On the Roles of Gravity, Turbulence, and the Magnetic Field in Angular Momentum Transfer within Molecular Clouds

Observations of molecular structures on scales of ∼0.1−50 pc show that the specific angular momentum ( j ) scales with radius ( R ) as j ∼ R ^3/2 . We study the effects of turbulence, gravity, and the magnetic field in shaping this scaling, by measuring the clump size and specific angular momentum in three smoothed particle hydrodynamics simulations of the formation of giant molecular clouds, progressively adding these three ingredients. In each simulation, we define “full” and “reduced” clump samples, the latter restricted to aspect ratios A < 3. We find that in the nonmagnetic runs, elongated clumps deviate the most from the j – R relation, which is best reproduced by the reduced sample in the gravity+turbulence run. In the purely hydrodynamic case, no dense elongated structures form, suggesting that turbulence alone is insufficient to generate dense filaments, although clumps have j magnitudes consistent with observations. In the gravity+turbulence+magnetic field run, most of the clumps are filamentary, yet the full sample appears to follow the observed j – R relation. This result, rather than being a real trend, could be due to a combination of the increase in j by the filamentary geometry and of its reduction by turbulence inhibition by the magnetic field. Finally, we measure the gravitational, magnetic, pressure gradient, and hydrodynamic torques (which involve turbulent viscosity) in our clump samples. We find that in magnitude, the hydrodynamic torques tend to be larger than the rest. This result is consistent with our previous work, where we proposed that gravity drives cloud formation and contraction, while turbulence redistributes angular momentum through fluid parcel exchanges.