Turbulence, Accretion Braking Torques and Efficient Jets Without Magnetocentrifugal Acceleration: Core Concepts

Abstract

I discuss three mutually-supportive notions or assumptions regarding jets and accretion. The first is magnetocentrifugal acceleration (MCA), the overwhelmingly favored mechanism for the production of jets in most steady accreting systems. The second is the zero-torque inner boundary condition. The third is that effective viscous dissipation is like real dissipation, leading directly to heating. All three assumptions fit nicely together in a manner that is simple, persuasive, and mutually-consistent. All, I argue, are incorrect. For concreteness I focus on protostars. Magnetohydrodynamic (MHD) turbulence in accretion is not a sink of energy, but a reservoir, capable of doing mechanical work directly and therefore efficiently, rather than solely through ohmic ("viscous") heating. Advection of turbulence energy reduces the effective radiative efficiency, and may help solve the missing boundary-layer emission problem. The angular momentum problem, whereby accretion spins up a protostar to breakup, is resolved by allowing direct viscous coupling to the protostar, permitting substantially greater energy to be deposited into the accretion flow than otherwise possible. This goes not into heat, but into a turbulent, tangled, buoyant toroidal magnetic field. I argue that there is neither an angular momentum problem nor an efficiency problem that MCA is needed to solve. Moreover, the turbulent magnetic field has ample ability not just to collimate but to accelerate gas, first radially inwards through tension forces and then vertically through pressure forces, without any MCA mechanism. I suggest then that jets, particularly the most powerful and well-collimated protostellar jets, are not magnetocentrifugally driven.