Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

Abstract

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = Rx, where the Keplerian angular speed of rotation Omegax equals the angular speed of the star Omega*. For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, Rx exceeds the radius of the star R* by a factor of a few, and the inner disk is effectively truncated at a radius Rt somewhat smaller than Rx. Where the closed field lines between Rt and Rx bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as Rx remains significantly greater than R*. Exterior to Rx field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate Mw equal to a definite fraction f of the disk accretion rate MD. For high disk accretion rates, Rx is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.