Protostellar disk formation by a non-rotating, non-axisymmetric collapsing cloud: model and comparison with observations

Abstract

Planet-forming disks are fundamental objects thought to be inherited from large scale rotation, through the conservation of angular momentum during the collapse of a prestellar dense core. We investigate the possibility for a protostellar disk to be formed from a motionless dense core which contains non-axisymmetric density fluctuations. The rotation is thus generated locally by the asymmetry of the collapse. We study the evolution of the angular momentum in a non-axisymmetric collapse of a dense core from an analytical point of view. To test the theory, we perform three-dimensional simulations of a collapsing prestellar dense core using adaptative mesh refinement. We start from a non-axisymmetrical situation, considering a dense core with random density perturbations that follow a turbulence spectrum. We analyse the emerging disk comparing the angular momentum it contains with the one expected from our analytic development. We study the velocity gradients at different scales in the simulation as it is done with observations. We show that the angular momentum in the frame of a stellar object which is not located at the center of mass of the core is not conserved, due to inertial forces. Our simulations of such non-axisymmetrical collapse quickly produce accretion disks at the small scales in the core. The analysis of the kinematics at different scales in the simulated core reveals projected velocity gradients of amplitudes similar to the ones observed in protostellar cores, and which directions vary, sometimes even reversing when small and large scales are compared. These complex kinematics patterns appear in recent observations, and could be a discriminating feature with models where rotation is inherited from large scales. Our results from simulations without initial rotation are more consistent with these recent observations than when solid-body rotation is initially [abridged]