A supersonic turbulence origin of Larson's laws

Abstract

We revisit the origin of Larson's scaling laws describing the structure and\nkinematics of molecular clouds. Our analysis is based on recent observational\nmeasurements and data from a suite of six simulations of the interstellar\nmedium, including effects of self-gravity, turbulence, magnetic field, and\nmultiphase thermodynamics. Simulations of isothermal supersonic turbulence\nreproduce observed slopes in linewidth-size and mass-size relations. Whether or\nnot self-gravity is included, the linewidth-size relation remains the same. The\nmass-size relation, instead, substantially flattens below the sonic scale, as\nprestellar cores start to form. Our multiphase models with magnetic field and\ndomain size 200 pc reproduce both scaling and normalization of the first Larson\nlaw. The simulations support a turbulent interpretation of Larson's relations.\nThis interpretation implies that: (i) the slopes of linewidth-size and\nmass-size correlations are determined by the inertial cascade; (ii) none of the\nthree Larson laws is fundamental; (iii) instead, if one is known, the other two\nfollow from scale invariance of the kinetic energy transfer rate. It does not\nimply that gravity is dynamically unimportant. The self-similarity of structure\nestablished by the turbulence breaks in star-forming clouds due to the\ndevelopment of gravitational instability in the vicinity of the sonic scale.\nThe instability leads to the formation of prestellar cores with the\ncharacteristic mass set by the sonic scale. The high-end slope of the core mass\nfunction predicted by the scaling relations is consistent with the Salpeter\npower-law index.\n