaller) than the turnover rate, the compression drivestoo much (too little) turbulence, producing unrealistic cloud properties. Wefind that the density probability distribution function evolves from a doublelog-normal representing the two-phase ISM, to a skewed, single log-normal inthe dense, cold phase. For purely hydrodynamical simulations, we find that theeffective driving parameter of contracting cloud turbulence is natural tomildly compressive (\mbox{$b\sim0.4$--$0.5$}), while for MHD turbulence, wefind \mbox{$b\sim0.3$--$0.4$}, i.e., solenoidal to naturally mixed. Overall,the physical properties of the simulated clouds that contract at a rate equalto the turbulent turnover rate, indicate that large-scale contraction mayexplain the origin and evolution of turbulence in the ISM.

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