Evolution of the angular momentum during gravitational fragmentation of molecular clouds

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 substructures. To this end, we measure the angular momentum (AM) of "lagrangian" (fixed) sets of particles in an SPH simulation of the formation and collapse of giant MCs undergoing Global Hierarchical Collapse (GHC). We find that: i) The Lagrangian particle sets evolve from the past 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, similarly to accretion disks. iv) We conclude that the j-R relation arises from a global tendency towards 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.