Name/Affiliation: Peter Todd Williams (Agilent Technologies, Inc.)
Non-Magnetocentrifugal Protostellar Jets Created by the Magnetorotational Instability in Thick Accretion Flows
The dominant models for the creation of jets in a variety of astrophysical systems, including protostellar systems, include magnetocentrifugal acceleration by large-scale poloidal magnetic fields. This is true whether the material is thought to originate in a disk-wind, at an intermediate x-point, or from the star itself. Many alternative scenarios have been suggested in the literature however. Here we focus on what in our model is the manifestly non-magnetocentrifugal creation - that is, confinement, powering, launching, and collimation - of protostellar jets by turbulence driven by the magnetorotational instability (MRI). We re-examine our previous work on this matter in light of recent developments. Our scenario for MRI-driven protostellar jets requires that the dynamically-significant part of the accretion flow, where the jet is launched, completely envelop the nascent star. The star itself is completely surrounded by a flow separatrix that divides flow that is ultimately accreted onto the star from flow that is not. Power coupling to the outflow occurs by radially-outward viscous and thermal transport through the separatrix. Most of the power comes from the innermost regions of accretion, including the so-called boundary layer of the accretion flow. In this respect this mechanism differs from jet-launching mechanisms that rely upon thin-disk accretion. Inasmuch as geometrically-thick accretion correlates with high accretion rates, FU Ori objects are among the best candidates for this type of jet-production. One consequence of this model would be that the magnetic field in the jet-launching region be largely toroidal. Secondly, since the magnetic field is turbulent and tangled in nature, the field strength can decay faster with respect to axial distance from the central star than in magnetocentfiugal mechanisms. Finally, since the MRI saturates at a finite plasma beta, this model places lower limits on the gas pressure in the jet-launching region.