Evolution of the Angular Momentum during Gravitational Fragmentation of Molecular Clouds*

Abstract

We investigate the origin of the observed scaling j ∼ R 3/2 between the specific angular momentum j and the radius R of molecular clouds (MCs) and their their substructures, and of the observed near independence of β , the ratio of rotational to gravitational energy, from R . To this end, we measure the angular momentum (AM) of “Lagrangian” particle sets in a smoothed particle hydrodynamics (SPH) simulation of the formation, collapse, and fragmentation of giant MCs. The Lagrangian sets are initially defined as connected particle sets above a certain density threshold at a certain time t def , and then the same set of SPH particles is followed either forward or backward in time. We find the following. (i) The Lagrangian particle sets evolve along the observed j – R relation when the volume containing them also contains a large number of other “intruder” particles. Otherwise, they evolve with j ∼ cst. (ii) Tracking Lagrangian sets to the future, we find that a subset of the SPH particles participates in the collapse, while the rest disperses away. (iii) These results suggest that the Lagrangian sets of fluid particles exchange their AM with other neighboring fluid particles via turbulent viscosity. (iv) We conclude that the j – R relation arises from a global tendency toward gravitational contraction, mediated by AM loss via turbulent viscosity, which induces fragmentation into dense, low-AM clumps, and diffuse, high-AM envelopes, which disperse away, limiting the mass efficiency of the fragmentation.